src/reduce.js

import _reduce from './_reduce.js';
import {iter} from '@iterable-iterator/iter';

/**
 * Applies the accumulator function iteratively on the last return value of the
 * accumulator and the next value in the input iterable. The initial value is
 * the initializer parameter. If no initial value is given, the first element
 * of the input iterable is used.
 *
 * @example
 * _reduce( ( x , y ) => x + y , range( 10 ) , 0 ) ; // returns 45
 *
 * @example
 * _reduce( ( x , y ) => x + y , range( 10 ) , 100 ) ; // returns 145
 *
 * @param {Function} accumulator - The accumulator, a 2-ary function.
 * @param {Iterable} iterable - The input iterable.
 * @param {Object} [initializer=undefined] - The initial value of the reduction.
 * @returns {Object} - The reduction of the elements of <code>iterable</code>.
 */
export default function reduce(accumulator, iterable, initializer = undefined) {
	if (initializer === undefined) {
		const iterator = iter(iterable);
		const first = iterator.next();

		if (first.done) {
			return undefined;
		}

		return _reduce(accumulator, iterator, first.value);
	}

	return _reduce(accumulator, iterable, initializer);
}