reduce

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

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 );