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HTML

              
                <canvas id="myCanvas" width="500" height="500"></canvas>

              
            
!

CSS

              
                body, html {
  margin: 0;
}
              
            
!

JS

              
                //-------------------------//
// Initialization and 
// Global Variables
//-------------------------//


// Get the canvas.
var canvas = document.getElementById('myCanvas');

// Set the canvas to fill the screen.
canvas.width = window.outerWidth;
canvas.height = window.outerHeight;

// Get a 2d drawing context.
var ctx = canvas.getContext('2d');

// Used to keep track of active touches.
var currentTouches = new Array;


//-------------------------//
// Helper Methods
//-------------------------//


// Returns a random color from an array.
var randomColor = function () {
	var colors = ['#3F3F3F', '#929292', '#00A3EE', '#F5D908', '#D80351'];
	return colors[Math.floor(Math.random() * colors.length)];
};


// Finds the array index of a touch in the currentTouches array.
var findCurrentTouchIndex = function (id) {
	for (var i=0; i < currentTouches.length; i++) {
		if (currentTouches[i].id === id) {
			return i;
		}
	}

	// Touch not found! Return -1.
	return -1;
};


//-------------------------//
// Handler Methods
//-------------------------//


// Creates a new touch in the currentTouches array and draws the starting
// point on the canvas.
var touchStarted = function (event) {
	var touches = event.changedTouches;

	for (var i=0; i < touches.length; i++) {
		var touch = touches[i];
		var touchColor = randomColor();

		currentTouches.push({
			id: touch.identifier,
			pageX: touch.pageX,
			pageY: touch.pageY,
			color: touchColor
		});

		ctx.beginPath();
		ctx.arc(touch.pageX, touch.pageY, 2.5, Math.PI*2, false);
		ctx.fillStyle = touchColor;
		ctx.fill();
	}

};


// Draws a line on the canvas between the previous touch location and
// the new location.
var touchMoved = function (event) {
	var touches = event.changedTouches;

	for (var i=0; i < touches.length; i++) {
		var touch = touches[i];
		var currentTouchIndex = findCurrentTouchIndex(touch.identifier);

		if (currentTouchIndex >= 0) {
			var currentTouch = currentTouches[currentTouchIndex];

			ctx.beginPath();
			ctx.moveTo(currentTouch.pageX, currentTouch.pageY);
			ctx.lineTo(touch.pageX, touch.pageY);
			ctx.lineWidth = 4;
			ctx.strokeStyle = currentTouch.color;
			ctx.stroke();

			// Update the touch record.
			currentTouch.pageX = touch.pageX;
			currentTouch.pageY = touch.pageY;

			// Store the record.
			currentTouches.splice(currentTouchIndex, 1, currentTouch);
		} else {
			console.log('Touch was not found!');
		}

	}

};


// Draws a line to the final touch position on the canvas and then
// removes the touh from the currentTouches array.
var touchEnded = function (event) {
	var touches = event.changedTouches;

	for (var i=0; i < touches.length; i++) {
		var touch = touches[i];
		var currentTouchIndex = findCurrentTouchIndex(touch.identifier);

		if (currentTouchIndex >= 0) {
			var currentTouch = currentTouches[currentTouchIndex];

			ctx.beginPath();
			ctx.moveTo(currentTouch.pageX, currentTouch.pageY);
			ctx.lineTo(touch.pageX, touch.pageY);
			ctx.lineWidth = 4;
			ctx.strokeStyle = currentTouch.color;
			ctx.stroke();

			// Remove the record.
			currentTouches.splice(currentTouchIndex, 1);
		} else {
			console.log('Touch was not found!');
		}

	}

};


// Removes cancelled touches from the currentTouches array.
var touchCancelled = function (event) {
	var touches = event.changedTouches;

	for (var i=0; i < touches.length; i++) {
		var currentTouchIndex = findCurrentTouchIndex(touches[i].identifier);

		if (currentTouchIndex >= 0) {
			// Remove the touch record.
			currentTouches.splice(currentTouchIndex, 1);
		} else {
			console.log('Touch was not found!');
		}
	}
};


//-------------------------//
// Event Listeners
//-------------------------//


// Set up an event listener for new touches.
canvas.addEventListener('touchstart', function(e) {
	e.preventDefault();
	touchStarted(event);
});


// Set up an event listener for when a touch ends.
canvas.addEventListener('touchend', function(e) {
	e.preventDefault();
	touchEnded(e);
});


// Set up an event listener for when a touch leaves the canvas.
canvas.addEventListener('touchleave', function(e) {
	e.preventDefault();
	touchEnded(e);
});


// Set up an event listener for when the touch instrument is moved.
canvas.addEventListener('touchmove', function(e) {
	e.preventDefault();
	touchMoved(e);
});


// Set up an event listener to catch cancelled touches.
canvas.addEventListener('touchcancel', function(e) {
	touchCancelled(e);
});
              
            
!
999px

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