在画布上显示一条线，我在清晰的画布前绘制

I am clearing canvas using ctx.clearRect(0,0,canvas.width,canvas.height) and just for debug I redraw after 2 seconds, on second time I redraw all content instead of lines. but it still showing one line on canvas which I draw lastly.I am clearing canvas using ctx.clearRect(0,0,ca