simple reduce and map (from top of my head)

map-reduce.js

const reduce = (array, callback, initialValue) => {
    let result;
    
    for(let i = 0; i < array.length; i++) {
        result = callback(result || initialValue, array[i], i);
    }

    return result;
};

const map = (array, callback) => {
    return reduce(array, (res, item, index) => {
        res.push(callback(item, index));
        return res;
    }, []);
}