由于我可以扩展的原因，你应该考虑清除你的画布并重新绘制它，除非有性能或合成的原因不这样做。

For reasons that I could expand upon, you should consider clearing your canvas and redrawing it entirely unless there are performance or compositing reasons not to.

你想 clearRect ，如下所示：

//clear the canvas so we can draw a fresh clock
ctx.clearRect(0, 0, canvasWidth, canvasHeight);

//redraw your clock here
/* ... */