<canvas id="src" width="100" height="100"></canvas>
<canvas id="mask" width="100" height="100"></canvas>
<canvas id="dest" width="100" height="100"></canvas>

var src = document.getElementById('src');
var srcCtx = src.getContext('2d');

var mask = document.getElementById('mask');
var maskCtx = mask.getContext('2d');

var dest = document.getElementById('dest');
var destCtx = dest.getContext('2d');
  
function test(sz){
   srcCtx.drawImage(mask, 0, 0, sz, sz, 0, 0, sz, sz);
   destCtx.drawImage(src, 0, 0, sz, sz, 0, 0, sz, sz);
}