# 5. Seamless interoperation with the DOM

## 5.1. Practical example: a simple, responsive website using no HTML or CSS at all

To many programmers, using 'static' HTML and CSS feels like being locked up in a closet. As an alternative, responsiveness can simply be programmed using Transcrypt, as can be seen on this site written in Transcrypt. Note that it features sourcemaps. The site adapts to diverse screen formats, device types and landscape vs. portrait mode.

## 5.2. SVG example: Turtle graphics

Turtle graphics are a way to teach computer programming to children, invented by Seymour Papert in the 1960's. Lines are drawn as 'track' of a walking turtle, that can move ahead and turn. Children use turtle graphics intuitively by imagining what they would do if they were the turtle. This leads to a recipe of motion, indeed an algorithm, which is the basis of imperative (as opposed to e.g. declarative) programming.

SVG or Scalable Vector Graphics are a way to display high quality graphs, e.g. in the browser. SVG, as opposed to e.g. the HTML Canvas, bypasses the pixel paradigm and works with floating point coordinates directly. As a consequence, SVG plots can be zoomed without becoming ragged or 'pixelated'.

When looking under the hood of SVG, there's an amazing correspondence between the primitives in an SVG path and the primitives of turtle graphics. So both from an aestethical and from a conceptual point of view, turtle graphics and SVG form a happy mariage.

Turtle graphics in Transcrypt do not require the use of any graphics libraries. Below are two turtle graphics examples and the source code of Transcrypt's turtle module, which is quite compact. As can be seen from the code integration between Transcrypt and JavaScript is trivial.

Drawing a alternatingly floodfilled star

    # Free after the example from the Python 3.5 manual

    from turtle import *

    fillColor = 'yellow'

    while fillColor:
        print ('Will plot star with fill color: ', fillColor)
        print ()

        up ()
        goto (-250, -21)
        startPos = pos ()

        down ()
        color ('red', fillColor)
        begin_fill ()
        while True:
            forward (500)
            right (170)
            if distance (startPos) < 1:
                break
        end_fill ()
        done ()

        fillColor = input ('Give new fill color (blue, green, orange, lightblue, ...) :')

Click here to view the resulting zoomable star.

Drawing the contours of a snowflake

    from turtle import *

    josh = Turtle ()

    def draw (length):
        if length > 9:
            draw (length / 3)
            josh.left (60)
            draw (length / 3)
            josh.right (120)
            draw (length / 3)
            josh.left (60)
            draw (length / 3)
        else:
            josh.forward (length)

    length = 150
    josh.up ()
    josh.forward (length / 2)
    josh.left (90)
    josh.forward (length / 4)
    josh.right (90)
    josh.down ()

    for i in range (3):
        josh.right (120)
        draw (length)

    josh.done ()

Click here to view the resulting zoomable snowflake.

Transcrypt's turtle graphics module sits directly on top of SVG, no libraries needed, so a very compact download

    __pragma__ ('skip')
    document = Math = setInterval = clearInterval = 0
    __pragma__ ('noskip')

    _debug = False

    def abs (vec2D):
        return Math.sqrt (vec2D [0] * vec2D [0] + vec2D [1] * vec2D [1])

    _ns = 'http://www.w3.org/2000/svg'
    _svg = document.createElementNS (_ns, 'svg')

    _defaultElement = document.getElementById ('__turtlegraph__')
    if not _defaultElement:
        _defaultElement = document.body
    _defaultElement.appendChild (_svg)

    _width = None
    _height = None
    _offset = None

    def _rightSize (self):
        nonlocal _width
        nonlocal _height
        nonlocal _offset

        _width = _defaultElement.offsetWidth
        _height = _defaultElement.offsetHeight
        _offset = [_width // 2, _height // 2]

        _svg.setAttribute ('width', _width)
        _svg.setAttribute ('height', _height)

    window.onresize = _rightSize

    _rightSize ()

    def bgcolor (color):
        nonlocal _defaultElement

        _bgcolor = color
        _defaultElement.style.backgroundColor = _bgcolor

    bgcolor ('white')

    def setDefaultElement (element):
        nonlocal _defaultElement

        _defaultElement.removeChild (_svg)
        _defaultElement = element
        element.appendChild (_svg)

        _rightSize ()
        bgcolor ('white')

    _allTurtles = []

    class Turtle:
        def __init__ (self):
            _allTurtles.append (self)
            self._paths = []
            self.reset ()

        def reset (self):
            self._heading = Math.PI / 2
            self.pensize (1)
            self.color ('black', 'black')
            self.down ()
            self._track = []    # Need to make track explicitly because:
            self.home ()        #   Makes a position but needs a track to put in in
            self.clear ()       #   Makes a track but needs a position to initialize it with

        def clear (self):
            for path in self._paths:
                _svg.removeChild (path)
            self._paths = []

            self._track = []
            self._moveto (self._position)

        def _flush (self):
            if _debug:
                print ('Flush:', self._track)

            if len (self._track) > 1:
                path = document.createElementNS (_ns, 'path')
                path.setAttribute ('d', ' '.join (self._track))
                path.setAttribute ('stroke', self._pencolor if self._pencolor != None else 'none')
                path.setAttribute ('stroke-width', self._pensize)
                path.setAttribute ('fill', self._fillcolor if self._fill and self._fillcolor != None else 'none')
                path.setAttribute ('fill-rule', 'evenodd')
                _svg.appendChild (path)
                self._paths.append (path)

                self._track = []
                self._moveto (self._position)   # _track should start with a move command

        def done (self):
            self._flush ()

        def pensize (self, width):
            self._flush ()
            if width == None:
                return self._pensize
            else:
                self._pensize = width

        def color (self, pencolor, fillcolor = None):
            self._flush ()
            self._pencolor = pencolor

            if fillcolor != None:
                self._fillcolor = fillcolor

        def goto (self, x, y = None):
            if y == None:
                self._position = x
            else:
                self._position = [x, y]

            self._track.append ('{} {} {}'.format (
                'L' if self._down else 'M',
                self._position [0] + _offset [0],
                self._position [1] + _offset [1])
            )

        def _moveto (self, x, y = None):
            wasdown = self.isdown ()
            self.up ()
            self.goto (x, y)
            if wasdown:
                self.down ()

        def home (self):
            self._moveto (0, 0)

        def position (self):
            return self._position [:]

        def pos (self):
            return self.position ()

        def distance (self, x, y = None):
            if y == None:
                other = x
            else:
                other = [x, y]

            dX = other [0] - self._position [0]
            dY = other [1] - self._position [1]

            return Math.sqrt (dX * dX + dY * dY)

        def up (self):
            self._down = False

        def down (self):
            self._down = True

        def isdown (self):
            return self._down

        def _predict (self, length):
            delta = [Math.sin (self._heading), Math.cos (self._heading)]
            return [self._position [0] + length * delta [0], self._position [1] + length * delta [1]]

        def forward (self, length):
            self._position = self._predict (length)

            self._track.append ('{} {} {}'.format (
                'L' if self._down else 'M',
                self._position [0] + _offset [0],
                self._position [1] + _offset [1])
            )

        def back (self, length):
            self.forward (-length)

        def circle (self, radius):
            self.left (90)
            opposite = self._predict (2 * (radius + 1) + 1)
            self.right (90)

            self._track.append ('{} {} {} {} {} {} {} {}'.format (
                'A',
                radius,
                radius,
                0,
                1,
                0,
                opposite [0] + _offset [0],
                opposite [1] + _offset [1]
            ))

            self._track.append ('{} {} {} {} {} {} {} {}'.format (
                'A',
                radius,
                radius,
                0,
                1,
                0,
                self._position [0] + _offset [0],
                self._position [1] + _offset [1]
            ))

        def left (self, angle):
            self._heading = (self._heading + Math.PI * angle / 180) % (2 * Math.PI)

        def right (self, angle):
            self.left (-angle)

        def begin_fill (self):
            self._flush ()
            self._fill = True

        def end_fill (self):
            self._flush ()
            self._fill = False

        def speed (speed = None):
            pass

    _defaultTurtle = Turtle ()
    _timer = None

    def reset ():
        nonlocal _timer, _allTurtles
        if _timer:
            clearTimeout (_timer)
        bgcolor ('white')
        for turtle in _allTurtles:
            turtle.reset ()
            turtle.done ()

    def clear ():
        nonlocal _allTurtles
        for turtle in _allTurtles:
            turtle.clear ()

    def ontimer (fun, t = 0):
        nonlocal _timer
        _timer = setTimeout (fun, t)

    def done ():                            _defaultTurtle.done ()
    def pensize (width):                    _defaultTurtle.pensize (width)
    def color (pencolor, fillcolor = None): _defaultTurtle.color (pencolor, fillcolor)
    def home ():                            _defaultTurtle.home ()
    def goto (x, y = None):                 _defaultTurtle.goto (x, y)
    def position ():                        return _defaultTurtle.position ()
    def pos ():                             return _defaultTurtle.pos ()
    def distance (x, y = None):             return _defaultTurtle.distance (x, y)
    def up ():                              _defaultTurtle.up ()
    def down ():                            _defaultTurtle.down ()
    def forward (length):                   _defaultTurtle.forward (length)
    def back (length):                      _defaultTurtle.back (length)
    def circle (radius):                    _defaultTurtle.circle (radius)
    def left (angle):                       _defaultTurtle.left (angle)
    def right (angle):                      _defaultTurtle.right (angle)
    def begin_fill ():                      _defaultTurtle.begin_fill ()
    def end_fill ():                        _defaultTurtle.end_fill ()
    def speed (speed):                      _defaultTurtle.speed (speed)

Remark: In a later stage animation may be added. As a further step, for complicated fractals, transparent server side compilation of a relatively simple algorithm would allow on-line editing combined with fast client side rendering of high-resolution graphics.

