Hey there, let's work through this issue together—those two errors are probably linked, so we'll tackle them one by one to get your POST endpoint working.

First: Fix the "Can't set headers after they are sent" Error #

This error is a dead giveaway that your code is trying to send multiple responses for the same request. Once Express sends a response (like res.send(), res.json(), or res.end()), it locks in the response headers—any attempt to send another response after that will throw this error.

Here are the most common fixes:

• Add return after sending error responses: It's easy to accidentally let code run past a response. For example:

  // ❌ Bad: Sends two responses if the title is missing
  app.post('/api/posts', (req, res) => {
    if (!req.body.title) {
      res.status(400).send('Title is required');
    }
    res.json({ success: true }); // This triggers the header error
  });
  
  // ✅ Good: return stops execution after the error response
  app.post('/api/posts', (req, res) => {
    if (!req.body.title) {
      return res.status(400).send('Title is required');
    }
    res.json({ success: true });
  });
  

• Check middlewares for accidental responses: If you have global or route-specific middlewares, make sure none of them call res.send()/res.json() and then also call next(). That would send a response early, then let the request continue to your POST route—leading to the header error when the route tries to send another response.

Next: Fix the 404 on POST /api/posts #

Once the header error is fixed, let's make sure your POST route is actually being hit:

• Verify the route path matches exactly: Double-check that your POST route is defined as /api/posts (not /api/post or /posts). Even a single typo will cause a 404.
• Check route order: Express matches routes in the order they're registered. If your 404 fallback middleware (like app.use((req, res) => res.sendStatus(404))) comes before your POST /api/posts route, Express will hit the 404 first and never reach your endpoint. Move all your specific routes above the 404 fallback.
• Check router prefixes (if using express.Router()): If you're using a router to organize routes, make sure you're mounting it correctly. For example:

  // If you do this:
  const postsRouter = express.Router();
  postsRouter.post('/posts', (req, res) => { /* ... */ });
  app.use('/api', postsRouter); // The actual path becomes /api/posts (correct)
  
  // But if you do this by mistake:
  postsRouter.post('/post', (req, res) => { /* ... */ });
  // The path is /api/post, which will 404 when you hit /api/posts
  

Final Testing Steps #

1. After fixing the header issue, restart your server.
2. Send a test POST request using a tool like curl or Postman:

   curl -X POST http://localhost:3000/api/posts -H "Content-Type: application/json" -d '{"title": "Test Post"}'
   

3. Check your server logs to see if the POST route handler is being executed (add a console.log('POST /api/posts hit') at the start of the handler to confirm).

Once you've worked through these steps, your POST endpoint should start working as expected!

内容的提问来源于stack exchange，提问作者SJC