Canvas judgment content is empty

// verification canvas Whether the canvas is an empty function 
function isCanvasBlank(canvas) {
    var blank = document.createElement('canvas');// The system gets an empty canvas object 
    blank.width = canvas.width;
    blank.height = canvas.height;
    return canvas.toDataURL() == blank.toDataURL();// If the comparison value is equal, it is null 
}
// call 
/ Form non empty verification 
function checkEmpty() {
    var c=document.getElementById("canvas"); //  obtain html Of canvas object , This is me. id="canvas"
    if(isCanvasBlank(c)){
        alert(" Please draw a sketch and upload it ！");
        return;
    }
});