# 6. Mixed examples

## 6.1. Three ways of integration with JavaScript libraries

There are three ways to integrate Transcrypt applications with existing JavaScript libraries.

1. The simplest way is to use the library as is, without any encapsulation. In this way all symbols of that library will be in the global namespace. While many JavaScript programmers don't seem to mind that, many Python programmers do.
2. Another way is to encapsulate the JavaScript library as a whole in a Transcrypt module. In the distibution this is done for the fabric module, that encapsulates fabric.js. In this way the global namespace stays clean.
3. The third way is to write a complete Pythonic API for the JavaScript library. This is overkill in most cases and makes it harder to keep up with new versions of the library. Note that Transcrypt was desiged to make seamless cooperation between Transcrypt and JavaScript libraries possible without

In the Pong example below, approach 2 is choosen to encapsulate the fabric.js graphics library. In most cases this approach strikes a good balance between effort and yield. As can be seen below, the effort involved is minimal.

The encapsulation layer for fabric.js

    __pragma__ ('noanno')

    fabric = __pragma__ ('js',
        '''
    (function () {{
        var exports = {{}};
        {}  // Puts fabric in exports and in global window
        delete window.fabric;
        return exports;
    }}) () .fabric;
        ''',
        __include__ ('com/fabricjs/__javascript__/fabric.js')
    )

Note that __pragma__ ('js', <skeletoncode>, includes = [<file1>, <file2>, ..]) is used to achieve the encapsulation. It replaces the {} by the respective contents of the files. The fabric module is part of the download. Note that not all facilities were included in customizing fabric.js. You can drop-in replace the fabric.js in the __javascript__ subdirectory by another customized version without changing anything. Preferably download a development version, since that enables easy debugging. Transcryp will minify it for you on the fly.

## 6.2. Example: Pong

In using the fabric.js JavaScript library this example, the only thing differing from plain JavaScipt is that new <constructor> is replaced by __new__ (<constructor>).

pong.py
    __pragma__ ('skip')
    document = window = Math = Date = 0 # Prevent complaints by optional static checker
    __pragma__ ('noskip')

    __pragma__ ('noalias', 'clear')

    from com.fabricjs import fabric

    orthoWidth = 1000
    orthoHeight = 750
    fieldHeight = 650

    enter, esc, space = 13, 27, 32

    window.onkeydown = lambda event: event.keyCode != space # Prevent scrolldown on spacebar press

    class Attribute:    # Attribute in the gaming sense of the word, rather than of an object
        def __init__ (self, game):
            self.game = game                    # Attribute knows game it's part of
            self.game.attributes.append (self)  # Game knows all its attributes
            self.install ()                     # Put in place graphical representation of attribute
            self.reset ()                       # Reset attribute to start position

        def reset (self):       # Restore starting positions or score, then commit to fabric
            self.commit ()      # Nothing to restore for the Attribute base class

        def predict (self):
            pass

        def interact (self):
            pass

        def commit (self):
            pass

    class Sprite (Attribute):   # Here, a sprite is an attribute that can move
        def __init__ (self, game, width, height):
            self.width = width
            self.height = height
            Attribute.__init__ (self, game)

        def install (self):     # The sprite holds an image that fabric can display
            self.image = __new__ (fabric.Rect ({
                'width': self.game.scaleX (self.width), 'height': self.game.scaleY (self.height),
                'originX': 'center', 'originY': 'center', 'fill': 'white'
            }))

        __pragma__ ('kwargs')
        def reset (self, vX = 0, vY = 0, x = 0, y = 0):
            self.vX = vX        # Speed
            self.vY = vY

            self.x = x          # Predicted position, can be commit, no bouncing initially
            self.y = y

            Attribute.reset (self)
        __pragma__ ('nokwargs')

        def predict (self):     # Predict position, do not yet commit, bouncing may alter it
            self.x += self.vX * self.game.deltaT
            self.y += self.vY * self.game.deltaT

        def commit (self):      # Update fabric image for asynch draw
            self.image.left = self.game.orthoX (self.x)
            self.image.top = self.game.orthoY (self.y)

        def draw (self):
            self.game.canvas.add (self.image)

    class Paddle (Sprite):
        margin = 30 # Distance of paddles from walls
        width = 10
        height = 100
        speed = 400 # / s

        def __init__ (self, game, index):
            self.index = index  # Paddle knows its player index, 0 == left, 1 == right
            Sprite.__init__ (self, game, self.width, self.height)

        def reset (self):       # Put paddle in rest position, dependent on player index
            Sprite.reset (
                self,
                x = orthoWidth // 2 - self.margin if self.index else -orthoWidth // 2 + self.margin,
                y = 0
            )

        def predict (self): # Let paddle react on keys
            self.vY = 0

            if self.index:                          # Right player
                if self.game.keyCode == ord ('K'):  # Letter K pressed
                    self.vY = self.speed
                elif self.game.keyCode == ord ('M'):
                    self.vY = -self.speed
            else:                                   # Left player
                if self.game.keyCode == ord ('A'):
                    self.vY = self.speed
                elif self.game.keyCode == ord ('Z'):
                    self.vY = -self.speed

            Sprite.predict (self)                   # Do not yet commit, paddle may bounce with walls

        def interact (self):    # Paddles and ball assumed infinitely thin
            # Paddle touches wall
            self.y = Math.max (self.height // 2 - fieldHeight // 2, Math.min (self.y, fieldHeight // 2 - self.height // 2))

            # Paddle hits ball
            if (
                (self.y - self.height // 2) < self.game.ball.y < (self.y + self.height // 2)
                and (
                    (self.index == 0 and self.game.ball.x < self.x) # On or behind left paddle
                    or
                    (self.index == 1 and self.game.ball.x > self.x) # On or behind right paddle
                )
            ):
                self.game.ball.x = self.x               # Ball may have gone too far already
                self.game.ball.vX = -self.game.ball.vX  # Bounce on paddle
                self.game.ball.speedUp (self)

    class Ball (Sprite):
        side = 8
        speed = 300 # / s

        def __init__ (self, game):
            Sprite.__init__ (self, game, self.side, self.side)

        def reset (self):   # Launch according to service direction with random angle offset from horizontal
            angle =  (
                self.game.serviceIndex * Math.PI    # Service direction
                +
                (1 if Math.random () > 0.5 else -1) * Math.random () * Math.atan (fieldHeight / orthoWidth)
            )

            Sprite.reset (
                self,
                vX = self.speed * Math.cos (angle),
                vY = self.speed * Math.sin (angle)
            )

        def predict (self):
            Sprite.predict (self)           # Integrate velocity to position

            if self.x < -orthoWidth // 2:   # If out on left side
                self.game.scored (1)        #   Right player scored
            elif self.x > orthoWidth // 2:
                self.game.scored (0)

            if self.y > fieldHeight // 2:   # If it hits top wall
                self.y = fieldHeight // 2   #   It may have gone too far already
                self.vY = -self.vY          #   Bounce
            elif self.y < -fieldHeight // 2:
                self.y = -fieldHeight // 2
                self.vY = -self.vY

        def speedUp (self, bat):
            factor = 1 + 0.15 * (1 - Math.abs (self.y - bat.y) / (bat.height // 2)) ** 2    # Speed will increase more if paddle hit near centre

            if Math.abs (self.vX) < 3 * self.speed:
                self.vX *= factor
                self.vY *= factor

    class Scoreboard (Attribute):
        nameShift = 75
        hintShift = 25

        def install (self): # Graphical representation of scoreboard are four labels and a separator line
            self.playerLabels = [__new__ (fabric.Text ('Player {}'.format (name), {
                    'fill': 'white', 'fontFamily': 'arial', 'fontSize': '{}' .format (self.game.canvas.width / 30),
                    'left': self.game.orthoX (position * orthoWidth), 'top': self.game.orthoY (fieldHeight // 2 + self.nameShift)
            })) for name, position in (('AZ keys:', -7/16), ('KM keys:', 1/16))]

            self.hintLabel = __new__ (fabric.Text ('[spacebar] starts game, [enter] resets score', {
                    'fill': 'white', 'fontFamily': 'arial', 'fontSize': '{}'.format (self.game.canvas.width / 70),
                    'left': self.game.orthoX (-7/16 * orthoWidth), 'top': self.game.orthoY (fieldHeight // 2 + self.hintShift)
            }))

            self.image = __new__ (fabric.Line ([
                    self.game.orthoX (-orthoWidth // 2), self.game.orthoY (fieldHeight // 2),
                    self.game.orthoX (orthoWidth // 2), self.game.orthoY (fieldHeight // 2)
                ],
                {'stroke': 'white'}
            ))

        def increment (self, playerIndex):
            self.scores [playerIndex] += 1

        def reset (self):
            self.scores = [0, 0]
            Attribute.reset (self)  # Only does a commit here

        def commit (self):          # Committing labels is adapting their texts
            self.scoreLabels = [__new__ (fabric.Text ('{}'.format (score), {
                    'fill': 'white', 'fontFamily': 'arial', 'fontSize': '{}'.format (self.game.canvas.width / 30),
                    'left': self.game.orthoX (position * orthoWidth), 'top': self.game.orthoY (fieldHeight // 2 + self.nameShift)
            })) for score, position in zip (self.scores, (-2/16, 6/16))]

        def draw (self):
            for playerLabel, scoreLabel in zip (self.playerLabels, self.scoreLabels):
                self.game.canvas.add (playerLabel)
                self.game.canvas.add (scoreLabel)
                self.game.canvas.add (self.hintLabel)
            self.game.canvas.add (self.image)

