now-and-later

Map over an array or object of values in parallel or series, passing each through the async iterator, with optional lifecycle hooks.

Usage

var nal = require('now-and-later');

function iterator(value, key, cb) {
  // called with each value in sequence
  // also passes the key
  cb(null, value * 2);
}

function create(value, key) {
  // called at the beginning of every iteration
  // return a storage object to be passed to each lifecycle method
  return { key: key, value: value };
}

function before(storage) {
  // called before the iterator function of every iteration
  // receives the storage returned from `create`
}

function after(result, storage) {
  // called after a success occurs in the iterator function of any iteration
  // receives the `result` of the iterator and the storage returned from `create`
}

function error(error, storage) {
  // called after an error occurs in the iterator function of any iteration
  // receives the `error` of the iterator and the storage returned from `create`
}

function done(error, results) {
  // called after all iterations complete or an error occurs in an iterator
  // receives an `error` if one occurred and all results (or partial results upon error) of the iterators
}

/*
  Calling mapSeries with an object can't guarantee order
  It uses Object.keys to get an order
  It is better to use an array if order must be guaranteed
 */
nal.mapSeries(
  [1, 2, 3],
  iterator,
  {
    create: create,
    before: before,
    after: after,
    error: error,
  },
  done
);

nal.map(
  {
    iter1: 1,
    iter2: 2,
  },
  iterator,
  {
    create: create,
    before: before,
    after: after,
    error: error,
  },
  done
);

API

`map(values, iterator[, options][, callback])`

Takes an object or array of values and an iterator function to execute with each value. Optionally, takes an options object and a callback function that is called upon completion of the iterations.

All iterations run in parallel.

`values`

An array or object of values to iterate over.

If values is an array, iterations are started in order by index. If values is an object, iterations are started in order by the order returned by Object.keys (order is not guaranteed).

If values is an array, the results of each iteration will be mapped to an array. If values is an object, the results of each iteration will be mapped to an object with corresponding keys.

`iterator(value, key, done)`

An async function called per iteration. All iterations are run in parallel.

The iterator function is called once with each value, key and a function (done(error, result)) to call when the async work is complete.

If done is passed an error as the first argument, the iteration will fail and the sequence will be ended; however, any iterations in progress will still complete. If done is passed a result value as the second argument, it will be added to the final results array or object.

`options`

The options object is primarily used for specifying functions that give insight into the lifecycle of each iteration. The possible extension points are create, before, after and error. If an extension point is not specified, it defaults to a no-op function.

The options object for map also allows specifying concurrency in which to run your iterations. By default, your iterations will run at maximum concurrency.

`options.concurrency`

Limits the amount of iterations allowed to run at a given time.

`options.create(value, key)`

Called at the very beginning of each iteration with the value being iterated and the key from the array or object. If create returns a value (storage), it is passed to the before, after and error extension points.

If a value is not returned, an empty object is used as storage for each other extension point.

This is useful for tracking information across an iteration.

`options.before(storage)`

Called immediately before each iteration with the storage value returned from the create extension point.

`options.after(result, storage)`

Called immediately after each iteration with the result of the iteration and the storage value returned from the create extension point.

`options.error(error, storage)`

Called immediately after a failed iteration with the error of the iteration and the storage value returned from the create extension point.

`callback(error, results)`

A function that is called after all iterations have completed or one iteration has errored.

If all iterations completed successfully, the error argument will be empty and the results will be a mapping of the iterator results.

If an iteration errored, the error argument will be passed from that iteration and the results will be whatever partial results had completed successfully before the error occurred.

`mapSeries(values, iterator[, options][, callback])`