https://github.com/stdlib-js/utils-async-if-then
If a predicate function returns a truthy value, invoke `x`; otherwise, invoke `y`.
https://github.com/stdlib-js/utils-async-if-then
async asynchronous control defer deferred delay delayed else if if-else if-then javascript node node-js nodejs stdlib util utilities utility utils
Last synced: 6 days ago
JSON representation
If a predicate function returns a truthy value, invoke `x`; otherwise, invoke `y`.
- Host: GitHub
- URL: https://github.com/stdlib-js/utils-async-if-then
- Owner: stdlib-js
- License: apache-2.0
- Created: 2021-06-13T19:27:25.000Z (about 5 years ago)
- Default Branch: main
- Last Pushed: 2026-07-14T03:58:56.000Z (13 days ago)
- Last Synced: 2026-07-14T05:24:50.062Z (13 days ago)
- Topics: async, asynchronous, control, defer, deferred, delay, delayed, else, if, if-else, if-then, javascript, node, node-js, nodejs, stdlib, util, utilities, utility, utils
- Language: JavaScript
- Homepage: https://github.com/stdlib-js/stdlib
- Size: 1.26 MB
- Stars: 2
- Watchers: 2
- Forks: 0
- Open Issues: 0
-
Metadata Files:
- Readme: README.md
- Changelog: CHANGELOG.md
- Contributing: CONTRIBUTING.md
- License: LICENSE
- Code of conduct: CODE_OF_CONDUCT.md
- Citation: CITATION.cff
- Security: SECURITY.md
- Notice: NOTICE
Awesome Lists containing this project
README
About stdlib...
We believe in a future in which the web is a preferred environment for numerical computation. To help realize this future, we've built stdlib. stdlib is a standard library, with an emphasis on numerical and scientific computation, written in JavaScript (and C) for execution in browsers and in Node.js.
The library is fully decomposable, being architected in such a way that you can swap out and mix and match APIs and functionality to cater to your exact preferences and use cases.
When you use stdlib, you can be absolutely certain that you are using the most thorough, rigorous, well-written, studied, documented, tested, measured, and high-quality code out there.
To join us in bringing numerical computing to the web, get started by checking us out on GitHub, and please consider financially supporting stdlib. We greatly appreciate your continued support!
# ifthenAsync
[![NPM version][npm-image]][npm-url] [![Build Status][test-image]][test-url] [![Coverage Status][coverage-image]][coverage-url]
> If a predicate function returns a truthy value, invoke `x`; otherwise, invoke `y`.
## Installation
```bash
npm install @stdlib/utils-async-if-then
```
Alternatively,
- To load the package in a website via a `script` tag without installation and bundlers, use the [ES Module][es-module] available on the [`esm`][esm-url] branch (see [README][esm-readme]).
- If you are using Deno, visit the [`deno`][deno-url] branch (see [README][deno-readme] for usage intructions).
- For use in Observable, or in browser/node environments, use the [Universal Module Definition (UMD)][umd] build available on the [`umd`][umd-url] branch (see [README][umd-readme]).
The [branches.md][branches-url] file summarizes the available branches and displays a diagram illustrating their relationships.
To view installation and usage instructions specific to each branch build, be sure to explicitly navigate to the respective README files on each branch, as linked to above.
## Usage
```javascript
var ifthenAsync = require( '@stdlib/utils-async-if-then' );
```
#### ifthenAsync( predicate, x, y, done )
If a `predicate` function returns a truthy value, invokes `x`; otherwise, invokes `y`.
```javascript
var randu = require( '@stdlib/random-base-randu' );
function predicate( clbk ) {
setTimeout( onTimeout, 0 );
function onTimeout() {
clbk( null, randu() > 0.5 );
}
}
function x( clbk ) {
setTimeout( onTimeout, 0 );
function onTimeout() {
clbk( null, 1.0 );
}
}
function y( clbk ) {
setTimeout( onTimeout, 0 );
function onTimeout() {
clbk( null, -1.0 );
}
}
function done( error, result ) {
if ( error ) {
throw error;
}
console.log( result );
}
ifthenAsync( predicate, x, y, done );
```
The `predicate` function is provided a single argument:
- `clbk`: callback to invoke upon `predicate` function completion.
The callback accepts two arguments:
- `error`: error object.
- `bool`: condition used to determine whether to invoke `x` or `y`.
Both `x` and `y` are provided a single argument:
- `clbk`: callback to invoke upon function completion.
The callback function accepts any number of arguments, with the first argument reserved for providing an error. If the error argument is falsy, the `done` callback is invoked with its first argument as `null` and all other provided arguments. If the error argument is truthy, the `done` callback is invoked with only an error argument.
