drawImage（）无效 - HTML5画布

Question

我正在尝试为我的绘图应用创建撤消和重做功能。我用console.log检查了函数的执行情况。 单击撤消/重做按钮时，函数会执行，但drawImage函数不会绘制相关图像（较旧的画布状态，较新的画布状态，如此）。

我不确定我哪里出错了。

//Calls this function every time a piece is drawn - mouseup event
function cPush() {
    cStep += 1;
    var length = cPushArray.length;
    if (cStep < cPushArray.length) { 
        cPushArray.length = cStep; 
    }
  cPushArray.push(document.getElementById("canvas").toDataURL("image/png"));
}

//Undo 
function undoClick() {
    if (cStep > 0) {
        cStep -= 1;
        var canvasPic = new Image();
        canvasPic.onload = function () { 
            ctx.drawImage(canvasPic, 0, 0); 
        };
        canvasPic.src = cPushArray[cStep];
    }
}

#Redo
function redoClick() {
    if (cStep < cPushArray.length - 1) {
        cStep += 1;
        var canvasPic = new Image();
        canvasPic.onload = function () { 
            ctx.drawImage(canvasPic, 0, 0); 
        };
        canvasPic.src = cPushArray[cStep];
     }
}