Node basics

Node lets JS run server-side. Standard library gives you filesystem, HTTP, child processes, streams, crypto. The async-by-default API and the event loop are the things that take getting used to.

1 · Filesystem

js fs.mjs

import { readFile, writeFile } from "node:fs/promises";

const text = await readFile("notes.txt", "utf8");
console.log(text);

await writeFile("out.txt", text.toUpperCase());

Always use the node:fs/promises API (async). The callback API and the sync API both exist; the promise API is the cleanest.

2 · HTTP server — without a framework

js server.mjs

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import { createServer } from "node:http";

const server = createServer((req, res) => {
    if (req.url === "/health") {
        res.writeHead(200, { "Content-Type": "application/json" });
        res.end(JSON.stringify({ ok: true }));
        return;
    }
    res.writeHead(404);
    res.end("not found");
});

server.listen(3000, () => console.log("listening on :3000"));

Most apps use Express, Koa, Fastify, or Hono — but the stdlib server is enough for small services and is what frameworks build on.

3 · Process and env

js env.mjs

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console.log(process.platform);    // 'darwin', 'linux', 'win32'
console.log(process.version);     // 'v20.10.0' etc.
console.log(process.argv);         // [node, script, ...args]
console.log(process.env.PORT);     // env var

// Exit
if (!process.env.API_KEY) {
    console.error("API_KEY required");
    process.exit(1);
}

4 · Useful built-in modules

Module	For
`node:fs/promises`	Filesystem (async)
`node:path`	Path joining, parsing
`node:url`	URL parsing
`node:http` / `node:https`	Servers and clients
`node:crypto`	Hashing, HMAC, encryption
`node:stream`	Streaming I/O
`node:child_process`	Run sub-processes
`node:worker_threads`	CPU-bound work off the main thread
`node:test`	Built-in test runner (Node 20+)

5 · Common mistakes

Sync filesystem on a server. readFileSync blocks the event loop. Use the promise API.
Forgetting to handle errors on streams. Unhandled errors crash the process.
Buffering full files instead of streaming. A 1 GB file readFile'd into memory will exhaust it.
Mutating process.env on the fly — works, but most config libraries snapshot at startup.

6 · When it clicks

You reach for the promise API before checking the docs.
You stream large files; you don't read them whole.
You know which work belongs in a worker_thread.

Next · Day 6 · Concept 18

The browser DOM

document, fetch, addEventListener. Vanilla JS in the browser.

Next concept

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