iterable-iterator/reduce

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

/**
 * Returns the largest element of the input iterable according
 * to some comparison function.
 *
 * @example
 * max( ( a , b ) => a - b , range( 10 ) ) ; // returns 9
 *
 * @example
 * max( ( a , b ) => a - b , range( 0 ) ) ; // returns undefined
 *
 * @param {Function} compare - The comparison function to use. This function
 * must be 2-ary. It must return -1, 0, or 1 depending whether the first
 * parameter is, respectively, less than, equal to, or greater than the second
 * parameter.
 * @param {Iterable} iterable - The input iterable.
 * @param {Object} [dflt=undefined] - The default value to return in the case
 * that the input iterable is empty.
 * @returns {Object} The largest element of <code>iterable</code> according to
 * <code>compare</code>.
 */
export default function max(compare, iterable, dflt = undefined) {
    const iterator = iter(iterable);

    const first = iterator.next();

    if (first.done) {
        return dflt;
    }

    let largest = first.value;

    for (const candidate of iterator) {
        if (compare(candidate, largest) > 0) {
            largest = candidate;
        }
    }

    return largest;
}