Check if canvas is blank

HTML

<canvas id='editor' style='border:solid'></canvas>

<canvas id='blank' style='display:none'></canvas>
<button id='save'>Save</button>

JavaScript

var canvas = document.getElementById('editor'),
    ctx = canvas.getContext('2d'),
    blankURL = document.getElementById('blank').toDataURL();

canvas.addEventListener('mousemove', function(e){
    ctx.lineTo(e.pageX, e.pageY);
    ctx.stroke();
}, false);

document.getElementById('save').addEventListener('click', function(){
    if(canvas.toDataURL() === blankURL){
        alert('It is blank');
    }
    else{
        alert('Save it!');
    }
}, false);