前端对图片压缩处理

//第一个参数就是原来的字符串，第二个是宽度，第三个就是回调方法
export const  cutImageBase64=(base64, w, callback)=> {
  let newImage = new Image();
  let 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
  let imgWidth, imgHeight;
  newImage.onload = function () {
      imgWidth = this.width;
      imgHeight = this.height;
      let canvas = document.createElement("canvas");
      let 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);
      let base64 = canvas.toDataURL("image/jpeg", quality); //压缩语句
      // 如想确保图片压缩到自己想要的尺寸,如要求在50-150kb之间，请加以下语句，quality初始值根据情况自定
      while (base64.length / 1024 > 150) {
          quality -= 0.01;
          base64 = canvas.toDataURL("image/jpeg", quality);
      }
      // 防止最后一次压缩低于最低尺寸，只要quality递减合理，无需考虑
      while (base64.length / 1024 < 50) {
          quality += 0.001;
          base64 = canvas.toDataURL("image/jpeg", quality);
      }
      callback(base64);//必须通过回调函数返回，否则无法及时拿到该值
  }
}