Function composition.
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> [Function composition][function-composition].
``bash`
npm install @stdlib/utils-async-compose
`javascript`
var composeAsync = require( '@stdlib/utils-async-compose' );
#### composeAsync( ...fcn )
Returns a composite function. Starting from the right, the composite function evaluates each function and passes the result as the first argument to the next function. The result of the leftmost function is the result of the whole.
`javascript
function a( x, next ) {
setTimeout( onTimeout, 0 );
function onTimeout() {
next( null, 2*x );
}
}
function b( x, next ) {
setTimeout( onTimeout, 0 );
function onTimeout() {
next( null, x+3 );
}
}
function c( x, next ) {
setTimeout( onTimeout, 0 );
function onTimeout() {
next( null, x/5 );
}
}
var f = composeAsync( c, b, a );
function done( error, result ) {
if ( error ) {
throw error;
}
console.log( result );
// => 3
}
f( 6, done ); // ((2*x)+3)/5
`
The last argument provided to each composed function is a next callback which accepts two arguments:
- error: error argument.
- result: function result.
Only the rightmost function is explicitly permitted to accept multiple arguments. All other functions are evaluated as binary functions.
`javascript
function a( x, y, next ) {
setTimeout( onTimeout, 0 );
function onTimeout() {
next( null, (x5) + (y3) );
}
}
function b( r, next ) {
setTimeout( onTimeout, 0 );
function onTimeout() {
next( null, r+12 );
}
}
var f = composeAsync( b, a );
function done( error, result ) {
if ( error ) {
throw error;
}
console.log( result );
// => 50
}
f( 4, 6, done );
`
- The function will throw if provided fewer than 2 input arguments.next
- If a composed function calls the callback with a truthy error argument, the function suspends execution and immediately calls the done callback for subsequent error handling.done
- Execution is not guaranteed to be asynchronous. To guarantee asynchrony, wrap the 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).
`javascript
var composeAsync = require( '@stdlib/utils-async-compose' );
function a( x, y, next ) {
setTimeout( onTimeout, 0 );
function onTimeout() {
next( null, x*y );
}
}
function b( z, next ) {
setTimeout( onTimeout, 0 );
function onTimeout() {
next( null, z+5 );
}
}
function c( r, next ) {
setTimeout( onTimeout, 0 );
function onTimeout() {
next( null, r/10 );
}
}
var f = composeAsync( c, b, a );
function done( error, result ) {
if ( error ) {
throw error;
}
console.log( result );
// => 2
}
f( 5, 3, done );
`
*
- [@stdlib/utils-compose][@stdlib/utils/compose]: function composition.
*
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].
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---
See [LICENSE][stdlib-license].
Copyright © 2016-2026. The Stdlib [Authors][stdlib-authors].
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[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-compose/tree/deno
[deno-readme]: https://github.com/stdlib-js/utils-async-compose/blob/deno/README.md
[umd-url]: https://github.com/stdlib-js/utils-async-compose/tree/umd
[umd-readme]: https://github.com/stdlib-js/utils-async-compose/blob/umd/README.md
[esm-url]: https://github.com/stdlib-js/utils-async-compose/tree/esm
[esm-readme]: https://github.com/stdlib-js/utils-async-compose/blob/esm/README.md
[branches-url]: https://github.com/stdlib-js/utils-async-compose/blob/main/branches.md
[stdlib-license]: https://raw.githubusercontent.com/stdlib-js/utils-async-compose/main/LICENSE
[function-composition]: https://en.wikipedia.org/wiki/Function_composition_%28computer_science%29
[@stdlib/utils/compose]: https://www.npmjs.com/package/@stdlib/utils-compose