elementui input框点击触发placeholder清空

<el-form-item prop="tel" class="topform">
              <el-input  maxlength="11" v-model.number="ReginForm.tel" @focus="blurSearchFor()" @blur="blurSear" :placeholder="placeholder"></el-input>
</el-form-item>

data{
   return{
       placeholder:'请您输入手机号'
   }
}

methods:{
        //获取焦点时触发
        blurSearchFor(){
          console.log(this.placeholder)
          if(this.placeholder=='请您输入手机号'){
              this.placeholder='';
          }
      },
        //失去焦点时触发
      blurSear(){
          this.placeholder='请您输入手机号';
      }
}