    class Game:
        def __init__ (self):
            self.serviceIndex = 1 if Math.random () > 0.5 else 0    # Index of player that has initial service
            self.pause = True                           # Start game in paused state
            self.keyCode = None

            self.textFrame = document.getElementById ('text_frame')
            self.canvasFrame = document.getElementById ('canvas_frame')
            self.buttonsFrame = document.getElementById ('buttons_frame')

            self.canvas = __new__ (fabric.Canvas ('canvas', {'backgroundColor': 'black', 'originX': 'center', 'originY': 'center'}))
            self.canvas.onWindowDraw = self.draw        # Install draw callback, will be called asynch
            self.canvas.lineWidth = 2
            self.canvas.clear ()

            self.attributes = []                        # All attributes will insert themselves here
            self.paddles = [Paddle (self, index) for index in range (2)]    # Pass game as parameter self
            self.ball = Ball (self)
            self.scoreboard = Scoreboard (self)

            window.setInterval (self.update, 10)    # Install update callback, time in ms
            window.setInterval (self.draw, 20)      # Install draw callback, time in ms
            window.addEventListener ('keydown', self.keydown)
            window.addEventListener ('keyup', self.keyup)

            self.buttons = []

            for key in ('A', 'Z', 'K', 'M', 'space', 'enter'):
                button = document.getElementById (key)
                button.addEventListener ('mousedown', (lambda aKey: lambda: self.mouseOrTouch (aKey, True)) (key))  # Returns inner lambda
                button.addEventListener ('touchstart', (lambda aKey: lambda: self.mouseOrTouch (aKey, True)) (key))
                button.addEventListener ('mouseup', (lambda aKey: lambda: self.mouseOrTouch (aKey, False)) (key))
                button.addEventListener ('touchend', (lambda aKey: lambda: self.mouseOrTouch (aKey, False)) (key))
                button.style.cursor = 'pointer'
                button.style.userSelect = 'none'
                self.buttons.append (button)

            self.time = + __new__ (Date)

            window.onresize = self.resize
            self.resize ()

        def install (self):
            for attribute in self.attributes:
                attribute.install ()

        def mouseOrTouch (self, key, down):
            if down:
                if key == 'space':
                    self.keyCode = space
                elif key == 'enter':
                    self.keyCode = enter
                else:
                    self.keyCode = ord (key)
            else:
                self.keyCode = None

        def update (self):                          # Note that update and draw are not synchronized
            oldTime = self.time
            self.time = + __new__ (Date)
            self.deltaT = (self.time - oldTime) / 1000.

            if self.pause:                          # If in paused state
                if self.keyCode == space:           #   If spacebar hit
                    self.pause = False              #         Start playing
                elif self.keyCode == enter:         #   Else if enter hit
                    self.scoreboard.reset ()        #         Reset score
            else:                                   # Else, so if in active state
                for attribute in self.attributes:   #   Compute predicted values
                    attribute.predict ()

                for attribute in self.attributes:   #   Correct values for bouncing and scoring
                    attribute.interact ()

                for attribute in self.attributes:   #   Commit them to pyglet for display
                    attribute.commit ()

        def scored (self, playerIndex):             # Player has scored
            self.scoreboard.increment (playerIndex) # Increment player's points
            self.serviceIndex = 1 - playerIndex     # Grant service to the unlucky player

            for paddle in self.paddles:             # Put paddles in rest position
                paddle.reset ()

            self.ball.reset ()                      # Put ball in rest position
            self.pause = True                       # Wait for next round

        def commit (self):
            for attribute in self.attributes:
                attribute.commit ()

        def draw (self):
            self.canvas.clear ()
            for attribute in self.attributes:
                attribute.draw ()

        def resize (self):
            self.pageWidth = window.innerWidth
            self.pageHeight = window.innerHeight

            self.textTop = 0

            if self.pageHeight > 1.2 * self.pageWidth:
                self.canvasWidth = self.pageWidth
                self.canvasTop = self.textTop + 300
            else:
                self.canvasWidth = 0.6 * self.pageWidth
                self.canvasTop = self.textTop + 200

            self.canvasLeft = 0.5 * (self.pageWidth - self.canvasWidth)
            self.canvasHeight = 0.6 * self.canvasWidth

            self.buttonsTop = self.canvasTop + self.canvasHeight + 50
            self.buttonsWidth = 500

            self.textFrame.style.top = self.textTop;
            self.textFrame.style.left = self.canvasLeft + 0.05 * self.canvasWidth
            self.textFrame.style.width = 0.9 * self.canvasWidth

            self.canvasFrame.style.top = self.canvasTop
            self.canvasFrame.style.left = self.canvasLeft
            self.canvas.setDimensions ({'width': self.canvasWidth, 'height': self.canvasHeight})

            self.buttonsFrame.style.top = self.buttonsTop
            self.buttonsFrame.style.left = 0.5 * (self.pageWidth - self.buttonsWidth)
            self.buttonsFrame.style.width = self.canvasWidth

            self.install ()
            self.commit ()
            self.draw ()

        def scaleX (self, x):
            return x * (self.canvas.width / orthoWidth)

        def scaleY (self, y):
            return y * (self.canvas.height / orthoHeight)

        def orthoX (self, x):
            return self.scaleX (x + orthoWidth // 2)

        def orthoY (self, y):
            return self.scaleY (orthoHeight - fieldHeight // 2 - y)

