如何使用HTML5画布将DIV保存为带有扩展名的图像？

<div id = &rdquo;cpimg&rdquo;   style = &rdquo;padding:  25px ; &rdquo; >     <h4>Welcome</h4></div>

html2canvas(document.querySelector(“#cpimg”)).then(canvas {        document.body.appendChild(canvas) });