0.1.7 • Public • Published

Extend native Promise in es6 with additional helper methods


// inject methods to native Promise
import 'promise-addition'; 
// promisify the function
const readFile = Promise.promisify(require('fs').readFile);
readFile(__filename, 'utf8')
  .then(content => console.log(content));
// run function with callback injected
Promise.fromCallback(function (callback) {
  callback(null, result); // or callback(err);
}).then((result) => {});
// wait 10ms
  .then(() => console.log('10ms past'));
// run each item promise one by one
Promise.each([1, 2, 3], (val, index) => val + 1)
  .then(result => console.log(result));// [2, 3, 4]
// run all promises concurrently[1, 2, 3, 4], (val, index) => val + 1)
  .then(result => console.log(result));// [2, 3, 4, 5]
// run 2 promises concurrently[1, 2, 3, 4], (val, index) => val + 1, {concurrency: 2})
  .then(result => console.log(result));// [2, 3, 4, 5]
// if query not completed in 10 seconds, reject as timeout
// if query completed less than 10 seconds, wait then go next
// use finally() to release resources
  .finally(() => db.disconnect())
  .then(users => console.log(users))
  .catch(err => console.log(`query error: ${err.message}`));

Static Methods

.delay(ms: int)

Promise version of setTimeout()

.promisify(input: function|array, options?: object)

Alias Promise.denodeify()
Return a promified function for node callback-style function. options:

  • context: object
  • multiArgs: boolean, default false
const readFile = Promise.promisify(fs.readFile)
const content = await readFile('/path/to/file', 'utf8');

.promisifyAll(obj: object, options?: object)

Alias Promise.denodeifyAll()
Modify the object, and create new functions with the suffix for all functions by promisify(). options:

  • suffix: string, default 'Async'
  • multiArgs: boolean, default false
  • filter: function, (value, key, obj)=>bool, default ()=>true
const fs = Promise.promisifyAll(require('fs'));
const content = await fs.readFileAsync('/path/to/file', 'utf8');

.fromCallback(func: (cb)=>any, options?: {multiArgs?: boolean})

Alias Promise.fromNode()
func is a function that called cb at the end, either cb(null, result) or cb(err).

const content = await Promise.fromCallback(cb=>fs.readFile('/path/to/file', 'utf8', cb));
//compare to promisify()
const content = await Promise.promisify(fs.readFile)('/path/to/file', 'utf8');

.each(input: array|promise, iterator: (item, index)=>any)

Run each item in the array with the iterator one by one, as opposed to Promise.all(). And return the result array with the same order of input. input can be an array or a promise that returns array. If someone in the iterator rejects/throws, promise will be rejected immediately and the remains will not be run. This function is equal to, iterator, {concurrency: 1})

.map(input: array|promise, iterator: (item, index)=>any, options?: {concurrency?:Infinity})

Run the specified number(concurrency) items promises with the iterator concurrently. If one resolved, then run next one. Reject immediately if some one failed and remains will not be run. If concurrency is Infinity or less than 1, then it equals to Promise.all(

.reduce(input: array|promise, iterator: (prev, item, index)=>any, initialValue?: any)

Same like array.reduce(). Run each item in the array with the iterator one by one. Reject immediately if some one failed.

.wait(checker: function, options?: {timeout: number, timeoutMessage: string, interval: number})

Poll and fulfilled when checker returns ture or rejected when checker rejects or timeout occurs.

Instance Methods

.timeout(ms: int, message?: string)

If the promise is fulfilled less than the specified time(ms), no timeout occurs. Otherwise it will be rejected with the timeout message(default timeout);

.delay(ms: int)

If the promise is fulfilled less than the specified time(ms), wait.

await Promise.delay(5).delay(15).delay(25)
//about 25 ms elapsed, but less than 30ms(25+5) and 45ms(25+5+15)

.finally(callback: ()=>any)

If the promise fulfilled, run the finally callback first, then run .then() or .catch(). If the callback returns a rejected promise or throws an error, it will trigger the global unhandledRejection event(process.on('unhandledRejection', cb)). Others return will be ignored.

.asCallback(cb: function)

Alias .nodeify(), .toCallback()
Restore promise to callback style.

const readFile = Promise.promisify(fs.readFile);
function originalReadFile(path, options, cb){
  readFile(path, options).asCallback(cb)

Why not create a sub class for es6 native Promise?

First, I think extending native Promise is easy to use and safe enough for me. Creating a sub class need to consider the static methods(like resolve(), reject(), all()), and there is a problem to do that. See:


Licensed under MIT

Copyright (c) 2017 Tian Jian

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