HTML5 Canvas 100% Width Height of Viewport?

In order to make the canvas full-screen width and height always, meaning even when the browser is resized, you need to run your draw loop within a function that resizes the canvas to the window.innerHeight and window.innerWidth.

Example: http://jsfiddle.net/jaredwilli/qFuDr/

HTML

<canvas id="canvas"></canvas>

JavaScript

(function() {  const canvas = document.getElementById('canvas');  const context = canvas.getContext('2d');  // resize the canvas to fill browser window dynamically  window.addEventListener('resize', resizeCanvas, false);          function resizeCanvas() {    canvas.width = window.innerWidth;    canvas.height = window.innerHeight;                    /**     * Your drawings need to be inside this function otherwise they will be reset when      * you resize the browser window and the canvas goes will be cleared.     */    drawStuff();   }    resizeCanvas();          function drawStuff() {    // do your drawing stuff here  }})();

CSS

* { margin:0; padding:0; } /* to remove the top and left whitespace */html, body { width:100%; height:100%; } /* just to be sure these are full screen*/canvas { display:block; } /* To remove the scrollbars */

That is how you properly make the canvas full width and height of the browser. You just have to put all the code for drawing to the canvas in the drawStuff() function.

You can try viewport units (CSS3):

canvas {   height: 100vh;   width: 100vw;   display: block;}

I'll answer the more general question of how to have a canvas dynamically adapt in size upon window resize. The accepted answer appropriately handles the case where width and height are both supposed to be 100%, which is what was asked for, but which also will change the aspect ratio of the canvas. Many users will want the canvas to resize on window resize, but while keeping the aspect ratio untouched. It's not the exact question, but it "fits in", just putting the question into a slightly more general context.

The window will have some aspect ratio (width / height), and so will the canvas object. How you want these two aspect ratios to relate to each other is one thing you'll have to be clear about, there is no "one size fits all" answer to that question - I'll go through some common cases of what you might want.

Most important thing you have to be clear about: the html canvas object has a width attribute and a height attribute; and then, the css of the same object also has a width and a height attribute. Those two widths and heights are different, both are useful for different things.

Changing the width and height attributes is one method with which you can always change the size of your canvas, but then you'll have to repaint everything, which will take time and is not always necessary, because some amount of size change you can accomplish via the css attributes, in which case you do not redraw the canvas.

I see 4 cases of what you might want to happen on window resize (all starting with a full screen canvas)

1: you want the width to remain 100%, and you want the aspect ratio to stay as it was. In that case, you do not need to redraw the canvas; you don't even need a window resize handler. All you need is

$(ctx.canvas).css("width", "100%");

where ctx is your canvas context.fiddle: resizeByWidth

2: you want width and height to both stay 100%, and you want the resulting change in aspect ratio to have the effect of a stretched-out image. Now, you still don't need to redraw the canvas, but you need a window resize handler. In the handler, you do

$(ctx.canvas).css("height", window.innerHeight);

fiddle: messWithAspectratio

3: you want width and height to both stay 100%, but the answer to the change in aspect ratio is something different from stretching the image. Then you need to redraw, and do it the way that is outlined in the accepted answer.

fiddle: redraw

4: you want the width and height to be 100% on page load, but stay constant thereafter (no reaction to window resize.

fiddle: fixed

All fiddles have identical code, except for line 63 where the mode is set. You can also copy the fiddle code to run on your local machine, in which case you can select the mode via a querystring argument, as ?mode=redraw