q

What is q?

The 'q' npm package is a library for creating and managing promises in JavaScript. It provides a robust set of tools for working with asynchronous operations, allowing developers to write cleaner, more maintainable code by avoiding the 'callback hell' that can occur with deeply nested callbacks.

What are q's main functionalities?

Creating Promises

This feature allows the creation of a new promise using Q.defer(). The deferred object has a promise property and methods for resolving or rejecting the promise.

const Q = require('q');
const deferred = Q.defer();

function asyncOperation() {
  // Perform some asynchronous operation
  setTimeout(() => {
    // Resolve the promise after 1 second
    deferred.resolve('Operation completed');
  }, 1000);
  return deferred.promise;
}

asyncOperation().then(result => console.log(result));

Promise Chaining

This feature demonstrates how promises can be chained together, with the output of one promise being passed as input to the next.

const Q = require('q');

function firstAsyncOperation() {
  var deferred = Q.defer();
  setTimeout(() => deferred.resolve(1), 1000);
  return deferred.promise;
}

function secondAsyncOperation(result) {
  var deferred = Q.defer();
  setTimeout(() => deferred.resolve(result + 1), 1000);
  return deferred.promise;
}

firstAsyncOperation()
  .then(secondAsyncOperation)
  .then(result => console.log('Final result:', result));

Error Handling

This feature shows how to handle errors in promise-based workflows. The catch method is used to handle any errors that occur during the promise's execution.

const Q = require('q');

function mightFailOperation() {
  var deferred = Q.defer();
  setTimeout(() => {
    if (Math.random() > 0.5) {
      deferred.resolve('Success!');
    } else {
      deferred.reject(new Error('Failed!'));
    }
  }, 1000);
  return deferred.promise;
}

mightFailOperation()
  .then(result => console.log(result))
  .catch(error => console.error(error.message));

Other packages similar to q

bluebird

Bluebird is a fully-featured promise library with a focus on innovative features and performance. It is known for being one of the fastest promise libraries and includes utilities for concurrency, such as Promise.map and Promise.reduce, which are not present in 'q'.

when

When is another lightweight Promise library that offers similar functionality to 'q'. It provides a solid API for creating and managing promises but is generally considered to have a smaller footprint and to be more modular than 'q'.

promise

This package is a simple implementation of Promises/A+. It is smaller and may be more straightforward than 'q' for those who only need basic promise functionality without the additional utilities provided by 'q'.

README

Thenables

The Q API supports CommonJS/Promises/A, Kris Zyp's proposal for "thenable" promises. A thenable is any object with a "then(callback, errback)" method. The "then" method, in turn, returns a promise for whatever value the callback's eventually return. Thus, promises are chainable.

Let's review Tim's example. This illustrates the same concept as the first example. We asynchronously read our own program and print it out in all capitals.

var Q = require("q");
var FS = require("q-fs");

Q.when(FS.read(__filename))
.then(function (text) {
    return text.toUpperCase();
}).then(function (text) {
    console.log(text);
});

/!\ IMPORTANT

The call to "Q.when" is not necessary but provides many assurances that, even if the FS API is poorly written or even if it is maliciously written, that the promise returned will behave consistently. That means that your callbacks will all occur in future turns of the event loop (so the state in your closure doesn't change), and that one callback for each "thenable" will ever be called (so that the state in your closure doesn't change more than once).

This example uses the "q/util" module again, because it has that lovely "deep" method for turning objects with promises inside-out (to a promise for an object).

var Q = require("q/util");
var FS = require("q-fs");

Q.when(Q.deep({
    "self": FS.read(__filename),
    "passwd": FS.read("/etc/passwd")
})).then(function (texts) {
    console.log(__filename + ":" + texts.self.length);
    console.log("/ext/passwd:" + texts.passwd.length);
});

In this case, we've simultaneously read the text of our own program, and the Unix user database, and then printed out their corresponding file sizes.

Finally, to read all of the files in the examples directory and note the lengths of each one, we can use a three step thenable:

var Q = require("q/util");
var FS = require("q-fs");

Q.when(FS.list(__dirname))
.then(function (fileNames) {
    return Q.deep(fileNames.map(function (fileName) {
        return {
            "name": fileName,
            "text": FS.read(FS.join(__dirname, fileName))
        };
    }));
}).then(function (files) {
    files.forEach(function (file) {
        console.log(file.name, file.text.length);
    });
});

Again, all of these examples are in the examples directory with the names then{1,2,3}.js.

Quacks Like a Duck:

Any object with a "then(callback, errback)" method will be treated as a promise, and all promises provided by the Q API have "then" methods so they can be used by any API that accepts thenables.