const canvas = document.createElement("canvas");
const context = canvas.getContext("2d");
context.fillStyle = "black";
context.fillRect(0, 0, canvas.width, canvas.height);
const imageElement = document.createElement("img");
imageElement.crossOrigin = "Anonymous";
imageElement.onload = () => {
 context.drawImage(
 imageElement,
 params.left,
 params.top,
 canvas.width,
 canvas.height,
 0,
 0,
 canvas.width,
 canvas.height
 );

 const dataUrl = canvas.toDataURL("image/jpeg", 1);
}
imageElement.src = 'xxx';

const reader = new FileReader();
reader.readAsDataURL(canvas.msToBlob());
reader.onloadend = () => {
 const base64data = reader.result;
};

// 之前的代码
// ...
// 最后一行 imageElement.src = 'xxx' 替换：
getDataUrlBySrc('xxx').then(b64 => (imageElement.src = b64));

function getDataUrlBySrc(src: string) {
 return new Promise<string>((resolve, reject) => {
 if (Cache.localGet("isIE")) {
  const xmlHTTP = new XMLHttpRequest();
  xmlHTTP.open("GET", src, true);

  // 以 ArrayBuffer 的形式返回数据
  xmlHTTP.responseType = "arraybuffer";

  xmlHTTP.onload = function(e) {
  
  // 1. 将返回的数据存储在一个 8 位无符号整数值的类型化数组里面
  const arr = new Uint8Array(xmlHTTP.response);
  
  // 2. 转为 charCode 字符串
  const raw = Array.prototype.map
   .call(arr, charCode => String.fromCharCode(charCode))
   .join("");
   
  // 3. 将二进制字符串转为 base64 编码的字符串
  const b64 = btoa(raw);
  
  const dataURL = "data:image/jpeg;base64," + b64;
  resolve(dataURL);
  };
  xmlHTTP.onerror = function(err) {
  reject(err);
  };
  xmlHTTP.send();
 } else {
  resolve(src);
 }
 });
}