js前端实现图片压缩上传的示例代码

<el-upload
  class="avatar-uploader"
  :action="GLOBAL.serverFileUrl"
  name="file"
  drag
  :show-file-list="false"
  :on-change="beforeAvatarUpload"
 >
  <i class="el-icon-plus avatar-uploader-icon"></i>
</el-upload>

//图片上传之前处理事件
  beforeAvatarUpload(file) {
    console.log(file);
    const isJpgPng =
        file.raw.type === "image/jpeg" || file.raw.type === "image/png";
    if (!isJpgPng ) {
      this.GLOBAL.messageEvent("error"，"上传头像图片只能是 JPG/PNG 格式!");
    } else {
        this.compressImg(file.raw);
    }
    return isJpgPng;
  },

compressImg(file) {
      let that = this;
      // ?通过FormData构造函数创建一个空对象
      let formData = new FormData();
      let reader = new FileReader();
      // ?将读取到的文件编码成DataURL
      reader.readAsDataURL(file);
      // ?压缩图片
      reader.onload = function(ev) {
         try {
             // ?读取图片来获得上传图片的宽高
             let img = new Image();
             img.src = ev.target.result;
             img.onload = function(ev) {
             // ?将图片绘制到canvas画布上进行压缩
             let canvas = document.createElement("canvas");
             let context = canvas.getContext("2d");
             let imgwidth = img.width;
             let imgHeight = img.height;
             // ?按比例缩放后图片宽高;
             let targetwidth = imgwidth;
             let targetHeight = imgHeight;
             // ?/如果原图宽大于最大宽度
             if (targetWidth > targetHeight) {
                  // ?原图宽高比例
                  let scale = targetHeight / 1280;
                  targetHeight = 1280;
                  targetWidth' = targetwidth / scale;
              } else {
                    // ?原图宽高比例
                    let scale = targetWidth / 1280;
                    targetWidth = 1280;
                    targetHeight = targetHeight / scale;
               }
               // ?缩放后高度仍然大于最大高度继续按比例缩小
               canvas.width = targetwidth; //canvas的宽=图片的宽
               canvas.height = targetHeight; //canvas的高=图片的高
               context.clearRect(0,0, canvas.width, canvas.height);
               context.drawImage(this, 0, 0, canvas.width, canvas.height);
               let data = "":
               // ?如果图片小于0.6Mb，不进行压缩，并返回二进制流
               if (file.size <= 628288) {
                   data = canvas.toDataURL("image/jpeg");
                   formData.append("file"， file);
                   that.handleChange(file);
                }
                // ?如果图片大于e.6Mb，进行压缩，并返回二进制流
                else {
                   // todo 压缩文件大小比例
                   data = canvas.toDataURL("image/jpeg",0.4);
                   let paper = that.GLOBAL.dataURLtoFile(data, file.name);
                   formData.append("file", paper);
                   that.handleChange(paper);
                }
              };
            } catch (error) {
                  console.log("出现错误"，error);
             }
         };
      },

// todo 调用上传接口 文件提交给后台
      handleChange(file) (
          let formData = new FormData( );
          formData.append("file",file.raw || file);
          console.log(formData);
          brandServices.uploadFile(formData).then(res => {
             if (res.data.errno === 0) {
                 this.imgUrl = res.data.data;
                 this.dialogImageUrl = URL.createObjectURL(file);
                 this.GLOBAL.messageEvent("success"，res.data.message);
             }  else { 
                  this.GLOBAL .messageEvent("error"，res .data.message);
              }
           });
       }