reducePolyfill.js

function reduce(array, fn, initial) {
    if (!(array instanceof Array)) {
        throw new TypeError('Illegal first argument, (not an array)');
    } else if (!(fn instanceof Function)) {
        throw new TypeError('Illegal second argument, (not a function)');
    }

    let startIndex = 1, 
        current = array[0];

    if (initial) {
        startIndex = 0;
        current = initial;
    }

    for (let i = startIndex; i < array.length; i++) {
        current = fn(current, array[i], i, array);
    }

    return current;
}