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The 'when' npm package is a robust library for working with asynchronous programming in JavaScript, particularly using promises. It provides utilities for creating, managing, and composing promises, making it easier to handle asynchronous operations and their potential complexities.
Creating Promises
This feature allows the creation of new promises. The code sample demonstrates how to create a simple promise that resolves with 'Hello, World!' after 1 second.
const when = require('when');
const promise = when.promise(function(resolve, reject) {
setTimeout(() => resolve('Hello, World!'), 1000);
});
promise.then(response => console.log(response));
Chaining Promises
This feature demonstrates chaining multiple promises. It shows how to perform a series of tasks sequentially, where each task starts only after the previous one has completed.
const when = require('when');
const cleanRoom = () => when.promise(resolve => resolve('Room cleaned'));
const removeTrash = () => when.promise(resolve => resolve('Trash removed'));
const winIcecream = () => when.promise(resolve => resolve('Won ice cream'));
cleanRoom()
.then(result => {
console.log(result);
return removeTrash();
})
.then(result => {
console.log(result);
return winIcecream();
})
.then(result => console.log(result));
Handling Errors
This feature involves error handling in promises. The code sample shows how to catch and handle errors that occur during the execution of promises.
const when = require('when');
const failTask = () => when.promise((resolve, reject) => reject('Failed task'));
failTask()
.then(result => console.log('Success:', result))
.catch(error => console.log('Error:', error));
Bluebird is a full-featured promise library with a focus on innovative features and performance. It is similar to 'when' but often cited for its superior performance and additional features like cancellation, progress tracking, and more detailed stack traces.
Q is one of the earliest promise libraries that influenced many others. It offers a similar API to 'when' but is generally considered to be less performant in modern applications. It provides a straightforward approach to handling asynchronous operations with promises.
The 'promise' package provides a minimalist implementation similar to the ES6 Promise specification. It is more lightweight compared to 'when' but lacks some of the more advanced features and utilities provided by 'when'.
When.js is cujojs's lightweight Promises/A+ and when()
implementation, derived from the async core of wire.js, cujojs's IOC Container. It features:
It passes the Promises/A+ Test Suite, is very fast, is under 1.5k when compiled with Google Closure + gzip, and has no external dependencies.
when/keys
module with all()
and map()
for object keys/values.promise.ensure
as a better, and safer, replacement for promise.always
. See discussion as to why promise.always
is mistake-prone.
promise.always
lift()
is now the preferred name for what was bind()
in when/function, when/node/function, and when/callbacks.
bind()
in when/function
, when/node/function
, and when/callbacks
. Use lift()
instead.Get it
bower install when
or yeoman install when
, orgit clone https://github.com/cujojs/when
or git submodule add https://github.com/cujojs/when
Configure your loader with a package:
packages: [
{ name: 'when', location: 'path/to/when/', main: 'when' },
// ... other packages ...
]
define(['when', ...], function(when, ...) { ... });
or require(['when', ...], function(when, ...) { ... });
npm install when
var when = require('when');
ringo-admin install cujojs/when
var when = require('when');
git clone https://github.com/cujojs/when
or git submodule add https://github.com/cujojs/when
Add a transient define
shim, and a <script>
element for when.js
<script>
window.define = function(factory) {
try{ delete window.define; } catch(e){ window.define = void 0; } // IE
window.when = factory();
};
</script>
<script src="path/to/when/when.js"></script>
when
will be available as window.when
Note that when.js includes the Promises/A+ Test Suite. Running unit tests in Node will run both when.js's own test suite, and the Promises/A+ Test Suite.
npm install
npm test
npm install
npm start
- starts buster server & prints a urllocalhost:1111/capture
npm run-script test-browser
Much of this code was inspired by the async innards of wire.js, and has been influenced by the great work in Q, Dojo's Deferred, and uber.js.
2.0.0
when/keys
module with all()
and map()
for object keys/values.promise.ensure
as a better, and safer, replacement for promise.always
. See discussion as to why promise.always
is mistake-prone.
promise.always
lift()
is now the preferred name for what was bind()
in when/function, when/node/function, and when/callbacks.
bind()
in when/function
, when/node/function
, and when/callbacks
. Use lift()
instead.FAQs
A lightweight Promises/A+ and when() implementation, plus other async goodies.
The npm package when receives a total of 1,087,952 weekly downloads. As such, when popularity was classified as popular.
We found that when demonstrated a not healthy version release cadence and project activity because the last version was released a year ago. It has 1 open source maintainer collaborating on the project.
Did you know?
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