        def keydown (self, event):
            self.keyCode = event.keyCode

        def keyup (self, event):
            self.keyCode = None

    game = Game ()  # Create and run game
| pong.mod.js
        (function () {
            var fabric = __init__ (__world__.com.fabricjs).fabric;
            var orthoWidth = 1000;
            var orthoHeight = 750;
            var fieldHeight = 650;
            var __left0__ = tuple ([13, 27, 32]);
            var enter = __left0__ [0];
            var esc = __left0__ [1];
            var space = __left0__ [2];
            window.onkeydown = (function __lambda__ (event) {
                return event.keyCode != space;
            });
            var Attribute = __class__ ('Attribute', [object], {
                get __init__ () {return __get__ (this, function (self, game) {
                    self.game = game;
                    self.game.attributes.append (self);
                    self.install ();
                    self.reset ();
                }, '__init__');},
                get reset () {return __get__ (this, function (self) {
                    self.commit ();
                }, 'reset');},
                get predict () {return __get__ (this, function (self) {
                    // pass;
                }, 'predict');},
                get interact () {return __get__ (this, function (self) {
                    // pass;
                }, 'interact');},
                get commit () {return __get__ (this, function (self) {
                    // pass;
                }, 'commit');}
            });
            var Sprite = __class__ ('Sprite', [Attribute], {
                get __init__ () {return __get__ (this, function (self, game, width, height) {
                    self.width = width;
                    self.height = height;
                    Attribute.__init__ (self, game);
                }, '__init__');},
                get install () {return __get__ (this, function (self) {
                    self.image = new fabric.Rect (dict ({'width': self.game.scaleX (self.width), 'height': self.game.scaleY (self.height), 'originX': 'center', 'originY': 'center', 'fill': 'white'}));
                }, 'install');},
                get reset () {return __get__ (this, function (self, vX, vY, x, y) {
                    if (typeof vX == 'undefined' || (vX != null && vX .__class__ == __kwargdict__)) {;
                        var vX = 0;
                    };
                    if (typeof vY == 'undefined' || (vY != null && vY .__class__ == __kwargdict__)) {;
                        var vY = 0;
                    };
                    if (typeof x == 'undefined' || (x != null && x .__class__ == __kwargdict__)) {;
                        var x = 0;
                    };
                    if (typeof y == 'undefined' || (y != null && y .__class__ == __kwargdict__)) {;
                        var y = 0;
                    };
                    if (arguments.length) {
                        var __ilastarg0__ = arguments.length - 1;
                        if (arguments [__ilastarg0__] && arguments [__ilastarg0__].__class__ == __kwargdict__) {
                            var __allkwargs0__ = arguments [__ilastarg0__--];
                            for (var __attrib0__ in __allkwargs0__) {
                                switch (__attrib0__) {
                                    case 'self': var self = __allkwargs0__ [__attrib0__]; break;
                                    case 'vX': var vX = __allkwargs0__ [__attrib0__]; break;
                                    case 'vY': var vY = __allkwargs0__ [__attrib0__]; break;
                                    case 'x': var x = __allkwargs0__ [__attrib0__]; break;
                                    case 'y': var y = __allkwargs0__ [__attrib0__]; break;
                                }
                            }
                        }
                    }
                    else {
                    }
                    self.vX = vX;
                    self.vY = vY;
                    self.x = x;
                    self.y = y;
                    Attribute.reset (self);
                }, 'reset');},
                get predict () {return __get__ (this, function (self) {
                    self.x += self.vX * self.game.deltaT;
                    self.y += self.vY * self.game.deltaT;
                }, 'predict');},
                get commit () {return __get__ (this, function (self) {
                    self.image.left = self.game.orthoX (self.x);
                    self.image.top = self.game.orthoY (self.y);
                }, 'commit');},
                get draw () {return __get__ (this, function (self) {
                    self.game.canvas.add (self.image);
                }, 'draw');}
            });
            var Paddle = __class__ ('Paddle', [Sprite], {
                margin: 30,
                width: 10,
                height: 100,
                speed: 400,
                get __init__ () {return __get__ (this, function (self, game, index) {
                    self.index = index;
                    Sprite.__init__ (self, game, self.width, self.height);
                }, '__init__');},
                get reset () {return __get__ (this, function (self) {
                    Sprite.reset (self, __kwargdict__ ({x: (self.index ? Math.floor (orthoWidth / 2) - self.margin : Math.floor (-(orthoWidth) / 2) + self.margin), y: 0}));
                }, 'reset');},
                get predict () {return __get__ (this, function (self) {
                    self.vY = 0;
                    if (self.index) {
                        if (self.game.keyCode == ord ('K')) {
                            self.vY = self.speed;
                        }
                        else {
                            if (self.game.keyCode == ord ('M')) {
                                self.vY = -(self.speed);
                            }
                        }
                    }
                    else {
                        if (self.game.keyCode == ord ('A')) {
                            self.vY = self.speed;
                        }
                        else {
                            if (self.game.keyCode == ord ('Z')) {
                                self.vY = -(self.speed);
                            }
                        }
                    }
                    Sprite.predict (self);
                }, 'predict');},
                get interact () {return __get__ (this, function (self) {
                    self.y = Math.max (Math.floor (self.height / 2) - Math.floor (fieldHeight / 2), Math.min (self.y, Math.floor (fieldHeight / 2) - Math.floor (self.height / 2)));
                    if ((self.y - Math.floor (self.height / 2) < self.game.ball.y && self.game.ball.y < self.y + Math.floor (self.height / 2)) && (self.index == 0 && self.game.ball.x < self.x || self.index == 1 && self.game.ball.x > self.x)) {
                        self.game.ball.x = self.x;
                        self.game.ball.vX = -(self.game.ball.vX);
                        self.game.ball.speedUp (self);
                    }
                }, 'interact');}
            });
            var Ball = __class__ ('Ball', [Sprite], {
                side: 8,
                speed: 300,
                get __init__ () {return __get__ (this, function (self, game) {
                    Sprite.__init__ (self, game, self.side, self.side);
                }, '__init__');},
                get reset () {return __get__ (this, function (self) {
                    var angle = self.game.serviceIndex * Math.PI + ((Math.random () > 0.5 ? 1 : -(1)) * Math.random ()) * Math.atan (fieldHeight / orthoWidth);
                    Sprite.reset (self, __kwargdict__ ({vX: self.speed * Math.cos (angle), vY: self.speed * Math.sin (angle)}));
                }, 'reset');},
                get predict () {return __get__ (this, function (self) {
                    Sprite.predict (self);
                    if (self.x < Math.floor (-(orthoWidth) / 2)) {
                        self.game.scored (1);
                    }
                    else {
                        if (self.x > Math.floor (orthoWidth / 2)) {
                            self.game.scored (0);
                        }
                    }
                    if (self.y > Math.floor (fieldHeight / 2)) {
                        self.y = Math.floor (fieldHeight / 2);
                        self.vY = -(self.vY);
                    }
                    else {
                        if (self.y < Math.floor (-(fieldHeight) / 2)) {
                            self.y = Math.floor (-(fieldHeight) / 2);
                            self.vY = -(self.vY);
                        }
                    }
                }, 'predict');},
                get speedUp () {return __get__ (this, function (self, bat) {
                    var factor = 1 + 0.15 * Math.pow (1 - Math.abs (self.y - bat.y) / (Math.floor (bat.height / 2)), 2);
                    if (Math.abs (self.vX) < 3 * self.speed) {
                        self.vX *= factor;
                        self.vY *= factor;
                    }
                }, 'speedUp');}
            });
            var Scoreboard = __class__ ('Scoreboard', [Attribute], {
                nameShift: 75,
                hintShift: 25,
                get install () {return __get__ (this, function (self) {
                    self.playerLabels = function () {
                        var __accu0__ = [];
                        for (var [py_name, position] of tuple ([tuple (['AZ keys:', -(7) / 16]), tuple (['KM keys:', 1 / 16])])) {
                            __accu0__.append (new fabric.Text ('Player {}'.format (py_name), dict ({'fill': 'white', 'fontFamily': 'arial', 'fontSize': '{}'.format (self.game.canvas.width / 30), 'left': self.game.orthoX (position * orthoWidth), 'top': self.game.orthoY (Math.floor (fieldHeight / 2) + self.nameShift)})));
                        }
                        return __accu0__;
                    } ();
                    self.hintLabel = new fabric.Text ('[spacebar] starts game, [enter] resets score', dict ({'fill': 'white', 'fontFamily': 'arial', 'fontSize': '{}'.format (self.game.canvas.width / 70), 'left': self.game.orthoX ((-(7) / 16) * orthoWidth), 'top': self.game.orthoY (Math.floor (fieldHeight / 2) + self.hintShift)}));
                    self.image = new fabric.Line (list ([self.game.orthoX (Math.floor (-(orthoWidth) / 2)), self.game.orthoY (Math.floor (fieldHeight / 2)), self.game.orthoX (Math.floor (orthoWidth / 2)), self.game.orthoY (Math.floor (fieldHeight / 2))]), dict ({'stroke': 'white'}));
                }, 'install');},
                get increment () {return __get__ (this, function (self, playerIndex) {
                    self.scores [playerIndex]++;
                }, 'increment');},
                get reset () {return __get__ (this, function (self) {
                    self.scores = list ([0, 0]);
                    Attribute.reset (self);
                }, 'reset');},
                get commit () {return __get__ (this, function (self) {
                    self.scoreLabels = function () {
                        var __accu0__ = [];
                        for (var [score, position] of zip (self.scores, tuple ([-(2) / 16, 6 / 16]))) {
                            __accu0__.append (new fabric.Text ('{}'.format (score), dict ({'fill': 'white', 'fontFamily': 'arial', 'fontSize': '{}'.format (self.game.canvas.width / 30), 'left': self.game.orthoX (position * orthoWidth), 'top': self.game.orthoY (Math.floor (fieldHeight / 2) + self.nameShift)})));
                        }
                        return __accu0__;
                    } ();
                }, 'commit');},
                get draw () {return __get__ (this, function (self) {
                    for (var [playerLabel, scoreLabel] of zip (self.playerLabels, self.scoreLabels)) {
                        self.game.canvas.add (playerLabel);
                        self.game.canvas.add (scoreLabel);
                        self.game.canvas.add (self.hintLabel);
                    }
                    self.game.canvas.add (self.image);
                }, 'draw');}
            });
            var Game = __class__ ('Game', [object], {
                get __init__ () {return __get__ (this, function (self) {
                    self.serviceIndex = (Math.random () > 0.5 ? 1 : 0);
                    self.pause = true;
                    self.keyCode = null;
                    self.textFrame = document.getElementById ('text_frame');
                    self.canvasFrame = document.getElementById ('canvas_frame');
                    self.buttonsFrame = document.getElementById ('buttons_frame');
                    self.canvas = new fabric.Canvas ('canvas', dict ({'backgroundColor': 'black', 'originX': 'center', 'originY': 'center'}));
                    self.canvas.onWindowDraw = self.draw;
                    self.canvas.lineWidth = 2;
                    self.canvas.clear ();
                    self.attributes = list ([]);
                    self.paddles = function () {
                        var __accu0__ = [];
                        for (var index = 0; index < 2; index++) {
                            __accu0__.append (Paddle (self, index));
                        }
                        return __accu0__;
                    } ();
                    self.ball = Ball (self);
                    self.scoreboard = Scoreboard (self);
                    window.setInterval (self.py_update, 10);
                    window.setInterval (self.draw, 20);
                    window.addEventListener ('keydown', self.keydown);
                    window.addEventListener ('keyup', self.keyup);
                    self.buttons = list ([]);
                    for (var key of tuple (['A', 'Z', 'K', 'M', 'space', 'enter'])) {
                        var button = document.getElementById (key);
                        button.addEventListener ('mousedown', (function __lambda__ (aKey) {
                            return (function __lambda__ () {
                                return self.mouseOrTouch (aKey, true);
                            });
                        }) (key));
                        button.addEventListener ('touchstart', (function __lambda__ (aKey) {
                            return (function __lambda__ () {
                                return self.mouseOrTouch (aKey, true);
                            });
                        }) (key));
                        button.addEventListener ('mouseup', (function __lambda__ (aKey) {
                            return (function __lambda__ () {
                                return self.mouseOrTouch (aKey, false);
                            });
                        }) (key));
                        button.addEventListener ('touchend', (function __lambda__ (aKey) {
                            return (function __lambda__ () {
                                return self.mouseOrTouch (aKey, false);
                            });
                        }) (key));
                        button.style.cursor = 'pointer';
                        button.style.userSelect = 'none';
                        self.buttons.append (button);
                    }
                    self.time = +(new Date);
                    window.onresize = self.resize;
                    self.resize ();
                }, '__init__');},
                get install () {return __get__ (this, function (self) {
                    for (var attribute of self.attributes) {
                        attribute.install ();
                    }
                }, 'install');},
                get mouseOrTouch () {return __get__ (this, function (self, key, down) {
                    if (down) {
                        if (key == 'space') {
                            self.keyCode = space;
                        }
                        else {
                            if (key == 'enter') {
                                self.keyCode = enter;
                            }
                            else {
                                self.keyCode = ord (key);
                            }
                        }
                    }
                    else {
                        self.keyCode = null;
                    }
                }, 'mouseOrTouch');},
                get py_update () {return __get__ (this, function (self) {
                    var oldTime = self.time;
                    self.time = +(new Date);
                    self.deltaT = (self.time - oldTime) / 1000.0;
                    if (self.pause) {
                        if (self.keyCode == space) {
                            self.pause = false;
                        }
                        else {
                            if (self.keyCode == enter) {
                                self.scoreboard.reset ();
                            }
                        }
                    }
                    else {
                        for (var attribute of self.attributes) {
                            attribute.predict ();
                        }
                        for (var attribute of self.attributes) {
                            attribute.interact ();
                        }
                        for (var attribute of self.attributes) {
                            attribute.commit ();
                        }
                    }
                }, 'update');},
                get scored () {return __get__ (this, function (self, playerIndex) {
                    self.scoreboard.increment (playerIndex);
                    self.serviceIndex = 1 - playerIndex;
                    for (var paddle of self.paddles) {
                        paddle.reset ();
                    }
                    self.ball.reset ();
                    self.pause = true;
                }, 'scored');},
                get commit () {return __get__ (this, function (self) {
                    for (var attribute of self.attributes) {
                        attribute.commit ();
                    }
                }, 'commit');},
                get draw () {return __get__ (this, function (self) {
                    self.canvas.clear ();
                    for (var attribute of self.attributes) {
                        attribute.draw ();
                    }
                }, 'draw');},
                get resize () {return __get__ (this, function (self) {
                    self.pageWidth = window.innerWidth;
                    self.pageHeight = window.innerHeight;
                    self.textTop = 0;
                    if (self.pageHeight > 1.2 * self.pageWidth) {
                        self.canvasWidth = self.pageWidth;
                        self.canvasTop = self.textTop + 300;
                    }
                    else {
                        self.canvasWidth = 0.6 * self.pageWidth;
                        self.canvasTop = self.textTop + 200;
                    }
                    self.canvasLeft = 0.5 * (self.pageWidth - self.canvasWidth);
                    self.canvasHeight = 0.6 * self.canvasWidth;
                    self.buttonsTop = (self.canvasTop + self.canvasHeight) + 50;
                    self.buttonsWidth = 500;
                    self.textFrame.style.top = self.textTop;
                    self.textFrame.style.left = self.canvasLeft + 0.05 * self.canvasWidth;
                    self.textFrame.style.width = 0.9 * self.canvasWidth;
                    self.canvasFrame.style.top = self.canvasTop;
                    self.canvasFrame.style.left = self.canvasLeft;
                    self.canvas.setDimensions (dict ({'width': self.canvasWidth, 'height': self.canvasHeight}));
                    self.buttonsFrame.style.top = self.buttonsTop;
                    self.buttonsFrame.style.left = 0.5 * (self.pageWidth - self.buttonsWidth);
                    self.buttonsFrame.style.width = self.canvasWidth;
                    self.install ();
                    self.commit ();
                    self.draw ();
                }, 'resize');},
                get scaleX () {return __get__ (this, function (self, x) {
                    return x * (self.canvas.width / orthoWidth);
                }, 'scaleX');},
                get scaleY () {return __get__ (this, function (self, y) {
                    return y * (self.canvas.height / orthoHeight);
                }, 'scaleY');},
                get orthoX () {return __get__ (this, function (self, x) {
                    return self.scaleX (x + Math.floor (orthoWidth / 2));
                }, 'orthoX');},
                get orthoY () {return __get__ (this, function (self, y) {
                    return self.scaleY ((orthoHeight - Math.floor (fieldHeight / 2)) - y);
                }, 'orthoY');},
                get keydown () {return __get__ (this, function (self, event) {
                    self.keyCode = event.keyCode;
                }, 'keydown');},
                get keyup () {return __get__ (this, function (self, event) {
                    self.keyCode = null;
                }, 'keyup');}
            });
            var game = Game ();
            __pragma__ ('<use>' +
                'com.fabricjs' +
            '</use>')
            __pragma__ ('<all>')
                __all__.Attribute = Attribute;
                __all__.Ball = Ball;
                __all__.Game = Game;
                __all__.Paddle = Paddle;
                __all__.Scoreboard = Scoreboard;
                __all__.Sprite = Sprite;
                __all__.enter = enter;
                __all__.esc = esc;
                __all__.fabric = fabric;
                __all__.fieldHeight = fieldHeight;
                __all__.game = game;
                __all__.orthoHeight = orthoHeight;
                __all__.orthoWidth = orthoWidth;
                __all__.space = space;
            __pragma__ ('</all>')
        }) ();

