参考：
https://www.cnblogs.com/ganws/p/11149668.html
https://www.jianshu.com/p/abd9e9c5ba20
使用html2canvas
npm install --save html2canvas
使用
import html2canvas from "html2canvas";
// 内容
<div id="lcode">变成图片</div>
// js
//创建一个新的canvas
let newCanvas= document.createElement("canvas");
w = window.getComputedStyle(document.getElementById("lcode")).width
h= window.getComputedStyle(document.getElementById("lcode")).height
newCanvas.width = w * 2;
newCanvas.height = h * 2;
newCanvas.style.width = w + "px";
newCanvas.style.height = h + "px";
let context: any = newCanvas.getContext("2d");
context.scale(2, 2);
html2canvas(document.getElementById("lcode") as any, {
canvas: newCanvas,
}).then(function(canvas) {
//canvas转换成url，然后利用a标签的download属性，直接下载
let a = document.createElement("a");
a.href = canvas.toDataURL();
//设置下载文件的名字
a.download = "下载";
//点击
a.click();
});
