Cheatsheet

Node.js

Node.js and npm for scripts and small servers: running code, modules, files, HTTP, async code and packages.

Run and manage

```
node app.js
```
Run a script
```
node --watch app.js
```
Restart when a file changes
```
node -e "console.log(process.version)"
```
Run one line of code
```
node --test
```
Run the built-in test runner on *.test.js files
```
node --inspect app.js
```
Start with the debugger listening (open chrome://inspect)

npm

```
npm init -y
```
Create a package.json
```
npm install express
```
Add a dependency (npm i -D for dev tools)
```
npm ci
```
Install exactly what package-lock.json says (use in CI and Docker)
```
npm run build
```
Run a script from package.json
```
npm outdated  |  npm audit
```
Packages with newer versions, and known vulnerabilities
```
npx prettier --write .
```
Run a tool without installing it globally

{ "scripts": { "start": "node server.js", "test": "node --test" } }

Scripts in package.json: npm start, npm test

Modules

```
const fs = require("node:fs");
```
CommonJS: load a module (built-ins have the node: prefix)
```
import { readFile } from "node:fs/promises";
```
ES modules: use "type": "module" in package.json or a .mjs file

module.exports = { add };  |  export const add = (a, b) => a + b;

Export from CommonJS / from an ES module

const path = require("node:path"); path.join(__dirname, "data", "a.json")

Build paths that work on every system

Files and process

const text = await fs.promises.readFile("a.txt", "utf8");

Read a file

await fs.promises.writeFile("out.json", JSON.stringify(data, null, 2));

Write a file

JSON.parse(text)  |  JSON.stringify(obj)

Text to object and back

```
process.argv.slice(2)
```
The command-line arguments
```
process.env.PORT || 3000
```
An environment variable with a default
```
process.exit(1)
```
Stop with an error code (0 means success)

Async code

const data = await fetch(url).then((r) => r.json());

Fetch JSON (fetch is built in)

```
const [a, b] = await Promise.all([getA(), getB()]);
```
Run two things at the same time and wait for both

try { await work(); } catch (err) { console.error(err.message); }

Handle a failure in async code

```
setTimeout(() => console.log("later"), 1000);
```
Run something after a delay (milliseconds)

process.on("unhandledRejection", (e) => { console.error(e); process.exit(1); });

Do not let a forgotten rejection pass silently

A small HTTP server

const http = require("node:http");
http.createServer((req, res) => {
  res.setHeader("Content-Type", "application/json");
  res.end(JSON.stringify({ ok: true, path: req.url }));
}).listen(process.env.PORT || 3000);

A server with no dependencies

```
curl -s localhost:3000/health
```
Try it from another terminal

res.writeHead(404); res.end("not found");

Reply with a status code

process.on("SIGTERM", () => server.close(() => process.exit(0)));

Shut down cleanly when a container stops

Cheatsheet for the Node.js path · Deboistech · All cheatsheets