Why does my express POST endpoint return 404 when Content-Type is set by requester?

Question

I have an express server:

import express from "express";

const app = express()
const port = 3000;

app.use(express.json()) 

app.use((req, res, next) => {
  res.header("Access-Control-Allow-Origin", "*");
  res.header("Access-Control-Allow-Headers", "*");
  next();
});

app.post(/city\/get-ids/g, (req, res) => {
  console.log(req.body);
  res.send('Hello World!')
})

app.listen(port);

as well as a front end fetch request:

const response = await fetch('http://localhost:3000/city/get-ids/', {
  method: 'POST',
  headers: {
    'Content-Type': 'application/json;charset=utf-8'
  },
  body: JSON.stringify({a: 1})
});

If I comment out Content-Type on the fetch, I get a "Hello World" response, but the server logs req.body as being an empty object.

If I keep the Content-Type on fetch, I get a 404.

---

Why does setting Content-Type make express return a 404 while not setting it returns "Hello World"?

Answer 1

Put in an issue, got a better response ( https://github.com/expressjs/express/issues/4590 )

The problem was the global g in app.post(/city\/get-ids/g .

You can see more about why here: Why does a RegExp with global flag give wrong results?

Anyways, the solution is to just remove the g :

app.post(/city\/get-ids/, (req, res) => {
  console.log(req.body);
  res.send('Hello World!')
});

---

Old post for comment posterity:

Alright. Fixed it, but it still makes absolutely zero sense.

Here's the fix:

app.post("/city/get-ids*", (req, res) => {
  console.log(req.body);
  res.send('Hello World!')
})

For some reason the RegExp version just doesn't work if Content-Type is set. The OPTIONS preflight returns correctly either way. If there's no Content-Type the POST route matches correctly. But RegExp for POST routes no longer works when Content-Type is set.

It makes absolutely no sense.