## 6.3. Joined minification

Minification is currently performed by the Google closure compiler, that's also part of the distribution. Rather than separately minifying libraries, the application is minified as a whole. In principle this enables a smaller total download size. Currently closures ADVANCED_OPTIMIZATIONS switch breaks the working strict code, however, so the SIMPLE_OPTIMIZATIONS switch is used by default.

As can be seen from the listings, pong.mod.js without libraries is only slightly longer than pong.py without libraries. The difference mainly comes from the expensive keyword arguments mechanism that is activated for the reset function, using __pragma__ ('kargs') and __pragma__ ('nokwargs'). The minified version is about half this size. The Transcrypt runtime itself in minified form is about 9kB. So the bulk of the total size of the minified file, 148kB comes from fabric.js. From this example it becomes clear that Transcrypt is extremely lightweight.

## 6.4. Example: jQuery

In contrast to the use of the fabric.js library in the Pong example, jQuery hasn't been encapsulated at all. It's just downloaded on the fly from a content delivery network and used as-is. Instead of the $ (that is not a valid Python identifier), an S is used as alias. This might have been any character sequence.

jquery_demo.py
    __pragma__ ('alias', 'S', '$')

    def start ():
        def changeColors ():
            for div in S__divs:
                S (div) .css ({
                    'color': 'rgb({},{},{})'.format (* [int (256 * Math.random ()) for i in range (3)]),
                })

        S__divs = S ('div')
        changeColors ()
        window.setInterval (changeColors, 500)
| jquery_demo.mod.js
        (function () {
            var start = function () {
                var changeColors = function () {
                    for (var div of $divs) {
                        $ (div).css (dict ({'color': 'rgb({},{},{})'.format (...function () {
                            var __accu0__ = [];
                            for (var i = 0; i < 3; i++) {
                                __accu0__.append (int (256 * Math.random ()));
                            }
                            return __accu0__;
                        } ())}));
                    }
                };
                var $divs = $ ('div');
                changeColors ();
                window.setInterval (changeColors, 500);
            };
            __pragma__ ('<all>')
                __all__.start = start;
            __pragma__ ('</all>')
        }) ();

## 6.5. Example: iOS web app with native look and feel

You can write full screen iOS web apps in Transcrypt with native look and feel. As example here's an app simulating 6 dice. While this example is kept very simple, you can in fact make apps of arbitrary complexity, with fast and beautiful graphics using any JS graphics library, e.g. multiplayer games working over the Internet. If you add the app to your homescreen it will be cached, so no Internet connection is needed to use it. Web apps for iOS can obtain and use location information from the user.

You can install this app on your iPhone from http://www.transcrypt.org/live/transcrypt/demos/ios_app/ios_app.html .

ios_app.py
    import random

    class Dice:
        def __init__ (self):
            document.body.addEventListener ('touchstart', lambda event: event.preventDefault ())
            document.body.addEventListener ('mousedown', lambda event: event.preventDefault ())
            document.body.style.margin = 0
            document.body.style.overflow = 'hidden';

            self.all = document.createElement ('div')
            self.all.style.color = 'white'
            self.all.style.backgroundColor = 'black'
            self.all.style.height = '100%'
            self.all.style.width = '100%'
            self.all.style.padding = 0
            self.all.style.margin = 0
            document.body.appendChild (self.all)

            self.dices = []

            for index in range (6):
                dice = document.createElement ('div')
                dice.normalColor = '#770000' if index < 3 else '#0000ff'
                dice.style.position = 'absolute'
                dice.style.backgroundColor = dice.normalColor
                dice.addEventListener ('touchstart', (lambda aDice: lambda: self.roll (aDice)) (dice))  # Returns inner lambda
                dice.addEventListener ('mousedown', (lambda aDice: lambda: self.roll (aDice)) (dice))
                self.dices.append (dice)
                self.all.appendChild (dice)

                dice.inner = document.createElement ('div')
                dice.inner.setAttribute ('unselectable', 'on')
                dice.inner.style.fontWeight = 'bold'
                dice.inner.style.textAlign = 'center'
                dice.inner.style.position = 'absolute'
                dice.inner.innerHTML = '?'
                dice.appendChild (dice.inner)

            self.banner = document.createElement ('div')
            self.banner.style.position = 'absolute'
            self.banner.style.cursor = 'pointer'
            self.banner.addEventListener ('touchstart', self.gotoTranscryptSite)
            self.banner.addEventListener ('mousedown', self.gotoTranscryptSite)
            self.banner.style.fontFamily = 'arial'
            self.banner.innerHTML = (
                '<span id="bannerLarge"><font color="777777">www.<b><i>' +
                '<font color="ff4422">T<font color="ffb000">r<font color="228822">a<font color="3366ff">n' +
                '<font color="ff4422">s<font color="ffb000">c<font color="228822">r<font color="3366ff">y<font color="ffb000">p<font color="228822">t' +
                '</i></b><font color="777777">.org<font size={}><font color="cccccc"></span>' +
                '<span id="bannerSmall"><i> Write your apps in Python for free!</i></span>'
            )
            self.all.appendChild (self.banner)

            self.bannerLarge = document.getElementById ('bannerLarge')
            self.bannerSmall = document.getElementById ('bannerSmall')

            self.audio = __new__ (Audio ('ios_app.mp3'))

            window.onresize = self.rightSize
            self.rightSize ()

        def gotoTranscryptSite (self):
            document.location.href = 'http://www.transcrypt.org'

        def roll (self, dice):
            frameIndex = 10

            self.audio.play ()

            def frame ():
                nonlocal frameIndex
                frameIndex -= 1

