# funseqAsync
> Function sequence.
## Usage
```javascript
var funseqAsync = require( '@stdlib/utils/async/function-sequence' );
```
#### funseqAsync( ...fcn )
Returns a pipeline function. Starting from the left, the pipeline function evaluates each function and passes the result as the first argument to the next function. The result of the rightmost 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 = funseqAsync( a, b, c );
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 function is a `next` callback which accepts two arguments:
- `error`: error argument
- `result`: function result
**Only** the leftmost 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 = funseqAsync( a, b );
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 provided function calls the `next` callback with a truthy `error` argument, the pipeline 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`).
- The difference between this function and [`composeAsync`][@stdlib/utils/async/compose] is that this function evaluates input arguments from left-to-right, rather than right-to-left.
## Examples
```javascript
var funseqAsync = require( '@stdlib/utils/async/function-sequence' );
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 = funseqAsync( a, b, c );
function done( error, result ) {
if ( error ) {
throw error;
}
console.log( result );
// => 2
}
f( 5, 3, done );
```
[@stdlib/utils/async/compose]: https://www.npmjs.com/package/@stdlib/utils/tree/main/async/compose