Express

Common errors

• Cannot GET /

  cause No route handler is defined for the requested path and HTTP method.
  fix Define a route using `app.get()`, `app.post()`, or `app.use()` that matches the incoming request path and method.

• Error: Can't set headers after they are sent to the client.

  cause An attempt was made to send a response or modify headers after the response has already been sent to the client. This often happens in asynchronous operations or when multiple response-sending calls are made.
  fix Ensure that response-sending methods like `res.send()` or `res.json()` are called only once per request. Use `return next()` or `return res.send()` to prevent further execution after sending a response.

• Error: listen EADDRINUSE: address already in use :::3000

  cause The port that Express is trying to listen on (e.g., 3000) is already in use by another process.
  fix Kill the process using the port (e.g., `kill -9 <PID>` on Linux/macOS, or find in Task Manager on Windows) or configure your Express application to listen on a different port (e.g., `process.env.PORT || 4000`).

• TypeError: app.use() requires a middleware function but got a Object

  cause An object or non-function value was passed to `app.use()` (or `app.get()`, `app.post()`, etc.) where a middleware function was expected.
  fix Ensure that all arguments passed to `app.use()` (and other route methods) are valid middleware functions, often imported or created as functions that take `(req, res, next)` as arguments.

• ReferenceError: express is not defined

  cause The `express` module was not correctly imported or required at the top of the file, or the variable name `express` was shadowed/redefined.
  fix Add `import express from 'express';` (for ESM) or `const express = require('express');` (for CommonJS) at the beginning of your file.

Warnings

• breaking Express v5.2.0 and v4.22.0 introduced an erroneous breaking change related to the extended query parser (CVE-2024-51999, later rejected). This was reverted in subsequent patch releases.
  Fix: Ensure you are using Express v5.2.1 or v4.22.1 (or newer) to avoid unexpected query parameter parsing behavior. Do not deploy applications with v5.2.0 or v4.22.0.
• breaking Express v5 requires Node.js v18 or newer. Older Node.js versions are no longer supported.
  Fix: Upgrade your Node.js environment to version 18 or higher before upgrading to Express v5. Express v4 continues to support older Node.js versions.
• deprecated The magic string 'back' for redirects (e.g., `res.redirect('back')`) has been deprecated and will be removed in a future major version.
  Fix: Replace `res.redirect('back')` with an explicit URL or dynamic referrer check, e.g., `res.redirect(req.get('Referer') || '/')`.
• gotcha Versions of Express prior to 5.0.1 and 4.21.1 contained a vulnerability in the underlying `cookie` dependency (CVE-2024-47764).
  Fix: Upgrade to Express v5.0.1 (or newer) or v4.21.1 (or newer) to ensure the patched `cookie` dependency is used and your application is not vulnerable.

Install

• npm install express npm
• yarn add express yarn
• pnpm add express pnpm

Imports

• express

  import express from 'express'

  For projects using `type: module` or modern bundlers. CommonJS projects can use `const express = require('express')`.

Quickstart

This demonstrates a basic 'Hello World' Express server listening on port 3000 (or specified by PORT environment variable), responding to GET requests on the root path '/'.

import express, { Express, Request, Response } from 'express';

const app: Express = express();
const port = process.env.PORT ?? 3000;

app.get('/', (req: Request, res: Response) => {
  res.send('Hello from Express!');
});

app.listen(port, () => {
  console.log(`⚡️[server]: Server is running at http://localhost:${port}`);
});