@stdlib/utils/async/parallel/README.md

Standard utilities.

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# parallel

> Execute functions in parallel and pass the results of all functions to a provided callback.

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## Usage

```javascript
var parallel = require( '@stdlib/utils/async/parallel' );
```

#### parallel( fcns, \[options,] done )

Executes a set of functions in parallel and passes the results of all functions to a provided callback.

```javascript
function a( clbk ) {
    setTimeout( onTimeout, 0 );
    function onTimeout() {
        clbk( null, 2 );
    }
}

function b( clbk ) {
    setTimeout( onTimeout, 0 );
    function onTimeout() {
        clbk( null, 4 );
    }
}

function done( error, out ) {
    if ( error ) {
        throw error;
    }
    console.log( out );
    // => [ 2, 4 ]
}

var fcns = [ a, b ];

parallel( fcns, done );
```

The function accepts the following `options`:

-   **limit**: maximum number of functions to execute concurrently. Default: `infinity`.
-   **thisArg**: execution context for each function.

To limit the maximum number of functions executing in parallel, set the `limit` option.

```javascript
function a( clbk ) {
    setTimeout( onTimeout, 0 );
    function onTimeout() {
        clbk( null, 2 );
    }
}

function b( clbk ) {
    setTimeout( onTimeout, 0 );
    function onTimeout() {
        clbk( null, 4 );
    }
}

function done( error, out ) {
    if ( error ) {
        throw error;
    }
    console.log( out );
    // => [ 2, 4 ]
}

var fcns = [ a, b ];

var opts = {
    'limit': 1
};

parallel( fcns, opts, done );
```

To set the `this` context for **all** `functions` in the provided function array, set the `thisArg` option.

```javascript
function a( clbk ) {
    this.idx += 1;
    clbk( null, 2 );
}

function b( clbk ) {
    this.idx += 1;
    clbk( null, 4 );
}

var fcns = [ a, b ];
var ctx = {
    'idx': 0
};
var opts = {
    'thisArg': ctx
};

parallel( fcns, opts, done );

function done( error, out ) {
    if ( error ) {
        throw error;
    }
    console.log( ctx.idx );
    // => 2
}
```

#### parallel.factory( fcns, \[options] )

Returns a reusable function which executes a set of functions in parallel.

```javascript
function a( clbk ) {
    setTimeout( onTimeout, 0 );
    function onTimeout() {
        clbk( null, 2 );
    }
}

function b( clbk ) {
    setTimeout( onTimeout, 0 );
    function onTimeout() {
        clbk( null, 4 );
    }
}

function done( error, out ) {
    if ( error ) {
        throw error;
    }
    console.log( out );
    // => [ 2, 4 ]
}

var fcns = [ a, b ];

var run = parallel.factory( fcns );

run( done );
run( done );
run( done );
```

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## Notes

-   The order of the results provided to the `done` callback corresponds to the order of the provided functions.
-   When executed, each provided function is invoked with a single callback argument. The callback should be invoked upon function completion. The first argument is reserved as an `error` argument (which can be `null`). The second argument is reserved for any results which should be passed to the `done` callback upon completion of all provided functions.
-   If any function fails to invoke the callback argument, the `done` callback will never be invoked.
-   This implementation is intended to start asynchronous tasks so that execution of each task runs concurrently. If provided a function which does not perform asynchronous tasks, the function will execute synchronously. Hence, this implementation does **not** guarantee that execution is asynchronous. To do so, 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 function executes provided functions in the same thread. Accordingly, the function does **not** spawn new threads.

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## Examples

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```javascript
var parallel = require( '@stdlib/utils/async/parallel' );

function foo( clbk ) {
    setTimeout( onTimeout, 300 );
    function onTimeout() {
        clbk( null, 'one' );
    }
}

function bar( clbk ) {
    setTimeout( onTimeout, 100 );
    function onTimeout() {
        clbk( null, 'two' );
    }
}

function done( error, results ) {
    if ( error ) {
        throw error;
    }
    console.log( results );
    // => [ 'one', 'two' ]
}

var fcns = [ foo, bar ];

parallel( fcns, done );
```

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