about the size of the img generated

[xingxingted]

hi, i wanna know why the code below is like this:
if(options.overlay != false) {
ctx.beginPath();
ctx.rect(0, 0, tempImg.width, tempImg.width);
ctx.fillStyle = options.overlay;
ctx.fill();
}
my point is that why u draw a rectrangle using the img'width to make it a squre?