解决方案

请参见******之间的代码！
后端-app.py:

from flask_cors import CORS

app = Flask(__name__)
***CORS(app, support_credentials=True)***

@app.route('/api/start', methods=['POST'])
***@cross_origin(supports_credentials=True)***
def dostuff_endpoint():
    global doingstuff_active
    
    if request.method == 'POST':
        if not doingstuff_active:
            doingstuff_active = True
            return 'Doingstuff activated.'
        else:
            return 'Doingstuff already active.'
    else:
        return 'Invalid request method.'

// Some code

if __name__ == '__main__':
    // Some code

    ***app.run(host='127.0.0.1', port=5000)***

前端- startButton.tsx：

'use client'

import { useEffect, useState } from 'react';
import axios from 'axios';

const StartButton = () => {
  
  const [data, setData] = useState('');

  const fetchData = async () => {
    try {
      const response = await axios.get('http://**127.0.0.1**:5000/api/dothat');
      const newEntry = response.data.new_entry;
      setData(newEntry || '');
    } catch (error) {
      console.log(error);
    }
  };

  const handleClick = async () => {
    try {
      await axios.post('http://***127.0.0.1***:5000/api/dothing');
      fetchData();
    } catch (error) {
      console.log(error);
    }
  };

  return (
    <div>
      <button onClick={handleClick}>Start</button>
      <p>{data}</p>
    </div>
  );
};

export default StartButton;

有许多与CORS相关的问题和解决方案，但在尝试了上述解决方案后，我仍然有问题。我将在本地运行这个应用程序，所以我不认为CORS是一个问题。有人能给我指个方向吗？TY！
我不想通过代理运行请求。
我尝试了以下提到的解决方案：（1）CORS error when making network call in useEffect in Next.Js，（2）Why does my JavaScript code receive a "No 'Access-Control-Allow-Origin' header is present on the requested resource" error, while Postman does not?
CORS错误消息：
CORS策略阻止了从源“http：//localhost：5000/api/start”访问XMLHttpRequest：请求的资源上不存在“Access-Control-Allow-Origin”标头。
后端-app.py:

from flask_cors import CORS

app = Flask(__name__)
CORS(app)

@app.route('/api/start', methods=['POST'])
def dostuff_endpoint():
    global doingstuff_active
    
    if request.method == 'POST':
        if not doingstuff_active:
            doingstuff_active = True
            return 'Doingstuff activated.'
        else:
            return 'Doingstuff already active.'
    else:
        return 'Invalid request method.'

// Some code

if __name__ == '__main__':
    // Some code

    app.run()

前端- startButton.tsx：

'use client'

import { useEffect, useState } from 'react';
import axios from 'axios';

const StartButton = () => {
  
  const [data, setData] = useState('');

  const fetchData = async () => {
    try {
      const response = await axios.get('http://localhost:5000/api/dothat');
      const newEntry = response.data.new_entry;
      setData(newEntry || '');
    } catch (error) {
      console.log(error);
    }
  };

  const handleClick = async () => {
    try {
      await axios.post('http://localhost:5000/api/dothing');
      fetchData();
    } catch (error) {
      console.log(error);
    }
  };

  return (
    <div>
      <button onClick={handleClick}>Start</button>
      <p>{data}</p>
    </div>
  );
};

export default StartButton;