vue+element input file 选择图片并预览

<el-form-item label="上传封面">
      <input type="file" id="cover" accept="image/png, image/jpeg, image/jpg" @change="preview(this)">
      <span style="display:inline-block;width:100px;height:100px;background:#e0e0e0;">
        <img :src="coversrc" alt="封面" id="coverImg" style="width:100%;height:100%">
      </span>
      <el-button type="success" size="small">上传</el-button>
</el-form-item>

preview(){
        var fr=new FileReader();
        var cover=document.getElementById(‘cover‘).files[0];
        fr.readAsDataURL(cover);
        fr.onload=function(e){
          document.getElementById(‘coverImg‘).src = this.result;
        }
      }