# reduceAsync
> Apply a function against an accumulator and each element in a collection and return the accumulated result.
## Usage
```javascript
var reduceAsync = require( '@stdlib/utils/async/reduce' );
```
#### reduceAsync( collection, initial, \[options,] reducer, done )
Applies a `function` against an accumulator and each element in a `collection` and returns the accumulated result.
```javascript
function reducer( acc, value, index, next ) {
setTimeout( onTimeout, value );
function onTimeout() {
console.log( value );
acc.sum += value;
next( null, acc );
}
}
function done( error, acc ) {
if ( error ) {
throw error;
}
console.log( acc.sum );
}
var arr = [ 3000, 2500, 1000 ];
var acc = {
'sum': 0
};
reduceAsync( arr, acc, reducer, done );
/*
3000
2500
1000
6500
*/
```
The `next` callback accepts two arguments: `error` and `accumulator`. The second argument to the `next` callback is passed as the first argument to the provided `reducer`.
```javascript
function reducer( acc, value, index, next ) {
setTimeout( onTimeout, value );
function onTimeout() {
next( null, acc );
}
}
function done( error, out ) {
if ( error ) {
throw error;
}
console.log( out === acc );
// => true
}
var arr = [ 3000, 2500, 1000 ];
var acc = {};
reduceAsync( arr, acc, reducer, done );
```
The function accepts the following `options`:
- `limit`: the maximum number of pending invocations at any one time. If provided, the function sets `options.series=false`. Default: `infinity`.
- `series`: `boolean` indicating whether to sequentially invoke `reducer` for each `collection` element. If `true`, the function sets `options.limit=1`. Default: `true`.
- `thisArg`: the execution context for `reducer`.
By default, all elements are processed **sequentially**, which means that the function **does** guarantee completion order. To process each `collection` element concurrently, set the `series` option to `false`.
```javascript
function reducer( acc, value, index, next ) {
setTimeout( onTimeout, value );
function onTimeout() {
console.log( value );
acc.sum += value;
next( null, acc );
}
}
function done( error, acc ) {
if ( error ) {
throw error;
}
console.log( acc.sum );
}
var arr = [ 3000, 2500, 1000 ];
var acc = {
'sum': 0
};
var opts = {
'series': false
};
reduceAsync( arr, acc, opts, reducer, done );
/* =>
1000
2500
3000
6500
*/
```
To limit the maximum number of pending function invocations, set the `limit` option.
```javascript
function reducer( acc, value, index, next ) {
setTimeout( onTimeout, value );
function onTimeout() {
console.log( value );
acc.sum += value;
next( null, acc );
}
}
function done( error, acc ) {
if ( error ) {
throw error;
}
console.log( acc.sum );
}
var arr = [ 3000, 2500, 1000 ];
var acc = {
'sum': 0
};
var opts = {
'limit': 2
};
reduceAsync( arr, acc, opts, reducer, done );
/* =>
2500
3000
1000
6500
*/
```
To set the execution context of `reducer`, set the `thisArg` option.
```javascript
function reducer( acc, value, index, next ) {
this.count += 1;
setTimeout( onTimeout, value );
function onTimeout() {
acc.sum += value;
next( null, acc );
}
}
var arr = [ 3000, 2500, 1000 ];
var acc = {
'sum': 0
};
var context = {
'count': 0
};
var opts = {
'thisArg': context
};
reduceAsync( arr, acc, opts, reducer, done );
function done( error, acc ) {
if ( error ) {
throw error;
}
console.log( acc.sum );
// => 6500
console.log( context.count );
// => 3
}
```
When invoked, `reducer` is provided a maximum of five arguments:
- `accumulator`: accumulated value.
- `value`: collection value.
- `index`: collection index.
- `collection`: the input `collection`.
- `next`: a callback which should be called once `reducer` has finished processing a collection `value`.
The actual number of provided arguments depends on function `length`. If `reducer` accepts three arguments, `reducer` is provided `accumulator`, `value` and `next`. If `reducer` accepts four arguments, `reducer` is provided `accumulator`, `value`, `index`, and `next`. For every other `reducer` signature, `reducer` is provided all five arguments.
```javascript
function reducer( acc, value, i, collection, next ) {
console.log( 'collection: %s. %d: %d', collection.join( ',' ), i, value );
setTimeout( onTimeout, value );
function onTimeout() {
console.log( value );
acc.sum += value;
next( null, acc );
}
}
function done( error, acc ) {
if ( error ) {
throw error;
}
console.log( acc.sum );
}
var arr = [ 3000, 2500, 1000 ];
var acc = {
'sum': 0
};
reduceAsync( arr, acc, reducer, done );
/* =>
collection: 3000,2500,1000. 0: 3000
collection: 3000,2500,1000. 1: 2500
collection: 3000,2500,1000. 2: 1000
3000
2500
1000
6500
*/
```
#### reduceAsync.factory( \[options,] reducer )
Returns a `function` which invokes a function once for each element in a `collection`.
```javascript
function reducer( acc, value, index, next ) {
setTimeout( onTimeout, value );
function onTimeout() {
console.log( value );
acc.sum += value;
next( null, acc );
}
}
function done( error, acc ) {
if ( error ) {
throw error;
}
console.log( acc.sum );
}
var f = reduceAsync.factory( reducer );
var arr1 = [ 3000, 2500, 1000 ];
var acc1 = {
'sum': 0
};
f( arr1, acc1, done );
/* =>
3000
2500
1000
6500
*/
var arr2 = [ 300, 250, 100 ];
var acc2 = {
'sum': 0
};
f( arr2, acc2, done );
/* =>
300
250
100
650
*/
```
The function accepts the same `options` as `reduceAsync()`.
## Notes
- A `collection` may be either an [`Array`][mdn-array], [`Typed Array`][mdn-typed-array], or an array-like [`Object`][mdn-object] (excluding `strings` and `functions`).
- If a provided function calls the `next` callback with a truthy `error` argument, the function suspends execution and immediately calls the `done` callback for subsequent `error` handling.
- The function invokes the `done` callback with the `accumulator` provided as the second argument. If provided an empty `collection`, the function invokes the `done` callback with the `initial` value as the second argument.
- The function does **not** support dynamic `collection` resizing.
- The function does **not** skip `undefined` elements.
- When processing `collection` elements concurrently, **beware** of race conditions when updating an accumulator. This is especially true when an accumulator is a primitive (e.g., a `number`). In general, prefer `object` accumulators, as objects are passed by reference, not by value.
- **Neither** `reduceAsync` nor the function returned by the `factory` method **guarantee** asynchronous execution. 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 resolve = require( 'path' ).resolve;
var readFile = require( '@stdlib/fs/read-file' );
var reduceAsync = require( '@stdlib/utils/async/reduce' );
var files = [
resolve( __dirname, 'package.json' ),
resolve( __dirname, 'README.md' )
];
function done( error, acc ) {
if ( error ) {
throw error;
}
console.log( acc.count );
}
function read( acc, file, next ) {
var opts = {
'encoding': 'utf8'
};
readFile( file, opts, onFile );
function onFile( error ) {
if ( error ) {
return next( null, acc );
}
acc.count += 1;
next( null, acc );
}
}
var acc = {
'count': 0
};
reduceAsync( files, acc, read, done );
```
[mdn-array]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array
[mdn-typed-array]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypedArray
[mdn-object]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object