Usage

`javascript
var forEachAsync = require( '@stdlib/utils-async-for-each' );
`

#### forEachAsync( collection, \[options,] fcn, done )

Invokes a function once for each element in a collection.

`javascript
function onDuration( value, next ) {
setTimeout( onTimeout, value );
function onTimeout() {
console.log( value );
next();
}
}

function done( error ) {
if ( error ) {
throw error;
}
}

var arr = [ 3000, 2500, 1000 ];

forEachAsync( arr, onDuration, done );
/* =>
1000
2500
3000
*/
`

The function accepts the following options:

- limit: the maximum number of pending invocations at any one time. Default: infinity.
- series: boolean indicating whether to sequentially invoke fcn for each collection element. If true, the function sets options.limit=1. Default: false.
- thisArg: the execution context for fcn.

By default, all elements are processed concurrently, which means that the function does not guarantee completion order. To process each collection element sequentially, set the series option to true.

`javascript
function onDuration( value, next ) {
setTimeout( onTimeout, value );
function onTimeout() {
console.log( value );
next();
}
}

function done( error ) {
if ( error ) {
throw error;
}
}

var arr = [ 3000, 2500, 1000 ];

var opts = {
'series': true
};

forEachAsync( arr, opts, onDuration, done );
/* =>
3000
2500
1000
*/
`

To limit the maximum number of pending function invocations, set the limit option.

`javascript
function onDuration( value, next ) {
setTimeout( onTimeout, value );
function onTimeout() {
console.log( value );
next();
}
}

function done( error ) {
if ( error ) {
throw error;
}
}

var arr = [ 3000, 2500, 1000 ];

var opts = {
'limit': 2
};

forEachAsync( arr, opts, onDuration, done );
/* =>
2500
3000
1000
*/
`

To set the execution context of fcn, set the thisArg option.

`javascript
function onDuration( value, next ) {
this.count += 1;
setTimeout( onTimeout, value );
function onTimeout() {
next();
}
}

var arr = [ 3000, 2500, 1000 ];

var context = {
'count': 0
};

var opts = {
'thisArg': context
};

forEachAsync( arr, opts, onDuration, done );

function done( error ) {
if ( error ) {
throw error;
}
console.log( context.count );
// => 3
}
`

When invoked, fcn is provided a maximum of four arguments:

- value: collection value.
- index: collection index.
- collection: the input collection.
- next: a callback which should be called once fcn has finished processing a collection value.

The actual number of provided arguments depends on function length. If fcn accepts two arguments, fcn is provided value and next. If fcn accepts three arguments, fcn is provided value, index, and next. For every other fcn signature, fcn is provided all four arguments.

`javascript
function onDuration( value, i, collection, next ) {
console.log( 'collection: %s. %d: %d', collection.join( ',' ), i, value );
setTimeout( onTimeout, value );
function onTimeout() {
console.log( value );
next();
}
}

function done( error ) {
if ( error ) {
throw error;
}
}

var arr = [ 3000, 2500, 1000 ];

forEachAsync( arr, onDuration, done );
/* =>
collection: 3000,2500,1000. 0: 3000
collection: 3000,2500,1000. 1: 2500
collection: 3000,2500,1000. 2: 1000
1000
2500
3000
*/
`

#### forEachAsync.factory( \[options,] fcn )

Returns a function which invokes a function once for each element in a collection.

`javascript
function onDuration( value, next ) {
setTimeout( onTimeout, value );
function onTimeout() {
console.log( value );
next();
}
}

function done( error ) {
if ( error ) {
throw error;
}
}

var f = forEachAsync.factory( onDuration );

var arr1 = [ 3000, 2500, 1000 ];

f( arr1, done );
/* =>
1000
2500
3000
*/

var arr2 = [ 300, 250, 100 ];

f( arr2, done );
/* =>
100
250
300
*/
`

The function accepts the same options as forEachAsync().