```javascript
var randu = require( '@stdlib/random-base-randu' );
function predicate( clbk ) {
setTimeout( onTimeout, 0 );
function onTimeout() {
clbk( null, randu() > 0.5 );
}
}
function x( clbk ) {
setTimeout( onTimeout, 0 );
function onTimeout() {
clbk( null, 1.0, 2.0, 3.0 );
}
}
function y( clbk ) {
setTimeout( onTimeout, 0 );
function onTimeout() {
clbk( null, 4.0, 5.0, 6.0 );
}
}
function done( error, a, b, c ) {
if ( error ) {
throw error;
}
console.log( a, b, c );
}
ifthenAsync( predicate, x, y, done );
```
## Notes
- The function is similar to [`ifelseAsync()`][@stdlib/utils/async/if-else], but allows deferred argument evaluation.
- Execution is **not** guaranteed to be asynchronous. To guarantee asynchrony, wrap the `done` callback in a function which either executes at the end of the current stack (e.g., `nextTick`) or during a subsequent turn of the event loop (e.g., `setImmediate`, `setTimeout`).
## Examples
```javascript
var randu = require( '@stdlib/random-base-randu' );
var ceil = require( '@stdlib/math-base-special-ceil' );
var repeatString = require( '@stdlib/string-repeat' );
var ifthenAsync = require( '@stdlib/utils-async-if-then' );
var i;
function next() {
ifthenAsync( predicate, x, y, done );
}
function predicate( clbk ) {
setTimeout( onTimeout, 0 );
function onTimeout() {
clbk( null, randu() > 0.9 );
}
}
function x( clbk ) {
setTimeout( onTimeout, 0 );
function onTimeout() {
clbk( null, repeatString( 'BOOP', ceil( randu()*3.0 ) ) );
}
}
function y( clbk ) {
setTimeout( onTimeout, 0 );
function onTimeout() {
clbk( null, repeatString( 'beep', ceil( randu()*5.0 ) ) );
}
}
function done( error, result ) {
if ( error ) {
throw error;
}
i += 1;
console.log( result );
if ( i < 100 ) {
return next();
}
}
i = 0;
next();
```
* * *
## See Also
- [`@stdlib/utils-async/if-else`][@stdlib/utils/async/if-else]: if a predicate function returns a truthy value, return `x`; otherwise, return `y`.
- [`@stdlib/utils-if-then`][@stdlib/utils/if-then]: if a condition is truthy, invoke `x`; otherwise, invoke `y`.
* * *
## Notice
This package is part of [stdlib][stdlib], a standard library for JavaScript and Node.js, with an emphasis on numerical and scientific computing. The library provides a collection of robust, high performance libraries for mathematics, statistics, streams, utilities, and more.
For more information on the project, filing bug reports and feature requests, and guidance on how to develop [stdlib][stdlib], see the main project [repository][stdlib].
#### Community
[![Chat][chat-image]][chat-url]
---
## License
See [LICENSE][stdlib-license].
## Copyright
Copyright © 2016-2026. The Stdlib [Authors][stdlib-authors].
[npm-image]: http://img.shields.io/npm/v/@stdlib/utils-async-if-then.svg
[npm-url]: https://npmjs.org/package/@stdlib/utils-async-if-then
[test-image]: https://github.com/stdlib-js/utils-async-if-then/actions/workflows/test.yml/badge.svg?branch=main
[test-url]: https://github.com/stdlib-js/utils-async-if-then/actions/workflows/test.yml?query=branch:main
[coverage-image]: https://img.shields.io/codecov/c/github/stdlib-js/utils-async-if-then/main.svg
[coverage-url]: https://codecov.io/github/stdlib-js/utils-async-if-then?branch=main
[chat-image]: https://img.shields.io/badge/zulip-join_chat-brightgreen.svg
[chat-url]: https://stdlib.zulipchat.com
[stdlib]: https://github.com/stdlib-js/stdlib
[stdlib-authors]: https://github.com/stdlib-js/stdlib/graphs/contributors
[umd]: https://github.com/umdjs/umd
[es-module]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Modules
[deno-url]: https://github.com/stdlib-js/utils-async-if-then/tree/deno
[deno-readme]: https://github.com/stdlib-js/utils-async-if-then/blob/deno/README.md
[umd-url]: https://github.com/stdlib-js/utils-async-if-then/tree/umd
[umd-readme]: https://github.com/stdlib-js/utils-async-if-then/blob/umd/README.md
[esm-url]: https://github.com/stdlib-js/utils-async-if-then/tree/esm
[esm-readme]: https://github.com/stdlib-js/utils-async-if-then/blob/esm/README.md
[branches-url]: https://github.com/stdlib-js/utils-async-if-then/blob/main/branches.md
[stdlib-license]: https://raw.githubusercontent.com/stdlib-js/utils-async-if-then/main/LICENSE
[@stdlib/utils/async/if-else]: https://github.com/stdlib-js/utils-async-if-else
[@stdlib/utils/if-then]: https://github.com/stdlib-js/utils-if-then