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 );