Q.js Promises Cheatsheet

[!NOTE] Historical Library: Q was a foundational Promises/A+ implementation before native ES2015 Promises and async/await were standardized in JavaScript engines.

Q is a promise library designed for Node.js and browsers.

Creating Promises

const Q = require('q');

function asyncAction() {
  const deferred = Q.defer();

  fs.readFile('data.txt', 'utf8', (err, data) => {
    if (err) deferred.reject(err);
    else deferred.resolve(data);
  });

  return deferred.promise;
}

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Converting Nodebacks (Node.js Callbacks)

Q provides utilities to convert standard Node callback signatures (err, result) into promises:

const fs = require('fs');

// Q.nfcall: Node Function Call
Q.nfcall(fs.readFile, 'file.txt', 'utf-8')
  .then((data) => console.log(data))
  .catch((err) => console.error(err));

// Q.denodeify: Wrap callback function into promise-returning function
const readFile = Q.denodeify(fs.readFile);
readFile('file.txt', 'utf-8').then(console.log);

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Array Combinators

// Q.all: Waits for all to resolve
Q.all([stepA(), stepB()])
  .then(([resA, resB]) => {
    console.log(resA, resB);
  });

// Q.allSettled: Waits for all to finish regardless of success
Q.allSettled([stepA(), stepB()])
  .then((results) => {
    results.forEach((r) => {
      if (r.state === 'fulfilled') console.log(r.value);
      else console.error(r.reason);
    });
  });

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References

• Q GitHub Repository
• JavaScript Promises Cheatsheet