element upload限制图片宽高

 beforeUpload(file) {
                // 上传图片前

                const isJPG = this.fileType(file);
                if (!isJPG) {
                    this.$message({
                        message: "请上传jpg,png,jpeg格式的图片",
                        type: "error"
                    });
                }
                const isLt5M = file.size / 1024 / 1024 < 5;
                if (!isLt5M) {
                    this.$message({
                        message: "文件不能大于5M!",
                        type: "error"
                    });
                }

                const isSize = new Promise(function (resolve, reject) {
                    let width = 375;
                    let height = 150;
                    let _URL = window.URL || window.webkitURL;
                    let img = new Image();
                    img.onload = function () {
                        console.log(img.width, img.height, "cccc");
                        let valid = img.width == width && img.height == height;
                        valid ? resolve() : reject();
                    }
                    img.src = _URL.createObjectURL(file);
                }).then(() => {
                    return file;
                }, () => {
                    this.$message({
                        message: '请上传尺寸为375*150的图片!',
                        type: 'error'
                    });
                    return Promise.reject()
                    return false;//必须加上return false; 才能阻止
                })
                return isLt5M && isSize;
            }