286 Favorite JavaScript Utilities

const castArray = value => Array.isArray(value) ? value : [value];

// Examples
castArray(1);               // [1]
castArray([1, 2, 3]);       // [1, 2, 3]

// `arr` is an array
const isEmpty = arr => !Array.isArray(arr) || arr.length === 0;

// Examples
isEmpty([]);            // true
isEmpty([1, 2, 3]);     // false

// `arr` is an array
const clone = arr => arr.slice(0);

// Or
const clone = arr => [...arr];

// Or
const clone = arr => Array.from(arr);

// Or
const clone = arr => arr.map(x => x);

// Or
const clone = arr => JSON.parse(JSON.stringify(arr));

// Or
const clone = arr => arr.concat([]);

// `a` and `b` are arrays
const isEqual = (a, b) => JSON.stringify(a) === JSON.stringify(b);

// Or
const isEqual = (a, b) => a.length === b.length && a.every((v, i) => v === b[i]);

// Examples
isEqual([1, 2, 3], [1, 2, 3]);      // true
isEqual([1, 2, 3], [1, '2', 3]);    // false

// `a` and `b` are arrays
const isEqual = (a, b) => JSON.stringify(a.sort()) === JSON.stringify(b.sort());

// Examples
isEqual([1, 2, 3], [1, 2, 3]);      // true
isEqual([1, 2, 3], [1, 3, 2]);      // true
isEqual([1, 2, 3], [1, '2', 3]);    // false

const toObject = (arr, key) => arr.reduce((a, b) => ({ ...a, [b[key]]: b }), {});

// Example
toObject(
    [
        { id: '1', name: 'Alpha', gender: 'Male' },
        { id: '2', name: 'Bravo', gender: 'Male' },
        { id: '3', name: 'Charlie', gender: 'Female' },
    ],
    'id'
);
/* 
{
    '1': { id: '1', name: 'Alpha', gender: 'Male' },
    '2': { id: '2', name: 'Bravo', gender: 'Male' },
    '3': { id: '3', name: 'Charlie', gender: 'Female' },
}
*/

const toNumbers = arr => arr.map(Number);

// Or
const toNumbers = arr => arr.map(x => +x);

// Example
toNumbers(['2', '3', '4']);     // [2, 3, 4]

const countBy = (arr, prop) => arr.reduce((prev, curr) => (prev[curr[prop]] = ++prev[curr[prop]] || 1, prev), {});

// Example
countBy([
    { branch: 'audi', model: 'q8', year: '2019' },
    { branch: 'audi', model: 'rs7', year: '2020' },
    { branch: 'ford', model: 'mustang', year: '2019' },
    { branch: 'ford', model: 'explorer', year: '2020' },
    { branch: 'bmw', model: 'x7', year: '2020' },
], 'branch');

// { 'audi': 2, 'ford': 2, 'bmw': 1 }

const countOccurrences = (arr, val) => arr.reduce((a, v) => (v === val ? a + 1 : a), 0);

// Or
const countOccurrences = (arr, val) => arr.filter(item => item === val).length;

// Examples
countOccurrences([2, 1, 3, 3, 2, 3], 2);                // 2
countOccurrences(['a', 'b', 'a', 'c', 'a', 'b'], 'a');  // 3

const countOccurrences = arr => arr.reduce((prev, curr) => (prev[curr] = ++prev[curr] || 1, prev), {});

// Examples
countOccurrences([2, 1, 3, 3, 2, 3]);               // { '1': 1, '2': 2, '3': 3 }
countOccurrences(['a', 'b', 'a', 'c', 'a', 'b']);   // { 'a': 3, 'b': 2, 'c': 1 }

const accumulate = arr => arr.map((sum => value => sum += value)(0));

// Or
const accumulate = arr => arr.reduce((a, b, i) => i === 0 ? [b] : [...a, b + a[i - 1]], []);

// Or
const accumulate = arr => arr.reduce((a, b, i) => i === 0 ? [b] : [...a, b + a[i - 1]], 0);

// Example
accumulate([1, 2, 3, 4]);   // [1, 3, 6, 10]
// 1             = 1
// 1 + 2         = 3
// 1 + 2 + 3     = 6
// 1 + 2 + 3 + 4 = 10

const range = (min, max) => [...Array(max - min + 1).keys()].map(i => i + min);

// Or
const range = (min, max) => Array(max - min + 1).fill(0).map((_, i) => min + i);

// Or
const range = (min, max) => Array.from({ length: max - min + 1 }, (_, i) => min + i);

// Example
range(5, 10);   // [5, 6, 7, 8, 9, 10]

const cartesian = (...sets) => sets.reduce((acc, set) => acc.flatMap((x) => set.map((y) => [...x, y])), [[]]);

// Example
cartesian([1, 2], [3, 4]);   // [ [1, 3], [1, 4], [2, 3], [2, 4] ]

/*
       3       4
   ---------------
1 | [1, 3]  [1, 4]
  |
2 | [2, 3]  [2, 4]

