Callbacks

Pass a function to be called when work finishes. Node's classic pattern — superseded by promises but still around.

3 min read Level 1/5 #nodejs#callbacks#async

What you'll learn

Recognize the (err, result) callback convention
Know why callbacks fell out of favor
Convert callbacks to promises with promisify

Before promises, Node used callbacks for everything async. You’d pass a function that the runtime calls when the work finishes.

The Pattern

import { readFile } from "node:fs";

readFile("data.txt", "utf8", (err, content) => {
  if (err) {
    console.error("read failed", err);
    return;
  }
  console.log(content);
});

Node’s convention: callback’s first argument is always the error (or null). Remaining args are the result. Same shape across the whole standard library.

The Problem — Nesting

Stack a few of them and you get the famous “callback pyramid”:

readFile("a.txt", "utf8", (err, a) => {
  if (err) return done(err);
  readFile("b.txt", "utf8", (err, b) => {
    if (err) return done(err);
    writeFile("c.txt", a + b, (err) => {
      if (err) return done(err);
      done(null, "all done");
    });
  });
});

Hard to read, hard to add error handling, hard to compose.

Convert to Promises

util.promisify wraps a Node-style callback function as a promise:

import { readFile, writeFile } from "node:fs";
import { promisify } from "node:util";

const read  = promisify(readFile);
const write = promisify(writeFile);

const a = await read("a.txt", "utf8");
const b = await read("b.txt", "utf8");
await write("c.txt", a + b);

For the standard library, just import from /promises — node:fs/promises, node:dns/promises, node:timers/promises.

When You’ll Still See Callbacks

Old library APIs (request, mongojs, etc.)
Stream events (stream.on("data", ...))
EventEmitter listeners
Some Node APIs (e.g. crypto.scrypt — has both)

Knowing the pattern means you can read older code and convert it.

Promises →