# reduce
> Apply a function against an accumulator and each element in a collection and return the accumulated result.
## Usage
```javascript
var reduce = require( '@stdlib/utils/reduce' );
```
#### reduce( collection, initial, reducer\[, thisArg ] )
Applies a `function` against an accumulator and each element in a `collection` and returns the accumulated result.
```javascript
function sum( accumulator, value ) {
return accumulator + value;
}
var arr = [ 1, 2, 3, 4 ];
var out = reduce( arr, 0, sum );
// returns 10
```
The `reducer` function is provided four arguments:
- `accumulator`: accumulated value
- `value`: collection element
- `index`: collection index
- `collection`: input collection
Basic support for dynamic collections is provided. Note, however, that index incrementation is monotonically increasing.
```javascript
function sum1( accumulator, value, index, collection ) {
if ( index === collection.length-1 && collection.length < 10 ) {
collection.push( index+2 );
}
return accumulator + value;
}
var arr = [ 1, 2, 3, 4 ];
var out = reduce( arr, 0, sum1 );
// returns 55
function sum2( accumulator, value, index, collection ) {
collection.shift();
return accumulator + value;
}
arr = [ 1, 2, 3, 4 ];
out = reduce( arr, 0, sum2 );
// returns 4
```
To set the function execution context, provide a `thisArg`.
```javascript
function sum( accumulator, value ) {
this.count += 1;
return accumulator + value;
}
var arr = [ 1, 2, 3, 4 ];
var context = {
'count': 0
};
var out = reduce( arr, 0, sum, context );
// returns 10
var mean = out / context.count;
// returns 2.5
```
## 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`).
- The function differs from [`Array.prototype.reduce`][mdn-array-reduce] in the following ways:
- The function **requires** an `initial` value for the `accumulator`. The `initial` value is used during the first invocation of the `reducer` function.
- The function does **not** skip the first element in the `collection`.
- The function does **not** skip `undefined` elements.
```javascript
function log( accumulator, value, index ) {
console.log( '%s: %s', index, value );
return accumulator;
}
var arr = [ 1, , , 4 ];
var out = reduce( arr, 0, log );
/* =>
0: 1
1: undefined
2: undefined
3: 4
*/
```
- The function provides limited support for dynamic collections (i.e., collections whose `length` changes during execution).
## Examples
```javascript
var reduce = require( '@stdlib/utils/reduce' );
var arr;
var s;
var i;
function sum( acc, value ) {
return acc + value;
}
arr = new Array( 1000 );
for ( i = 0; i < arr.length; i++ ) {
arr[ i ] = i;
}
s = reduce( arr, 0, sum );
console.log( s );
```
[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
[mdn-array-reduce]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/reduce