vue 文件上传判断文件尺寸级大小

一、vue 基于elementui实现文件或图片上传判断文件或图片的尺寸；本例基于图片上传的demo

             <el-form-item label="添加图片">
                     <div class="imgList">
                        <div class="imgBox" :class="{pitch:item.alter}" v-for="(item,indexs) in imageArr" :key="indexs" @mousemove="moveImg(item)" @mouseout="outImg(item)">
                            <img class="img" :preview="indexs" :src="item.url"/>
                            <img v-if="item.alter"  @click="deleteImg(indexs)" class="deleteImg"src="../../assets/img/deleteImg.png"/>
                        </div>
                        <el-upload
                        multiple
                        class="avatar-uploader"
                        :action="action"
                        accept="image/*"
                        :show-file-list="false"
                        :on-success="handleAvatarSuccess"
                        :before-upload="beforeAvatarUpload">
                        <i v-if="imageArr.length<1" class="el-icon-plus avatar-uploader-icon"></i>
                        </el-upload>
                        <div class="el-upload__tip" slot="tip">banner尺寸为750*388px , 支持jpg/png格式，不超过5M</div>
                    </div>
             </el-form-item>

export default {
    data(){
        return {
            picture:[],
            picname:[],
            imageArr:[],
       }
    },
    methods:{
      handleAvatarSuccess(res, file) {
            this.picture.push(res.data);
            this.picname.push(res.fileName);
            this.imageArr.push({url: URL.createObjectURL(file.raw), alter: false});
        },
        beforeAvatarUpload(file) {
            const isLt5M = file.size / 1024 / 1024 < 5;
            const isJPG = checkImg(file.name);
            if (!isLt5M) {
                this.$message.error('上传图片大小不能超过5MB!');
            }
            if(!isJPG){
                 this.$message.error('上传图片格式是jpg/jpeg/png!');
            }
　　　　　　　　//核心代码部分
            const isSize = new Promise(function(resolve, reject) {
                let width = 750;
                let height = 388;
                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.$message.error('banner尺寸必须是750*388!');
                return Promise.reject();
            });
            return isLt5M && isJPG && isSize;
        },
        moveImg(item){
          item.alter = true;
        },
        outImg(item){
          item.alter = false;
        },
        deleteImg(index){
            this.$confirm('您确定要删除作品图片吗？', '提示', {
                confirmButtonText: '确定',
                cancelButtonText: '取消'
            }).then(() => {
                this.imageArr.splice(index,1);
                this.picture.splice(index,1);
            }).catch(() => {

            });
        },
     }
}

 其中 checkImg（）方法是判断图片的格式，通过这种方式可判断出上传图片的尺寸及大小！