# trythenAsync
> If a function does not return an error, invoke a callback with the function result; otherwise, invoke a second function.
## Usage
```javascript
var trythenAsync = require( '@stdlib/utils/async/try-then' );
```
#### trythenAsync( x, y, done )
If a function `x` does not return an error, invokes a `done` callback with the function result; otherwise, invokes a second function `y`.
```javascript
var randu = require( '@stdlib/random/base/randu' );
function x( clbk ) {
setTimeout( onTimeout, 0 );
function onTimeout() {
if ( randu() > 0.5 ) {
return clbk( null, 1.0 );
}
clbk( new Error( 'oops' ) );
}
}
function y( clbk ) {
setTimeout( onTimeout, 0 );
function onTimeout() {
clbk( null, -1.0 );
}
}
function done( error, result ) {
if ( error ) {
throw error;
}
console.log( result );
}
trythenAsync( x, y, done );
```
The function `x` is 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.
```javascript
var randu = require( '@stdlib/random/base/randu' );
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 );
}
trythenAsync( x, y, done );
```
If the error argument is truthy, the function invokes `y`. The number of arguments provided to `y` depends on the function's `length`. If `y` is a unary function, `y` is provided a single argument:
- `clbk`: callback to invoke upon function completion
Otherwise, `y` is provided two arguments:
- `error`: the error from `x`
- `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 equal to `null` and all other provided arguments. If the error argument is truthy, the `done` callback is invoked with only the error argument provided by `y`.
```javascript
var randu = require( '@stdlib/random/base/randu' );
function x( clbk ) {
setTimeout( onTimeout, 0 );
function onTimeout() {
if ( randu() > 0.5 ) {
return clbk( null, 1.0, 2.0, 3.0 );
}
clbk( new Error( 'beep' ) );
}
}
function y( clbk ) {
setTimeout( onTimeout, 0 );
function onTimeout() {
if ( randu() > 0.5 ) {
return clbk( null, 4.0, 5.0, 6.0 );
}
clbk( new Error( 'boop' ) );
}
}
function done( error, a, b, c ) {
if ( error ) {
console.error( error.message );
}
console.log( a, b, c );
}
trythenAsync( x, y, done );
```
## Notes
- 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 trythenAsync = require( '@stdlib/utils/async/try-then' );
var i;
function next() {
trythenAsync( x, y, done );
}
function x( clbk ) {
setTimeout( onTimeout, 0 );
function onTimeout() {
if ( randu() > 0.9 ) {
return clbk( null, repeatString( 'BOOP', ceil( randu()*3.0 ) ) );
}
clbk( new Error( 'oops' ) );
}
}
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();
```