Check an array for non-zero values

var hasEmptyPixels = function(x,y,w,h)
{
    // Returns false if any value
    // in the array is greater than zero
    var imgData = context.getImageData(x,y,w,h);
    var length = imgData.data.length;
    for(var i = 0; i < length; ++i){
        if (imgData.data[i] > 0) return false;
    }
    return true;
}