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
• 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).