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 --save pify

Usage

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

// promisify a single function

pify(fs.readFile)('package.json', 'utf8').then(data => {
	console.log(JSON.parse(data).name);
	//=> 'pify'
});

// or promisify all methods in a module

pify(fs).readFile('package.json', 'utf8').then(data => {
	console.log(JSON.parse(data).name);
	//=> 'pify'
});

API

pify(input, [promiseModule], [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.

promiseModule

Type: function

Custom promise module to use instead of the native one.

Check out pinkie-promise if you need a tiny promise polyfill.

options

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.

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

pify(request, {multiArgs: true})('https://sindresorhus.com').then(result => {
	const [httpResponse, body] = result;
});

include

Type: array of (string|regex)

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

exclude

Type: array of (string|regex)
Default: [/.+Sync$/]

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

excludeMain

Type: boolean
Default: false

By default, if given module is a function itself, this function will be promisified. Turn this option on if you want to promisify only methods of the module.

const pify = require('pify');

function fn() {
	return true;
}

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

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

if (promiseFn()) {
	promiseFn.method('hi').then(data => {
		console.log(data);
	});
}

License

MIT © Sindre Sorhus