js实现压缩base64格式的图片

压缩方法，可以直接用亲测有效（刚写完需求所以肯定好使）

//压缩方法
function dealImage(base64, w, callback) {
  var newImage = new Image();
  var quality = 0.6; //压缩系数0-1之间
  newImage.src = base64;
  newImage.setAttribute("crossOrigin", "Anonymous"); //url为外域时需要
  var imgWidth, imgHeight;
  newImage.onload = function () {
    imgWidth = this.width;
    imgHeight = this.height;
    var canvas = document.createElement("canvas");
    var ctx = canvas.getContext("2d");
    if (Math.max(imgWidth, imgHeight) > w) {
      if (imgWidth > imgHeight) {
        canvas.width = w;
        canvas.height = (w * imgHeight) / imgWidth;
      } else {
        canvas.height = w;
        canvas.width = (w * imgWidth) / imgHeight;
      }
    } else {
      canvas.width = imgWidth;
      canvas.height = imgHeight;
      quality = 0.6;
    }
    ctx.clearRect(0, 0, canvas.width, canvas.height);
    ctx.drawImage(this, 0, 0, canvas.width, canvas.height);
    var base64 = canvas.toDataURL("image/jpeg", quality); //压缩语句
    callback(base64); //必须通过回调函数返回，否则无法及时拿到该值
  };
}