                dice.inner.innerHTML = random.randint (1, 6)

                if frameIndex:
                    dice.style.color = random.choice (('red', 'green', 'blue', 'yellow'))
                    setTimeout (frame, 100)
                else:
                    dice.style.backgroundColor = dice.normalColor
                    dice.style.color = 'white'

            frame ()

        def rightSize (self):
            self.pageWidth = window.innerWidth
            self.pageHeight = window.innerHeight
            portrait = self.pageHeight > self.pageWidth

            for index, dice in enumerate (self.dices):
                if self.pageHeight > self.pageWidth:    # Portrait
                    dice.style.height = 0.3 * self.pageHeight
                    dice.style.width = 0.4 * self.pageWidth
                    dice.style.top = (0.03 + (index if index < 3 else index - 3) * 0.32) * self.pageHeight
                    dice.style.left = (0.06 if index < 3 else 0.54) * self.pageWidth

                    charBoxSide = 0.3 * self.pageHeight
                    dice.inner.style.top = 0.15 * self.pageHeight - 0.6 * charBoxSide
                    dice.inner.style.left = 0.2 * self.pageWidth - 0.5 * charBoxSide

                    self.banner.style.top = 0.975 * self.pageHeight
                    self.banner.style.left = 0.06 * self.pageWidth
                    self.bannerLarge.style.fontSize = 0.017 * self.pageHeight
                    self.bannerSmall.style.fontSize = 0.014 * self.pageHeight
                else:                                   # Landscape
                    dice.style.height = 0.4 * self.pageHeight
                    dice.style.width = 0.3 * self.pageWidth
                    dice.style.top = (0.06 if index < 3 else 0.54) * self.pageHeight
                    dice.style.left = (0.03 + (index if index < 3 else index - 3) * 0.32) * self.pageWidth

                    charBoxSide = 0.4 * self.pageHeight
                    dice.inner.style.top = 0.2 * self.pageHeight - 0.6 * charBoxSide
                    dice.inner.style.left = 0.15 * self.pageWidth - 0.5 * charBoxSide

                    self.banner.style.top = 0.95 * self.pageHeight
                    self.banner.style.left = 0.03 * self.pageWidth
                    self.bannerLarge.style.fontSize = 0.015 * self.pageWidth
                    self.bannerSmall.style.fontSize = 0.012 * self.pageWidth

                dice.inner.style.height = charBoxSide
                dice.inner.style.width = charBoxSide
                dice.inner.style.fontSize = charBoxSide

    dice = Dice ()

ios_app.html
    <html manifest="cache.manifest">
        <!-- v45 -->
        <head>
            <meta name="apple-mobile-web-app-capable" content="yes">
            <meta name="apple-mobile-web-app-status-bar-style" content="black">
            <meta name="apple-mobile-web-app-title" content="Dice">
            <meta name="viewport" content="width=device-width, initial-scale=1, user-scalable=no">
            <link rel="apple-touch-icon" href="ios_app.png">
        </head>
        <body>
            <script src="__javascript__/ios_app.js"></script>
        </body>
    </html>

cache.manifest
    CACHE MANIFEST

    # v45

    CACHE:
    ios_app.html
    ios_app.mp3
    __javascript__/ios_app.js

N.B.1 Cache manifests have to be served with mime type text/cache-manifest.

N.B.2 For native behaviour, e.g. no visible address bar, the app must indeed be added to the home screen of your iOS device.

## 6.6. Example: D3.js

The D3.js graphics library offers animation by data driven DOM manipulation. It combines well with class based object oriented programming as supported by Trancrypt, leading to applications that are easy to understand and maintain.

d3js_demo.py
    class Spawn:
        def __init__ (self, width, height):
            self.width, self.height, self.spacing = self.fill = width, height, 100, d3.scale.category20 ()

            self.svg = d3.select ('body'
            ) .append ('svg'
            ) .attr ('width', self.width
            ) .attr ('height', self.height
            ) .on ('mousemove', self.mousemove
            ) .on ('mousedown', self.mousedown)

            self.svg.append ('rect'
            ) .attr ('width', self.width
            ) .attr ('height', self.height)

            self.cursor = self.svg.append ('circle'
            ) .attr ('r', self.spacing
            ) .attr ('transform', 'translate ({}, {})' .format (self.width / 2, self.height / 2)
            ) .attr ('class', 'cursor')

            self.force = d3.layout.force (
            ) .size ([self.width, self.height]
            ) .nodes ([{}]
            ) .linkDistance (self.spacing
            ) .charge (-1000
            ) .on ('tick', self.tick)

            self.nodes, self.links, self.node, self.link = self.force.nodes (), self.force.links (), self.svg.selectAll ('.node'), self.svg.selectAll ('.link')

            self.restart ()

        def mousemove (self):
            self.cursor.attr ('transform', 'translate (' + d3.mouse (self.svg.node ()) + ')')

        def mousedown (self):
            def pushLink (target):
                x, y = target.x - node.x, target.y - node.y
                if Math.sqrt (x * x + y * y) < self.spacing:
                    spawn.links.push ({'source': node, 'target': target})

            point = d3.mouse (self.svg.node ())
            node = {'x': point [0], 'y': point [1]}
            self.nodes.push (node)
            self.nodes.forEach (pushLink)
            self.restart ()

        def tick (self):
            self.link.attr ('x1', lambda d: d.source.x
            ) .attr ('y1', lambda d: d.source.y
            ) .attr ('x2', lambda d: d.target.x
            ) .attr ('y2', lambda d: d.target.y)

            self.node.attr ('cx', lambda d: d.x
            ) .attr ('cy', lambda d: d.y)

        def restart (self):
            self.link = self.link.data (self.links)

            self.link.enter (
            ) .insert ('line', '.node'
            ) .attr('class', 'link')

            self.node = self.node.data (self.nodes)

            self.node.enter (
            ) .insert ('circle', '.cursor'
            ) .attr ('class', 'node'
            ) .attr ('r', 7
            ) .call (self.force.drag)

            self.force.start ()

    spawn = Spawn (window.innerWidth, window.innerHeight)
| d3js_demo.mod.js
        (function () {
            var Spawn = __class__ ('Spawn', [object], {
                get __init__ () {return __get__ (this, function (self, width, height) {
                    var __left0__ = tuple ([width, height, 100, d3.scale.category20 ()]);
                    self.width = __left0__ [0];
                    self.height = __left0__ [1];
                    self.spacing = __left0__ [2];
                    self.fill = __left0__;
                    self.svg = d3.select ('body').append ('svg').attr ('width', self.width).attr ('height', self.height).on ('mousemove', self.mousemove).on ('mousedown', self.mousedown);
                    self.svg.append ('rect').attr ('width', self.width).attr ('height', self.height);
                    self.cursor = self.svg.append ('circle').attr ('r', self.spacing).attr ('transform', 'translate ({}, {})'.format (self.width / 2, self.height / 2)).attr ('class', 'cursor');
                    self.force = d3.layout.force ().size (list ([self.width, self.height])).nodes (list ([dict ({})])).linkDistance (self.spacing).charge (-(1000)).on ('tick', self.tick);
                    var __left0__ = tuple ([self.force.nodes (), self.force.links (), self.svg.selectAll ('.node'), self.svg.selectAll ('.link')]);
                    self.nodes = __left0__ [0];
                    self.links = __left0__ [1];
                    self.node = __left0__ [2];
                    self.link = __left0__ [3];
                    self.restart ();
                }, '__init__');},
                get mousemove () {return __get__ (this, function (self) {
                    self.cursor.attr ('transform', ('translate (' + d3.mouse (self.svg.node ())) + ')');
                }, 'mousemove');},
                get mousedown () {return __get__ (this, function (self) {
                    var pushLink = function (target) {
                        var __left0__ = tuple ([target.x - node.x, target.y - node.y]);
                        var x = __left0__ [0];
                        var y = __left0__ [1];
                        if (Math.sqrt (x * x + y * y) < self.spacing) {
                            spawn.links.push (dict ({'source': node, 'target': target}));
                        }
                    };
                    var point = d3.mouse (self.svg.node ());
                    var node = dict ({'x': point [0], 'y': point [1]});
                    self.nodes.push (node);
                    self.nodes.forEach (pushLink);
                    self.restart ();
                }, 'mousedown');},
                get tick () {return __get__ (this, function (self) {
                    self.link.attr ('x1', (function __lambda__ (d) {
                        return d.source.x;
                    })).attr ('y1', (function __lambda__ (d) {
                        return d.source.y;
                    })).attr ('x2', (function __lambda__ (d) {
                        return d.target.x;
                    })).attr ('y2', (function __lambda__ (d) {
                        return d.target.y;
                    }));
                    self.node.attr ('cx', (function __lambda__ (d) {
                        return d.x;
                    })).attr ('cy', (function __lambda__ (d) {
                        return d.y;
                    }));
                }, 'tick');},
                get restart () {return __get__ (this, function (self) {
                    self.link = self.link.data (self.links);
                    self.link.enter ().insert ('line', '.node').attr ('class', 'link');
                    self.node = self.node.data (self.nodes);
                    self.node.enter ().insert ('circle', '.cursor').attr ('class', 'node').attr ('r', 7).call (self.force.drag);
                    self.force.start ();
                }, 'restart');}
            });
            var spawn = Spawn (window.innerWidth, window.innerHeight);
            __pragma__ ('<all>')
                __all__.Spawn = Spawn;
                __all__.spawn = spawn;
            __pragma__ ('</all>')
        }) ();

