Promise utilities

Some utility functions for working with Promises in node.

npm install promise-util

wait( milliseconds, value )

setTimeout wrapped in a promise.

• milliseconds Integer, time to wait
• value value to return

var promiseUtil = require('promise-util');

promiseUtil.wait( 1000, value )
	.then( function(value) {
		// Do something 1 second later
	} );

defer()

Creates a closure so you don't have to. Returns a promise with resolve() and reject() methods.

Useful for converting callbacks to promises.

var promiseUtil = require('promise-util');

var defer = promiseUtil.defer();
fs.readFile( filename, function( e, contents ) {
	if (e) {
		defer.reject(e);
	}
	defer.resolve(contents);
} );

callback( context, fn, ...args )

Calls a function using the node callback pattern as a promise.

• context is the context to call the function in.
• fn can be the function name as a string, or the function itself.
• ...args arguments to pass to the function

If the callback returns more than one argument, they will be returned as an array.

var promiseUtil = require('promise-util');

promiseUtil.callback( fs, 'readFile', filename )
	.then( /* do stuff */ );

// Is equivalent to
promiseUtil.callback( fs, fs.readFile, filename )
	.then( /* do stuff */ );

// Is equivalent to (but in this case only because fs functions require no context)
promiseUtil.callback( null, fs.readFile, filename )
	.then( /* do stuff */ ); 

If you want to transform a callback function into a function that generates promises then do this.

var readFile = promiseUtil.callback.bind( null, fs, 'readFile' );

Queue()

Creates a reusable promise. This can be used to create a queue of tasks with an adjustable concurrency.

var Queue = require('promise-util').Queue;

// Create a new queue. new is optional.
var queue = new Queue();

// Add some tasks.
queue
	.then( function() {
		// do stuff
	})
	.then( function() {
		// do more stuff
	})
	.catch( function() {
		// catch an error
	})
	.then( 
		function() { /* do stuff */ },
		function() { /* catch an error */ }
	);

// Run the queue of tasks
queue.run()
	.then( function(value) {
		// Result of running the queue
	} );

// Run the queue in series against a collection
var queue = new Queue()
	.then( function(value) {
		return value*value;
	} )
	.runSeries( [1,2,3,4,5] )
		.then( function(value) {
			// value = [1,4,9,16,25];
		} );

// See below for concurrency options

Instance methods

Queue.prototype.then( resolveFn, rejectFn )

Add a task to the queue. Tasks will be run in the context of the queue.

• resolveFn optional function to resolve with
• rejectFn optional function to reject with

Returns the Queue instance for chaining.

Queue.prototype.catch( rejectFn )

Add a catch task to the queue

• rejectFn function to reject with

Returns the Queue instance for chaining.

Queue.prototype.run()

Run the set of tasks.

Returns a Promise.

Queue.prototype.runSeries( collection, options )

Run the set of tasks against a collection in series and return the result of all tasks as a promise.

• collection An array of values to run the queue against.
• options.collect Boolean, default=true, Collect the results of each run in an array
• options.parallel Integer, default=1, How many queues to run at once
• options.infinite Boolean, default=false, Keep running until finish is called.

Returns a Promise, with the additional methods abort and finish.

• abort() will abort running the series.
• finish() will end the series if the infinite option is used.

This will shift() the first value from the collection and then run() it. It will continue to do this until collection is empty. It then resolves the promise with an array of the results of each run.

parallel controls how many queues are allowed to run() at once.

If collect is false than undefined rather than an array of results will be returned in the resolved promise. This is useful if collection is very large.

Items can be added to collection while it runs. If infinite is true, the returned Promise will not resolve until finish is called. If infinite is true, collect defaults to false.

Events

The Queue instance is an event emitter and will emit the following events:

• start when a queue starts
• resolved when a queue resolves, with the result as the first argument
• rejected when a queue rejects, with the error as the fitst argument

fifo( options )

A first in first out queue. Runs tasks in the order they are given ensuring tasks complete before the next one starts.

• options.parallel Integer, the number of tasks to run in parallel. Defaults to 1.

Returns a Function that adds additional items to the queue. The function will return a Promise resolving when the item finishes. Items an be functions, values or promises.

// Create a fifo queue
var fifo = require('promise-util').fifo();

// Add a task
fifo( function() {	
		return someLongProcessReturningAPromise();
	} )
	.then( function() {
		// Do something else
	} );

sequence( ar, initial )

Given an array of functions run then as a sequence of Promises.

• ar The array of functions
• initial A starting value to pass to the first funtion

Returns a Promise resolving to the value of the final function.

spawn( generator )

Given a generator, run the generator resolving any yielded value as a promise and return the final value.

See the "Bonus round: Promises and Generators" section in www.html5rocks.com/en/tutorials/es6/promises/.