Express 4 and 5 req.get doesnt return header if not set in lowercase #5288

In the documentation, it's stated that req.get is intended to be case-insensitive. However, it appears that when you set a custom header in a middleware, this case-insensitivity breaks if the header is not in lowercase. Should this behavior be corrected, or should the guidance be to always set headers in lowercase? Perhaps, adding a setHeader function in the request object could be a solution to this issue.

The problem seems to originate from this part of the code: https://github.com/expressjs/express/blob/master/lib/request.js#L82, where the headers are not being converted to lowercase, while the input is.

Example code

const express = require('express');
const app = express();

app.use((req, res, next) => {
    req.headers['X-filipe'] = 'filipe';
    next();
});

app.get('/', (req, res) => {
    res.json({ header: req.get('x-filipe') }); 
});

app.listen(3000, () => {
    console.log('Server is listening on port 3000');
});

Expected output:

{"header": "filipe"}

This adjustment ensures that the headers are consistently in lowercase, aligning with the case-insensitive nature of req.get.