Promisify a callback-style function
npm install pifyimport 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'Returns a Promise wrapped version of the supplied function or module.
Type: Function | object
Callback-style function or module whose methods you want to promisify.
Type: object
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');Type: Array<string | RegExp>
Methods in a module to promisify. Remaining methods will be left untouched.
Type: Array<string | RegExp>
Default: [/.+(?:Sync|Stream)$/]
Methods in a module not to promisify. Methods with names ending with 'Sync' are excluded by default.
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'));
}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.
Type: Function
Custom promise module to use instead of the native one.
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.
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));If you're using TypeScript and your input has function overloads, then only the last overload will be chosen and promisified. This is a TypeScript limitation.
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>