# composeAsync
> [Function composition][function-composition].
## Usage
```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, (x*5) + (y*3) );
}
}
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 );
```
## Notes
- The function will throw if provided fewer than `2` input arguments.
- If a composed function calls the `next` callback with a truthy `error` argument, the function suspends execution and immediately calls the `done` callback for subsequent `error` handling.
- 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 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 );
```
[function-composition]: https://en.wikipedia.org/wiki/Function_composition_%28computer_science%29