我不能在图像上画一个矩形

Question

当我按下按钮＆＃34; Draw＆＃34;。我无法在图像上绘制矩形。

https://jsfiddle.net/ikat/5xzc13r9/

function drawRect() {
// creating a square
var w = endX - startX;
var h = endY - startY;
var offsetX = (w < 0) ? w : 0;
var offsetY = (h < 0) ? h : 0;
var width = Math.abs(w);
var height = Math.abs(h);

ctx.clearRect(0, 0, canvas.width, canvas.height);
ctx.save();
ctx.beginPath();
ctx.rect(startX + offsetX, startY + offsetY, width, height);
ctx.lineWidth = 7;
ctx.strokeStyle = 'red';
ctx.stroke();
ctx.closePath();
}