*/

const empty = arr => arr.length = 0;

// Or
arr = [];

// Find the number from `arr` which is closest to `n`
const closest = (arr, n) => arr.reduce((prev, curr) => Math.abs(curr - n) < Math.abs(prev - n) ? curr : prev);

// Or
const closest = (arr, n) => arr.sort((a, b) => Math.abs(a - n) - Math.abs(b - n))[0];

// Example
closest([29, 87, 8, 78, 97, 20, 75, 33, 24, 17], 50);   // 33

const lastIndex = (arr, predicate) => arr.reduce((prev, curr, index) => predicate(curr) ? index : prev, -1);

// Or
const lastIndex = (arr, predicate) => arr.map(item => predicate(item)).lastIndexOf(true);

// Example
lastIndex([1, 3, 5, 7, 9, 2, 4, 6, 8], i => i % 2 === 1);   // 4
lastIndex([1, 3, 5, 7, 9, 8, 6, 4, 2], i => i > 6);         // 5

const indexOfMax = arr => arr.reduce((prev, curr, i, a) => curr > a[prev] ? i : prev, 0);

// Examples
indexOfMax([1, 3, 9, 7, 5]);        // 2
indexOfMax([1, 3, 7, 7, 5]);        // 2

const indexOfMin = arr => arr.reduce((prev, curr, i, a) => curr < a[prev] ? i : prev, 0);

// Examples
indexOfMin([6, 4, 8, 2, 10]);       // 3
indexOfMin([6, 4, 2, 2, 10]);       // 2

const findLongest = words => Math.max(...(words.map(el => el.length)));

// Example
findLongest(['always','look','on','the','bright','side','of','life']);  // 6

const max = arr => Math.max(...arr);

const maxBy = (arr, key) => arr.reduce((a, b) => a[key] >= b[key] ? a : b, {});

// Example
const people = [
    { name: 'Bar', age: 24 },
    { name: 'Baz', age: 32 },
    { name: 'Foo', age: 42 },
    { name: 'Fuzz', age: 36 },
];
maxBy(people, 'age');   // { name: 'Foo', age: 42 }

const min = arr => Math.min(...arr);

const minBy = (arr, key) => arr.reduce((a, b) => a[key] < b[key] ? a : b, {});

// Example
const people = [
    { name: 'Bar', age: 24 },
    { name: 'Baz', age: 32 },
    { name: 'Foo', age: 42 },
    { name: 'Fuzz', age: 36 },
];
minBy(people, 'age');   // { name: 'Bar', age: 24 }

const flat = arr => [].concat.apply([], arr.map(a => Array.isArray(a) ? flat(a) : a));
// Or
const flat = arr => arr.reduce((a, b) => Array.isArray(b) ? [...a, ...flat(b)] : [...a, b], []);

// Or
// See the browser compatibility at https://caniuse.com/#feat=array-flat
const flat = arr => arr.flat();

// Example
flat(['cat', ['lion', 'tiger']]);   // ['cat', 'lion', 'tiger']

const getConsecutiveArrays = (arr, size) => size > arr.length ? [] : arr.slice(size - 1).map((_, i) => arr.slice(i, size + i));

// Examples
getConsecutiveArrays([1, 2, 3, 4, 5], 2);   // [[1, 2], [2, 3], [3, 4], [4, 5]]
getConsecutiveArrays([1, 2, 3, 4, 5], 3);   // [[1, 2, 3], [2, 3, 4], [3, 4, 5]]
getConsecutiveArrays([1, 2, 3, 4, 5], 6);   // []

const getNthItems = (arr, nth) => arr.filter((_, i) => i % nth === nth - 1);

// Examples
getNthItems([1, 2, 3, 4, 5, 6, 7, 8, 9], 2);    // [2, 4, 6, 8]
getNthItems([1, 2, 3, 4, 5, 6, 7, 8, 9], 3);    // [3, 6, 9]

const getSubsets = arr => arr.reduce((prev, curr) => prev.concat(prev.map(k => k.concat(curr))), [[]]);

// Examples
getSubsets([1, 2]);     // [[], [1], [2], [1, 2]]
getSubsets([1, 2, 3]);  // [[], [1], [2], [1, 2], [3], [1, 3], [2, 3], [1, 2, 3]]

const indices = (arr, value) => arr.reduce((acc, v, i) => (v === value ? [...acc, i] : acc), []);

// Or
const indices = (arr, value) => arr.map((v, i) => v === value ? i : false).filter(Boolean);

// Examples
indices(['h', 'e', 'l', 'l', 'o'], 'l');    // [2, 3]
indices(['h', 'e', 'l', 'l', 'o'], 'w');    // []

const average = arr => arr.reduce((a, b) => a + b, 0) / arr.length;

const getIntersection = (a, ...arr) => [...new Set(a)].filter(v => arr.every(b => b.includes(v)));

// Examples
getIntersection([1, 2, 3], [2, 3, 4, 5]);               // [2, 3]
getIntersection([1, 2, 3], [2, 3, 4, 5], [1, 3, 5]);    // [3]

const ranking = (arr) => arr.map((x, y, z) => z.filter(w => w > x).length + 1);

