resize_canvas html5

function resize_canvas(){
var sizecanvas = document.getElementById("myCanvas");
var sizecontext = sizecanvas.getContext('2d');
if (sizecanvas.width  < window.innerWidth){
sizecanvas.width  = window.innerWidth;
}

if (sizecanvas.height < window.innerHeight) {
sizecanvas.height = window.innerHeight;
}

innerWidth = window.innerWidth;
innerHeight = window.innerHeight;
outerWidth = window.outerWidth;
outerHeight = window.outerHeight;

sizeDiffW = innerWidth-canvasWidth;
sizeDiffH = innerHeight-canvasHeight;
chkscaleW = innerWidth/canvasWidth;
chkscaleH = innerHeight/canvasHeight;
scaleFix = Math.min(chkscaleW,chkscaleH);

sizecanvas.width=canvasWidth*scaleFix;
sizecanvas.height=canvasHeight*scaleFix;
context.scale(scaleFix, scaleFix);
sizecanvas.style.left=((innerWidth-canvasWidth*scaleFix)/2)+"px";
sizecanvas.style.top=((innerHeight-canvasHeight*scaleFix)/2)+"px";

sizecanvas.style.marginLeft=""+((innerWidth-canvasWidth*scaleFix)/2)+"px";
sizecanvas.style.marginTop=""+((innerHeight-canvasHeight*scaleFix)/2)+"px";
}