prom-queen

prom-queen

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prom-queen provides a number of small helper utilities to make working in a promise-based environment more pleasant. It has no external dependencies and is built for recent/modern javascript platforms.

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In order to meet the goal of zero extra dependencies yet remaining lightweight, this assumes a modern javascript environment exists -- In particular, this library makes no effort to support environments without a native promise implementation, arrow functions, and a few other ES6-y things.

All ES6 features used in this library are supported under Node 4.x LTS; babel should not be required for running under a recent node platform. No node-specific features are used other than that, so a babel+browserify/webpack type situation should work for running in a browser, though this has not yet been tested.

On _listish_ and _proc_

This documentation refers often to a _listish_. A listish thingy is either an array or an object that can be either treated as or transformed to/from an array. Currently supported listish thingies:

* a regular javascript Array
* any javascript iterable (has a object[Symbol.iterator]() method which returns an iterator)
* an array-like object (has .length with type number, has keys 0 and if length != 0, obj.length - 1)
* a regular javascript Object: transformed into an array for processing, transformed back into an object when resolved
* a promise: any promise passed as a _listish_ is assumed to eventually resolve to a _listish_

Many of prom-queen's utils take a proc argument, which is a function reference. This proc function should always return a promise in some way.

api

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These take all take arguments of (listish, proc), and all are comparable to a promise-based map or forEach -- they run all items within the listish using proc, which is a function that takes a single element from the listish and must return a promise.

##### queen.parallel(listish, proc) ⇒ Promise
* runs proc(item) on all items in parallel, roughly equivalent to Promise.all(), except it takes a listish while Promise.all() only takes an iterable

##### queen.sequential(listish, proc) ⇒ Promise
* runs proc(item) on all items in series, waiting for proc(item) to resolve before continuing to the next.

##### queen.batch(listish, num, proc) ⇒ Promise
* num must be an integer > 1
* creates num worker promises, running them in parallel. whenever any worker finishes its task, it queues up the next available item on the listish to run again.

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The repeater utilities are all various ways of running a single proc function more than once. In all of the repeater utilities:
* proc is called without any arguments
* anytime a function is passed as condition, this condition function is also called without any arguments
* anytime proc rejects, the repeater will also immediately reject, propagating the original error

##### queen.repeat(condition, proc) ⇒ Promise
This is the most generic form of .repeat() and the core of all the other repeater utilities in prom-queen.
condition should be a function that takes no arguments and returns a boolean. It's roughly equivalent to while (condition()) { proc(); }. If condition() or proc() ever rejects, repeat will reject immediately. If condition() ever returns a non-truthy value, the repeat will immediately resolve with an array containing the results from every iteration so far.

##### queen.repeat(number, proc) ⇒ Promise

In this form, proc will be invoked exactly number times (unless any invocation rejects, in which case execution will immediately stop).

##### queen.repeat(Infinity, proc) ⇒ Promise
##### queen.repeat(proc) ⇒ Promise
##### queen.infinite(proc) ⇒ Promise

These are all aliases for queen.repeat(() => true, proc). This is an infinite asynchronous loop which only exits if proc() ever rejects. In this case repeat will also reject, propagating the error.

##### queen.repeatWhile(condition, proc, opts) ⇒ Promise

Apart from opts, this is a synonym for queen.repeat(condition, proc) -- proc will execute repeatedly until condition() resolves to a non-truthy value. If opts.postCondition (or its alias opts.post) is defined and truthy then the first call to condition() is skipped; this makes queen.repeat(condition, proc) and queen.repeat(condition, proc, {post:false}) equivalent to while (condition()) { proc() }, while queen.repeat(condition, proc, {post:true}) is equivalent to do { proc() } while (condition());.

##### queen.repeatUntil(condition, proc, opts) ⇒ Promise
The same as queen.repeatWhile() except with its condition negated; proc will execute repeatedly until condition() resolves to a truthy value

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##### queen.tasks(initialState, taskArray) ⇒ Promise
##### queen.tasks(taskArray) ⇒ Promise

queen.tasks is a convenient way to run a handful of task functions in order, each of which will be called with a shared state object where they can store results and other data, and each of which should return a promise. queen.tasks() resolves with the final state after all of the tasks have run.

If any entry in taskArray is itself an array, it is assumed to be an array of task functions, which will be run in parallel.

The state object has one extra (non enumerable/writable/configurable) property called set; this is a small helper for setting things into the state. somePromise.then(state.set('myResult')) is equivalent to somePromise.then(result => {state.myResult = result; })

A small example:

``javascript
queen.tasks([
state => User.find({email:'someone@somedomain.com'}).then(state.set('user'))
[
state => Posts.count({user:state.user.id}).then(state.set('posts')),
state => Comments.count({user:state.user.id}).then(state.set('comments')),
]
]).then(results => console.log(results.user, results.posts, results.comments));
`

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##### queen.isPromise(thingy) ⇒ Boolean
* returns true if thingy seems to be a promise of some sort, either by being instanceof Promise, or having a .then method

##### queen.delayed(ms, val) ⇒ Promise
* returns a promise that resolves with val after ms milliseconds. Quick hack and available elsewhere but tiny and was useful during prototyping so was left in.

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##### queen.cb(resolve, reject) ⇒ (error, result) => null
* simple util for dealing with callbacks. This transforms a resolve/reject pair into a callback function of (roughly) the form (error, result) => { if (error) { return reject(error); } return resolve(result); }
`javascript
function read(file) {
return new Promise((resolve, reject) => {
fs.readFile(file, 'utf8', queen.cb(resolve, reject));
});
}
`

##### queen.adapt(func) ⇒ Promise
* very light wrapper around queen.cb() above, creates a promise and calls the passed function with a callback which triggers the rejection or resolution of the returned promise.
* used for adapting callback-based code into promises
``javascript

// run a callback-taking function (similar to call below)
queen.adapt(callback => fs.readFile(file, 'utf8', callback)).then(contents => console.log("my contents are", contents));

// wrap a callback-taking function
const _readFile = (fn, opts) => queen.adapt(cb => fs.readFile(fn, opts, cb));
`

##### queen.call(obj, method, argsArray) ⇒ Promise
* wraps a single call of a single method in a promise: queen.call(fs, 'readFile', ['package.json', 'utf8']).then(JSON.parse)`