JS utils

utils.js

const head = array => array[0];
const zip = (...arrays) => (
    head(arrays).map((value, index) => arrays.map(otherArray => otherArray[index]))
);
const map = (collection, mapFn) => (
    Object.keys(collection).map(key => mapFn(collection[key], key))
);
const reduce = (collection, reduceFn, seed) => (
    Object.keys(collection).reduce((acc, key) => {
        const value = collection[key];
        return reduceFn(acc, value, key);
    }, seed)
);
const mapKeys = (collection, mapFn) => (
    reduce(collection, (acc, value, key) => {
        acc[mapFn(value, key)] = value;
        return acc;
    }, {})
);
const contains = (array, itemToFind) => array.some(item => item === itemToFind);

const range = (start, end) => (
  Array.from({ length: end - start }, (x, i) => i + start)
);
// OR
const range = (start, end) => Array(end - start).fill().map((x, i) => i + start)

const flatten = (xs) => xs.reduce((acc, value) => acc.concat(value), [])