Set Canvas size using javascript

I have the following code in html:

<canvas  id="myCanvas" width =800 height=800>

I want, instead of specifying the width as 800, to call the JavaScript function getWidth() to get the width e.g.

 <canvas  id="myCanvas" width =getWidth() height=800>

What is the correct syntax to do it? Because what I'm doing doesn't work.

You can set the width like this :

function draw() {
  var ctx = (a canvas context);
  ctx.canvas.width  = window.innerWidth;
  ctx.canvas.height = window.innerHeight;
  //...drawing code...
}

function setWidth(width) {
  var canvas = document.getElementById("myCanvas");  
  canvas.width = width;
}

Try this:

var setCanvasSize = function() {
canvas.width = window.innerWidth;
canvas.height = window.innerHeight;
}

Try this:

var w = window,
d = document,
e = d.documentElement,
g = d.getElementsByTagName('body')[0],
xS = w.innerWidth || e.clientWidth || g.clientWidth,
yS = w.innerHeight|| e.clientHeight|| g.clientHeight;
alert(xS + ' × ' + yS);

document.write('')

works for iframe and well.