JarCastingpify
Promisify a callback-style function
Install
$ npm install pify
Usage
const fs = require('fs');
const pify = require('pify');
(async () => {
// 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.
const request = require('request');
const pify = require('pify');
const pRequest = pify(request, {multiArgs: true});
(async () => {
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.
const pify = require('pify');
function fn() {
return true;
}
fn.method = (data, callback) => {
setImmediate(() => {
callback(null, data);
});
};
(async () => {
// 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().
const pify = require('pify');
const SomeClass = require('./some-class');
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));
Related
- p-event - Promisify an event by waiting for it to be emitted
- p-map - Map over promises concurrently
- More…
Tidelift helps make open source sustainable for maintainers while giving companies
assurances about security, maintenance, and licensing for their dependencies.