Package Exports

@stdlib/utils-reduce

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Readme

reduce

Apply a function against an accumulator and each element in a collection and return the accumulated result.

Installation

npm install @stdlib/utils-reduce

Usage

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.

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.

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.

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, Typed Array, or an array-like Object (excluding strings and functions).
The function differs from Array.prototype.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.
```
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

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 );

Notice

This package is part of stdlib, a standard library for JavaScript and Node.js, with an emphasis on numerical and scientific computing. The library provides a collection of robust, high performance libraries for mathematics, statistics, streams, utilities, and more.

For more information on the project, filing bug reports and feature requests, and guidance on how to develop stdlib, see the main project repository.

License

See LICENSE.

@stdlib/utils-reduce