now-and-later

What is now-and-later?

The now-and-later npm package is designed to execute functions in a specific order, allowing for both series and parallel execution with support for mapping and iterating over collections. It provides a way to manage and coordinate asynchronous tasks, making it easier to handle complex flows in Node.js applications.

What are now-and-later's main functionalities?

Map Series

Executes an iterator function on each item in an array, in series. Each iterator is called with the item, key, and a callback function. The results are collected and passed to the final callback.

const nal = require('now-and-later');
nal.mapSeries(['a', 'b', 'c'], function(value, key, cb) {
  setTimeout(function() {
    cb(null, value.toUpperCase());
  }, 1000);
}, function(err, results) {
  console.log(results); // ['A', 'B', 'C']
});

Map

Similar to mapSeries, but executes the iterator functions in parallel. The final callback is called once all iterators have completed.

const nal = require('now-and-later');
nal.map(['a', 'b', 'c'], function(value, key, cb) {
  setTimeout(function() {
    cb(null, value.toUpperCase());
  }, 1000);
}, function(err, results) {
  console.log(results); // ['A', 'B', 'C']
});

Map Values

Executes an iterator function on each property of an object, in parallel. The iterator is called with the value, key, and a callback function. The results are collected into an object and passed to the final callback.

const nal = require('now-and-later');
const obj = { a: 1, b: 2, c: 3 };
nal.mapValues(obj, function(value, key, cb) {
  setTimeout(function() {
    cb(null, value * 2);
  }, 1000);
}, function(err, results) {
  console.log(results); // { a: 2, b: 4, c: 6 }
});

Map Values Series

Works like mapValues but executes the iterator functions in series.

const nal = require('now-and-later');
const obj = { a: 1, b: 2, c: 3 };
nal.mapValuesSeries(obj, function(value, key, cb) {
  setTimeout(function() {
    cb(null, value * 2);
  }, 1000);
}, function(err, results) {
  console.log(results); // { a: 2, b: 4, c: 6 }
});

Other packages similar to now-and-later

async

The async package is a comprehensive collection of asynchronous control-flow utilities. It provides more than 70 functions including various forms of parallel, series, and race, as well as other utility functions. It is more widely used and has a larger community compared to now-and-later.

bluebird

Bluebird is a fully-featured promise library with a focus on innovative features and performance. It allows for conversion of callback-based functions to promises, which can then be used to manage asynchronous control flow. Bluebird offers more features related to promises compared to now-and-later.

q

Q is a promise library for JavaScript that provides a robust way of working with asynchronous operations. It allows you to create and manage promises, which can be used to organize asynchronous code more effectively. Q is more focused on promises than now-and-later, which uses callbacks.

README

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])

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 serial.

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 serial.

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 without executing any more iterations. 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.

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.

License

MIT

Changelog

3.0.0 (2022-06-25)

⚠ BREAKING CHANGES

• Rename extensions to options and allow other configuration passed
• Normalize repository, dropping node <10.13 support (#18)

Features

• Allow max concurrency for parallel map with option (#15) (92ae356)

Miscellaneous Chores

• Normalize repository, dropping node <10.13 support (#18) (7dc89e9)
• Rename extensions to options and allow other configuration passed (92ae356)