elementui,el-upload中获取图片宽高

beforeImgUpload(file) {
            let reader = new FileReader();
            reader.onload = function (e) {
                let txt = e.target.result
                let img = document.createElement("img")
                img.src = txt
                img.onload = function () {
                    console.log("宽度：",img.width);
                    console.log("高度：",img.height);
                }
            };
            reader.readAsDataURL(file);
},

下面是别人写的一段代码，对图片进行校验的

beforeAvatarUpload(file) {
    // 上传图片前处理函数
    const isJPG =
        file.type === "image/jpeg" ||
        file.type === "image/png" ||
        file.type === "image/gif";
    const isLt2M = file.size / 1024 / 1024 < 2;
    let that = this;
    let isAllow = false;
    if (!isJPG) {
        this.$message.error("上传头像图片只能是 jpg、png、gif 格式!");
    }
    if (!isLt2M) {
        this.$message.error("上传头像图片大小不能超过 2MB!");
    }
    const isSize = new Promise(function(resolve, reject) {
        let width = 750;
        let height = 420;
        let _URL = window.URL || window.webkitURL;
        let image = new Image();
        image.onload = function() {
          let valid = image.width == width && image.height == height;
          valid ? resolve() : reject();
        };
        image.src = _URL.createObjectURL(file);
    }).then(
        () => {
          return file;
        },
        () => {
          this.$message.error("上传头像图片尺寸不符合,只能是750*420!");
          return Promise.reject();
        }
      );
    return isJPG && isLt2M && isSize;
}