1. When the page accesses the blad template page, the variable is undefined.
2. Code

<p>
            <form method="POST" action="importexcel" enctype="multipart/form-data">
                <input type="file" name="myfile" /><br/>
                <input type="submit" name="submit" value="提交" />
            </form>
        </p>
        @if($flag_result == true)
            <p style="color:darkgreen;margin-top:5px;">
                导入数据成功
            </p>
        @endif

        @if(!($fileType == 'xlsx'))
            <p style="color:red;margin-top:5px;">
                数据不合法
            </p>
        @endif  
    </p>

3. It will prompt Undefined variable: flag_result
4. How to solve it?
PS: You can return view('importexcel',compact('flag_result','fileType')); in the control layer. I enter this view first. Then, submit the data into the controller and return to the page.