Express.md

Express Cheatsheet

• Express Cheatsheet
  • Creating an App
  • Listening on a port
    • Using a separate server instance
  • Middleware
    • Standard middleware function signature
    • app.use
    • Routes - app.get, app.post, etc
    • Router
      • Making a new router
      • Letting app know about the router and it's prefix.
      • Defining Routes on the Router
  • Error handling
  • Common Pitfalls
    • Not sending anything back from a route

Creating an App

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

Listening on a port

const PORT = 4000;

app.listen(PORT, () => {
    console.log(`Listening on ${PORT}`);
});

Using a separate server instance

Sometimes you may want to separate the app from the server itself.

This makes testing easier, and is a good separation of concerns.

// In server.js
const http = require('http'); // This is node's built-in http module
const app = require('./app'); // Assuming we create our app in app.js

const server = http.createServer(app); // We can pass the app to createServer

server.listen(PORT, () => {
    console.log(`Listening on ${PORT`}`);
});

Middleware

Every callback we define in express is middleware.

Standard middleware function signature

    (req, res, next) => {}

• req - The Request object that represents the incoming Request to the server
• res - The Response object that represents the outgoing response to the browser
• next - A function that will move to the next middleware function in the chain

app.use

app.use is how you register a middleware function with express. Functions are added to the express app in the order the app.use() functions are invoked. The callback (middleware) functions aren't called until a new request comes in to the server.

app.use((req, res, next) => {
    // this runs when a request comes in.
});

app.use can accept a list of middleware functions OR an array of them.

// Two arrow functions.
app.use((req, res, next) => {}, (req, res, next) => {});

// Two named functions.
app.use(middleware1, middleware2);

// An array of middleware functions
app.use([middleware1, middleware2]);

// Or any combination. It's flexible!
app.use([middleware1, middleware2], middleware3, (req, res, next) => {
});

The important thing is, you are just giving express function references, the functions do not get called until a request is made to the server.

Routes - app.get, app.post, etc

Express routes are similar to app.use, but are conditional middleware.

They depend on the HTTP Method and the Path matching in order to run.

The first argument to these routes is the path to match.

app.get('/', (req, res, next) => {
    // Runs only for a GET request to / 
});

app.post('/users', (req, res, next) => {
    // Runs only for a POST request to /users
});

Router

As a way of organizing your router, express includes a router object.

Making a new router

    const express = require('express');
    const userRouter = express.Router();

Letting app know about the router and it's prefix.

We can use app.use to tell express about the router and what prefix all of it's routes will be under.

const app = express();
const userRouter = express.Router();

// every route we create on userRouter will have the prefix `/users`
app.use('/users', userRouter); 

Defining Routes on the Router

Routers contain the same .use() .get() and .post() methods that app does. However, the path matching matches everything after the router's prefix.

// Passing a middleware to the router
userRouter.use(middleware);

userRouter.get('/profile', (req, res, next) => {
    // Runs when the router path matches /profile
    // excluding the prefix

    // Example: If the userRouter's prefix was /users
    // then the full path that matches is /users/profile
});

Error handling

Error handling middleware is just like regular middleware but has four parameters to the callback function.

Make sure you log your errors to the console, otherwise if you have a bug in your express app, it might be completely hidden from you.

app.use((err, req, res, next) => {
    // The first argument will be an error.
    console.error(err); // This is a good idea
});

Common Pitfalls

Not sending anything back from a route

This will cause express to hang forever. Your browser will never get a response.

app.get('/', (req, res, next) => {
    // if I don't res.send or res.json
    // And I don't call next()
    // This will cause express to hang.
});