## 6.7. Example: React

React is a JavaScript library for easy creation of interactive UI's. Changes to the UI are made by fast manipulation of a light-weight virtual DOM. The real DOM, which is much slower to manipulate, is then compared with the altered virtual DOM and updated efficiently in a minimum number of steps. This way of working leads to good performance, at the same time keeping a straightforward structure of application UI code, since the complexities of optimizing DOM updates are left to the React library. React is unintrusive and mixes well with Transcrypt, allowing creation of extensive web applications that combine maintainability with speed. This example once again clearly illustrates the philosophy behind Transcrypt: rather than confining you to a "parallel" universe that could never keep up, Transcrypt offers you direct access to the ever expanding universe of innovative JavaScript libraries.

react_demo.py
    # Helper functions

    def h(elm_type, props='', *args):
        return React.createElement(elm_type, props, *args)

    def render(react_element, destination_id, callback=lambda: None):
        container = document.getElementById(destination_id)
        ReactDOM.render(react_element, container, callback)

    # Create a component

    Hello = React.createClass({
        'displayName': 'Hello',

        'getInitialState': lambda: {'counter': 0},

        'updateCounter': lambda: (this.setState({'counter': this.state['counter']+1})),

        'componentDidMount': lambda: (setInterval(this.updateCounter, 1000)),

        'render': lambda: h('div', {'className': 'maindiv'},
                              h('h1', None, 'Hello ', this.props['name']),
                              h('p', None, 'Lorem ipsum dolor sit ame.'),
                              h('p', None, 'Counter: ', this.state['counter'])
                            )
    })

    # Render the component in a 'container' div

    element = React.createElement(Hello, {'name': 'React!'})
    render(element, 'container')
| react_demo.mod.js
        (function () {
            var h = function (elm_type, props) {
                if (typeof props == 'undefined' || (props != null && props .__class__ == __kwargdict__)) {;
                    var props = '';
                };
                var args = tuple ([].slice.apply (arguments).slice (2));
                return React.createElement (elm_type, props, ...args);
            };
            var render = function (react_element, destination_id, callback) {
                if (typeof callback == 'undefined' || (callback != null && callback .__class__ == __kwargdict__)) {;
                    var callback = (function __lambda__ () {
                        return null;
                    });
                };
                var container = document.getElementById (destination_id);
                ReactDOM.render (react_element, container, callback);
            };
            var Hello = React.createClass (dict ({'displayName': 'Hello', 'getInitialState': (function __lambda__ () {
                return dict ({'counter': 0});
            }), 'updateCounter': (function __lambda__ () {
                return this.setState (dict ({'counter': this.state ['counter'] + 1}));
            }), 'componentDidMount': (function __lambda__ () {
                return setInterval (this.updateCounter, 1000);
            }), 'render': (function __lambda__ () {
                return h ('div', dict ({'className': 'maindiv'}), h ('h1', null, 'Hello ', this.props ['name']), h ('p', null, 'Lorem ipsum dolor sit ame.'), h ('p', null, 'Counter: ', this.state ['counter']));
            })}));
            var element = React.createElement (Hello, dict ({'name': 'React!'}));
            render (element, 'container');
            __pragma__ ('<all>')
                __all__.Hello = Hello;
                __all__.element = element;
                __all__.h = h;
                __all__.render = render;
            __pragma__ ('</all>')
        }) ();

## 6.8. Example: Riot

Riot is a UI framework that combines the use of custom tags and a virtual DOM, much like the one in React. Custom tags look like ordinary HTML tags, but whereas HTML tags only define structure, Riot tags define structure, style and behaviour. Custom tags are compiled to JavaScript by a compiler that comes with Riot. With these custom tags as reusable components, web pages can be built. Riot itself is tiny, and the virtual DOM allows for fast adaptation of page content.

riot_demo.html
    <!DOCTYPE html>
    <html>

      <head>
        <meta charset="UTF-8" />
        <title>Hello PyRiot ;-)</title>
        <!-- for transcrypted version we do not need the compiler.
             for the classic version you can use riot+compiler -->
        <script src="https://cdn.jsdelivr.net/riot/2.5/riot.js"></script>
        <style>
            body {font-family:arial;font-size:30px;padding:50px;}
            h1 {font-size:50px;color:#0000ff;}
        </style>
      </head>

      <body>

      <!-- classic riot (compiled on server, could be done in browser)         -->
            <sample id="s1"></sample>
            <script src="tags/sample.js"></script>
            <script>riot.mount('sample')</script>

      <!-- transcrypted version (transpiled on server, using python)           -->
            <sample2 id='s2' label="nr1"></sample2>
            <mp2     id='s3' label="nr2"></mp2>
            <!-- the transpiled tag's js -->
            <script src="__javascript__/riot_demo.js"></script>
            <script>
                // register
                var cls = riot_demo.Sample2
                riot.tag2('sample2', cls.template, cls.style, '', function(opts) {
                    new cls(this, opts)})
                document.t1 = riot.mount('sample2')[0].update()
                document.t2 = riot.mount('#s3', 'sample2')[0]
                document.t2.update()
            </script>
      </body>
    </html>

sample.tag, a classic Riot tag
    // vim: ft=html
    <sample>

        <h1>Riot Native Tag</h1>

        <h5 each="{lv}">name: {name} - counter: {this.count()}</h5>

        <script>
            this.counter = 0

            // riot's synctactic sugar:
            count() { this.counter += 1; return this.counter }

            // to test stuff on console:
            window.native_tag = this

            this.on('update', function() {
                this.lv = [{name: 'n1'}, {'name': 'n2'}]
                this.update()
            })
        </script>
        <style scoped>
        h1 {color: red}
        </style>

    </sample>
| sample.js, compiled by Riot
    // vim: ft=html
    riot.tag2('sample', '<h1>Riot Native Tag</h1> <h5 each="{lv}">name: {name} - counter: {this.count()}</h5>',
    'sample h1,[riot-tag="sample"] h1,[data-is="sample"] h1{color: red}', '', function(opts) {
            this.counter = 0

            this.count = function() { this.counter += 1; return this.counter }.bind(this)

            window.native_tag = this

            this.on('update', function() {
                this.lv = [{name: 'n1'}, {'name': 'n2'}]
                this.update()
            })
    });

riot_tag.py, baseclass of all Transcrypt Riot tags
    # Parent Class for a Transcrypt Riot Tag
    #
    # This binds the namespace of a riot tag at before-mount event 100% to that of
    # the a transcrypt instance, except members which begin with an underscore, those
    # are private to the transcrypt object.
    #
    # The 4 riot lifecycle events are bound to overwritable python functions.
    #
    # Immutables (strings, ints, ...) are bound to property functions within the tag,
    # so the templates work, based on state in the transcrypt tag.
    # State can be changed in the riot tag as well but take care to not create new
    # references - you won't find them in the Transcrypt tag.
    #
    #
    # Best Practices:
    #  - mutate state only in the transcrypt tag.
    #  - declare all variables so they are bound into the riot tag
    #  - IF you declare new variables to be used in templates, run
    #    self.bind_vars(self.riot_tag)
    # TODO: docstring format not accepted by the transpiler, strange.

    __author__ = "Gunther Klessinger, gk@axiros.com, Germany"

    # just a minihack to get some colors, mainly to test lamdas and imports:
    from color import colors, cprint as col_print
    c = colors
    M, I, L, R, B = c['purple'], c['orange'], c['gray'], c['red'], c['black']

    lifecycle_ev = ['before-mount', 'mount', 'update', 'unmount']

    cur_tag_col = 0
    class RiotTag:
        """
        taking care for extending the riot tag obj with
        functions and immutable(!) properties of derivations of us
        See counter.
        """
        debug = None
        # placeholders:
        template = '<h1>it worx</h1>'
        style = ''
        node_name = 'unmounted'
        opts = None
        def __init__(self, tag, opts):
            # opts into the python instance, why not:
            self.opts = opts
            self._setup_tag(tag)
            # giving ourselves a unique color:
            global cur_tag_col # working (!)
            cur_tag_col = (cur_tag_col + 1) % len(colors)
            # TODO values() on a dict
            self.my_col = colors.items()[cur_tag_col][1]

        def _setup_tag(self, tag):
            # keeping mutual refs
            tag.py_obj = self
            self.riot_tag = tag
            # making the event system call self's methods:
            handlers = {}
            for ev in lifecycle_ev:
                f = getattr(self, ev.replace('-', '_'))
                if f:
                    # this.on('mount', function() {...}):
                    # whats nicer?
                    tag.on(ev, f)

        def pp(self, *msg):
            # color flash in the console. one color per tag instance.
            col_print(
                #B(self.riot_tag._riot_id),
                L('<', self.my_col(self.node_name, self.my_col), '/> '),
                M(' '.join([s for s in msg])))

        def _lifecycle_ev(self, mode):
            if self.debug:
                self.pp(mode + 'ing')

        # overwrite these for your specific one:
        def update (self): self._lifecycle_ev('update')
        def mount  (self): self._lifecycle_ev('mount')
        def unmount(self): self._lifecycle_ev('unmount')

        def before_mount(self):
            self._lifecycle_ev('before-mount')
            return self.bind_vars()

        def bind_vars(self):
            tag = self.riot_tag
            self.node_name = tag.root.nodeName.lower()
            self.debug and self.pp('binding vars')
            # binding self's functions into the tag instance
            # binding writable properties to everything else (e.g. ints, strs...)
            tag._immutables = im = []
            lc = lifecycle_ev
            for k in dir(self):
                # private or lifecycle function? don't bind:
                if k[0] == '_' or k in lifecycle_ev or k == 'before_mount':
                    continue
                v = getattr(self, k)
                # these I can't write in python. Lets use JS then.
                # TODO there should be, maybe some mocking facility for code
                # testing w/o a js runtime:
                __pragma__('js', '{}', '''
                      typeof v === "function" || typeof v === "object" ?
                      tag[k] = self[k] : tag._immutables.push(k)''')

