# ifthenAsync
> If a predicate function returns a truthy value, invoke `x`; otherwise, invoke `y`.
## 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();
```
[@stdlib/utils/async/if-else]: https://www.npmjs.com/package/@stdlib/utils/tree/main/async/if-else