element ui Upload 上传 限制图片宽高

<el-upload
        class="avatar-uploader"
        :action="action"
        :show-file-list="false"
        :before-upload="beforeAvatarUpload">
      <i class="el-icon-plus avatar-uploader-icon"></i>
    </el-upload>

methods: {
    beforeAvatarUpload(file) {
      let width = 750;
      let height = 1642;
      const isSize = new Promise(function (resolve, reject) {
        let _URL = window.URL || window.webkitURL;
        let img = new Image();
        img.onload = function () {
          let valid = img.width == width && img.height == height;
          valid ? resolve() : reject();
        }
        img.src = _URL.createObjectURL(file);
      }).then(() => {
        return file;
      }, () => {
        this.$message({
          message: `上传图片尺寸只能是${width}*${height}px!请重新选择!`,
          type: 'warning'
        });
        return false;//必须加上return false; 才能阻止
      })
      return isSize;
    }
 },