            __pragma__('js', '{}', '''
            var i = tag._immutables, py = self
            i.forEach(function(k, j, i) {
                Object.defineProperty(tag, k, {
                    get: function()  { return self[k]},
                    set: function(v) { self[k] = v }
                })
            })''')
| riot_tag.mod.js, compiled by Transcrypt
        __nest__ (
            __all__,
            'riot_tag', {
                __all__: {
                    __inited__: false,
                    __init__: function (__all__) {
                        var __author__ = 'Gunther Klessinger, gk@axiros.com, Germany';
                        var colors = __init__ (__world__.color).colors;
                        var col_print = __init__ (__world__.color).cprint;
                        var c = colors;
                        var __left0__ = tuple ([c ['purple'], c ['orange'], c ['gray'], c ['red'], c ['black']]);
                        var M = __left0__ [0];
                        var I = __left0__ [1];
                        var L = __left0__ [2];
                        var R = __left0__ [3];
                        var B = __left0__ [4];
                        var lifecycle_ev = list (['before-mount', 'mount', 'update', 'unmount']);
                        var cur_tag_col = 0;
                        var RiotTag = __class__ ('RiotTag', [object], {
                            debug: null,
                            template: '<h1>it worx</h1>',
                            style: '',
                            node_name: 'unmounted',
                            opts: null,
                            get __init__ () {return __get__ (this, function (self, tag, opts) {
                                self.opts = opts;
                                self._setup_tag (tag);
                                cur_tag_col = __mod__ (cur_tag_col + 1, len (colors));
                                self.my_col = colors.py_items () [cur_tag_col] [1];
                            }, '__init__');},
                            get _setup_tag () {return __get__ (this, function (self, tag) {
                                tag.py_obj = self;
                                self.riot_tag = tag;
                                var handlers = dict ({});
                                for (var ev of lifecycle_ev) {
                                    var f = getattr (self, ev.py_replace ('-', '_'));
                                    if (f) {
                                        tag.on (ev, f);
                                    }
                                }
                            }, '_setup_tag');},
                            get pp () {return __get__ (this, function (self) {
                                var msg = tuple ([].slice.apply (arguments).slice (1));
                                col_print (L ('<', self.my_col (self.node_name, self.my_col), '/> '), M (' '.join (function () {
                                    var __accu0__ = [];
                                    for (var s of msg) {
                                        __accu0__.append (s);
                                    }
                                    return __accu0__;
                                } ())));
                            }, 'pp');},
                            get _lifecycle_ev () {return __get__ (this, function (self, mode) {
                                if (self.debug) {
                                    self.pp (mode + 'ing');
                                }
                            }, '_lifecycle_ev');},
                            get py_update () {return __get__ (this, function (self) {
                                self._lifecycle_ev ('update');
                            }, 'update');},
                            get mount () {return __get__ (this, function (self) {
                                self._lifecycle_ev ('mount');
                            }, 'mount');},
                            get unmount () {return __get__ (this, function (self) {
                                self._lifecycle_ev ('unmount');
                            }, 'unmount');},
                            get before_mount () {return __get__ (this, function (self) {
                                self._lifecycle_ev ('before-mount');
                                return self.bind_vars ();
                            }, 'before_mount');},
                            get bind_vars () {return __get__ (this, function (self) {
                                var tag = self.riot_tag;
                                self.node_name = tag.root.nodeName.lower ();
                                self.debug && self.pp ('binding vars');
                                var __left0__ = list ([]);
                                tag._immutables = __left0__;
                                var im = __left0__;
                                var lc = lifecycle_ev;
                                for (var k of dir (self)) {
                                    if (k [0] == '_' || __in__ (k, lifecycle_ev) || k == 'before_mount') {
                                        continue;
                                    }
                                    var v = getattr (self, k);

                                                      typeof v === "function" || typeof v === "object" ?
                                                      tag[k] = self[k] : tag._immutables.push(k)
                                }

                                        var i = tag._immutables, py = self
                                        i.forEach(function(k, j, i) {
                                            Object.defineProperty(tag, k, {
                                                get: function()  { return self[k]},
                                                set: function(v) { self[k] = v }
                                            })
                                        })
                            }, 'bind_vars');}
                        });
                        __pragma__ ('<use>' +
                            'color' +
                        '</use>')
                        __pragma__ ('<all>')
                            __all__.B = B;
                            __all__.I = I;
                            __all__.L = L;
                            __all__.M = M;
                            __all__.R = R;
                            __all__.RiotTag = RiotTag;
                            __all__.__author__ = __author__;
                            __all__.c = c;
                            __all__.col_print = col_print;
                            __all__.colors = colors;
                            __all__.cur_tag_col = cur_tag_col;
                            __all__.lifecycle_ev = lifecycle_ev;
                        __pragma__ ('</all>')
                    }
                }
            }
        );

riot_demo.py, a derived Transcrypt Riot tag class
    # an example user tag, using RiotTag

    from riot_tag import RiotTag

    class P(RiotTag):
        debug = 1
        # never do mutables on class level. this is just to check if transpiler
        # creates the same behaviour - and it does, a second tag instance gets
        # the same lv object:
        lv = [{'name': 'n0'}]
        # immuatble on class level. does a second instance start at 1?
        # answer: yes, perfect:
        counter = 1

        template = ''' <div><h1>Riot Transcrypt Tag Instance {label}</h1>
                             <div>INNER</div></div> '''
        def count_up(self):
            self.counter = self.counter + 1
            self.pp('counter:', self.counter, 'len lv:', len(self.lv), 'adding one lv' )
            self.lv.append({'name': 'n' + self.counter})
            return self.counter

    # try some inheritance...
    class Sample2(P):
        # ... and change the state at every update, just for fun:
        template = P.template.replace('INNER', '''
        <div>
        <h5 each="{lv}">name: {name} - counter: {count_up()}</h5>
        </div>
        ''')

        # no scoped styles currently
        style = '''sample2 h5 {color: green}'''

        def __init__(self, tag, opts):
            self.label = opts.label.capitalize()  # this rocks so much.
            # alternative to super:
            RiotTag.__init__(self, tag, opts)
            # uncomment next line and chrome will stop:
            # debugger
            self.pp('tag init', 'adding 2 lv')
            # mutating the lv object:
            self.lv.extend([{'name': 'n1'}, {'name': 'n2'}])

        def update(self):
            self.pp('update handler in the custom tag, calling super')
            RiotTag.update(self)
| riot_demo.mod.js, compiled by Transcrypt
        (function () {
            var RiotTag = __init__ (__world__.riot_tag).RiotTag;
            var P = __class__ ('P', [RiotTag], {
                debug: 1,
                lv: list ([dict ({'name': 'n0'})]),
                counter: 1,
                template: ' <div><h1>Riot Transcrypt Tag Instance {label}</h1>\n                         <div>INNER</div></div> ',
                get count_up () {return __get__ (this, function (self) {
                    self.counter = self.counter + 1;
                    self.pp ('counter:', self.counter, 'len lv:', len (self.lv), 'adding one lv');
                    self.lv.append (dict ({'name': 'n' + self.counter}));
                    return self.counter;
                }, 'count_up');}
            });
            var Sample2 = __class__ ('Sample2', [P], {
                template: P.template.py_replace ('INNER', '\n    <div>\n    <h5 each="{lv}">name: {name} - counter: {count_up()}</h5>\n    </div>\n    '),
                style: 'sample2 h5 {color: green}',
                get __init__ () {return __get__ (this, function (self, tag, opts) {
                    self.label = opts.label.capitalize ();
                    RiotTag.__init__ (self, tag, opts);
                    self.pp ('tag init', 'adding 2 lv');
                    self.lv.extend (list ([dict ({'name': 'n1'}), dict ({'name': 'n2'})]));
                }, '__init__');},
                get py_update () {return __get__ (this, function (self) {
                    self.pp ('update handler in the custom tag, calling super');
                    RiotTag.py_update (self);
                }, 'update');}
            });
            __pragma__ ('<use>' +
                'riot_tag' +
            '</use>')
            __pragma__ ('<all>')
                __all__.P = P;
                __all__.RiotTag = RiotTag;
                __all__.Sample2 = Sample2;
            __pragma__ ('</all>')
        }) ();

## 6.9. Example: Using input and print in a DOM __terminal__ element in your browser

Without special measures, Transcrypt's print function prints to the debugging console. However if there's an element with id __terminal__ in your DOM tree, the print function prints to this element. Moreover, the input function also prints its prompt message to the terminal element. Input is collected using a dialog box and echoed to the terminal element.

This means that you can write applications with blocking I/O, rather than event driven behaviour, e.g. for simple activities or, since they are intuitively easy to comprehend, for educational purposes.

terminal_demo.html
    <html>
        <head>
            <style>
                #__terminal__ {
                    background-color:black;
                    color:white;
                    font-family:arial;
                    font-size:12px;
                    height: 400px;
                    width: 640px;
                }
            </style>
        </head>
        <body>
            <div id="__terminal__"></div>
            <script src="__javascript__/terminal_demo.js"></script>
        </body>
    </html>

terminal_demo.py
    name = input ('What''s your name? ')
    print ('Hi ', name, 'I am your computer.')

    age = float (input ('How old are you? '))
    if age < 18:
        print ('Sorry ', name, age, 'is to young to drive a car in the Netherlands.')
    else:
        print ('OK ', name, age, 'is old enough to drive a car in the Netherlands.')