pify

What is pify?

The pify npm package is a utility module that converts callback-based functions or methods to return Promises. This is particularly useful when working with older Node.js or JavaScript libraries that do not natively support Promises, allowing developers to write cleaner, more modern asynchronous code using async/await or .then() chaining.

What are pify's main functionalities?

Promisifying a single function

This code sample demonstrates how to promisify Node.js's fs.readFile function using pify. The resulting readFileAsync function returns a Promise that resolves with the file's contents or rejects with an error.

const pify = require('pify');
const fs = require('fs');
const readFileAsync = pify(fs.readFile);

readFileAsync('file.txt', 'utf8').then(data => {
  console.log(data);
}).catch(error => {
  console.error(error);
});

Promisifying an entire module

This code sample shows how to promisify all the functions of the fs module. After promisification, methods like fs.readFile return Promises.

const pify = require('pify');
const fs = pify(require('fs'));

fs.readFile('file.txt', 'utf8').then(data => {
  console.log(data);
}).catch(error => {
  console.error(error);
});

Custom promisification options

This code sample illustrates how to use pify with custom options. The 'exclude' option prevents certain functions from being promisified, while 'multiArgs' allows the promise to resolve with an array of values if the original callback returns multiple arguments.

const pify = require('pify');
const someModule = require('some-module');

const promisifiedModule = pify(someModule, {
  exclude: ['nonAsyncFunction'],
  multiArgs: true
});

promisifiedModule.someFunction().then(result => {
  const [firstResult, secondResult] = result;
  console.log(firstResult, secondResult);
});

Other packages similar to pify

util.promisify

Built into Node.js, util.promisify is a core method that converts a callback-based function into a Promise-based one. It is similar to pify but does not offer the same level of customization, such as excluding functions or handling multiple callback arguments.

bluebird

Bluebird is a comprehensive promise library that includes a promisify and promisifyAll method, which are similar to pify's functionality. Bluebird promises offer additional features such as cancellation, progress, and long stack traces, which pify does not.

es6-promisify

The es6-promisify package is another alternative that converts callback-based functions into Promises. It is similar to pify but with a slightly different API and does not provide as many options for customization.

README

pify

Promisify a callback-style function

Install

npm install pify

Usage

import fs from 'fs';
import pify from 'pify';

// Promisify a single function.
const data = await pify(fs.readFile)('package.json', 'utf8');
console.log(JSON.parse(data).name);
//=> 'pify'

// Promisify all methods in a module.
const data2 = await pify(fs).readFile('package.json', 'utf8');
console.log(JSON.parse(data2).name);
//=> 'pify'

API

pify(input, options?)

Returns a Promise wrapped version of the supplied function or module.

input

Type: Function | object

Callback-style function or module whose methods you want to promisify.

options

Type: object

multiArgs

Type: boolean
Default: false

By default, the promisified function will only return the second argument from the callback, which works fine for most APIs. This option can be useful for modules like request that return multiple arguments. Turning this on will make it return an array of all arguments from the callback, excluding the error argument, instead of just the second argument. This also applies to rejections, where it returns an array of all the callback arguments, including the error.

import request from 'request';
import pify from 'pify';

const pRequest = pify(request, {multiArgs: true});

const [httpResponse, body] = await pRequest('https://sindresorhus.com');

include

Type: Array<string | RegExp>

Methods in a module to promisify. Remaining methods will be left untouched.

exclude

Type: Array<string | RegExp>
Default: [/.+(?:Sync|Stream)$/]

Methods in a module not to promisify. Methods with names ending with 'Sync' are excluded by default.

excludeMain

Type: boolean
Default: false

If the given module is a function itself, it will be promisified. Enable this option if you want to promisify only methods of the module.

import pify from 'pify';

function fn() {
	return true;
}

fn.method = (data, callback) => {
	setImmediate(() => {
		callback(null, data);
	});
};

// Promisify methods but not `fn()`.
const promiseFn = pify(fn, {excludeMain: true});

if (promiseFn()) {
	console.log(await promiseFn.method('hi'));
}

errorFirst

Type: boolean
Default: true

Whether the callback has an error as the first argument. You'll want to set this to false if you're dealing with an API that doesn't have an error as the first argument, like fs.exists(), some browser APIs, Chrome Extension APIs, etc.

promiseModule

Type: Function

Custom promise module to use instead of the native one.

FAQ

How is this different from Node.js's util.promisify?

• Pify existed long before util.promisify.
• Pify is faster.
• Pify supports wrapping a whole module/object, not just a specific method.
• Pify has useful options like the ability to handle multiple arguments (multiArgs).
• Pify does not have magic behavior for certain Node.js methods and instead focuses on predictability.

How can I promisify a single class method?

Class methods are not bound, so when they're not called on the class itself, they don't have any context. You can either promisify the whole class or use .bind().

import pify from 'pify';
import SomeClass from './some-class.js';

const someInstance = new SomeClass();

// ❌ `someFunction` can't access its class context.
const someFunction = pify(someClass.someFunction);

// ✅ The whole class is promisified and the `someFunction` method is called on its class.
const someClassPromisified = pify(someClass);
someClassPromisified.someFunction();

// ✅ `someFunction` is bound to its class before being promisified.
const someFunction = pify(someClass.someFunction.bind(someClass));

Why is pify choosing the last function overload when using it with TypeScript?

If you're using TypeScript and your input has function overloads, then only the last overload will be chosen and promisified.

If you need to choose a different overload, consider using a type assertion:

function overloadedFunction(input: number, callback: (error: unknown, data: number => void): void
function overloadedFunction(input: string, callback: (error: unknown, data: string) => void): void {
	/* … */
}

const fn = pify(overloadedFunction as (input: number, callback: (error: unknown, data: number) => void) => void)
// ^ ? (input: number) => Promise<number>

Related

• p-event - Promisify an event by waiting for it to be emitted
• p-map - Map over promises concurrently
• More…

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