`util.promisify`

promisify converts Node-style (err, result) callback functions to promise-returning ones.

2 min read Level 1/5 #nodejs#promisify#async

What you'll learn

Use promisify on legacy APIs
Recognize when standard /promises modules exist

util.promisify converts a Node-style callback function — one that takes (err, result) as the last argument — into one that returns a promise.

The Pattern

import { promisify } from "node:util";
import { exec } from "node:child_process";

const run = promisify(exec);

const { stdout, stderr } = await run("git status");
console.log(stdout);

The original exec(cmd, callback) becomes run(cmd) returning a promise.

When You Need It

Modern Node std-lib usually offers /promises versions already — prefer those over promisify:

// instead of:
import { readFile } from "node:fs";
import { promisify } from "node:util";
const readFileAsync = promisify(readFile);

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

You’ll reach for promisify when:

A specific function has no built-in promise version (exec, crypto.scrypt)
A third-party callback-based lib hasn’t published a promise API yet

A Crypto Example

import { scrypt } from "node:crypto";
import { promisify } from "node:util";

const scryptAsync = promisify(scrypt);

const key = await scryptAsync("password", "salt", 32);
console.log(key.toString("hex"));

When NOT To

If you wrote the callback function yourself, just rewrite it to return a promise directly. promisify is a bridge for legacy APIs.

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