// Examples
ranking([80, 65, 90, 50]);      // [2, 3, 1, 4]
ranking([80, 80, 70, 50]);      // [1, 1, 3, 4]
ranking([80, 80, 80, 50]);      // [1, 1, 1, 4]

const sum = arr => arr.reduce((a, b) => a + b, 0);

const unique = arr => [...new Set(arr)];

// Or
const unique = arr => arr.filter((el, i, array) => array.indexOf(el) === i);

// Or
const unique = arr => arr.reduce((acc, el) => acc.includes(el) ? acc : [...acc, el], []);

const union = (...arr) => [...new Set(arr.flat())];

// Example
union([1, 2], [2, 3], [3]);     // [1, 2, 3]

const groupBy = (arr, key) => arr.reduce((acc, item) => ((acc[item[key]] = [...(acc[item[key]] || []), item]), acc), {});

// Example
groupBy([
    { branch: 'audi', model: 'q8', year: '2019' },
    { branch: 'audi', model: 'rs7', year: '2020' },
    { branch: 'ford', model: 'mustang', year: '2019' },
    { branch: 'ford', model: 'explorer', year: '2020' },
    { branch: 'bmw', model: 'x7', year: '2020' },
], 'branch');

/*
{
    audi: [
        { branch: 'audi', model: 'q8', year: '2019' },
        { branch: 'audi', model: 'rs7', year: '2020' }
    ],
    bmw: [
        { branch: 'bmw', model: 'x7', year: '2020' }
    ],
    ford: [
        { branch: 'ford', model: 'mustang', year: '2019' },
        { branch: 'ford', model: 'explorer', year: '2020' }
    ],
}
*/

// Merge but don't remove the duplications
const merge = (a, b) => a.concat(b);
// Or
const merge = (a, b) => [...a, ...b];

// Merge and remove the duplications
const merge = [...new Set(a.concat(b))];
// Or
const merge = [...new Set([...a, ...b])];

const partition = (arr, criteria) => arr.reduce((acc, i) => (acc[criteria(i) ? 0 : 1].push(i), acc), [[], []]);

// Example
partition([1, 2, 3, 4, 5], n => n % 2);     // [[2, 4], [1, 3, 5]]

const removeDuplicate = arr => arr.filter(i => arr.indexOf(i) === arr.lastIndexOf(i));

// Example
removeDuplicate(['h', 'e', 'l', 'l', 'o', 'w', 'o', 'r', 'l', 'd']); //  ['h', 'e', 'w', 'r', 'd']

const removeFalsy = arr => arr.filter(Boolean);

// Example
removeFalsy([0, 'a string', '', NaN, true, 5, undefined, 'another string', false]); // ['a string', true, 5, 'another string']

// `arr` is an array
const repeat = (arr, n) => [].concat(...Array(n).fill(arr));

// Or
const repeat = (arr, n) => Array(n).fill(arr).flat();

// Or
const repeat = (arr, n) => Array(arr.length * n).fill(0).map((_, i) => arr[i % arr.length]);

// Or
const repeat = (arr, n) => Array.from({ length: arr.length * n }, (_, i) => arr[i % arr.length]);

// Examples
repeat([1, 2, 3], 3);       // [1, 2, 3, 1, 2, 3, 1, 2, 3]

const shuffle = arr => arr.map(a => ({ sort: Math.random(), value: a })).sort((a, b) => a.sort - b.sort).map(a => a.value);

// Or
const shuffle = arr => arr.sort(() => .5 - Math.random());

// Example
shuffle([1, 2, 3, 4, 5, 6, 7, 8, 9, 10]);   // [9, 1, 10, 6, 8, 5, 2, 3, 7, 4]

const sortBy = (arr, k) => arr.concat().sort((a, b) => (a[k] > b[k]) ? 1 : ((a[k] < b[k]) ? -1 : 0));

// Example
const people = [
    { name: 'Foo', age: 42 },
    { name: 'Bar', age: 24 },
    { name: 'Fuzz', age: 36 },
    { name: 'Baz', age: 32 },
];
sortBy(people, 'age');

// returns
//  [
//      { name: 'Bar', age: 24 },
//      { name: 'Baz', age: 32 },
//      { name: 'Fuzz', age: 36 },
//      { name: 'Foo', age: 42 },
//  ]

const sort = arr => arr.sort((a, b) => a - b);

// Example
sort([1, 5, 2, 4, 3]);      // [1, 2, 3, 4, 5]

const chunk = (arr, size) => arr.reduce((acc, e, i) => (i % size ? acc[acc.length - 1].push(e) : acc.push([e]), acc), []);

// Examples
chunk([1, 2, 3, 4, 5, 6, 7, 8], 3);     // [[1, 2, 3], [4, 5, 6], [7, 8]]
chunk([1, 2, 3, 4, 5, 6, 7, 8], 4);     // [[1, 2, 3, 4], [5, 6, 7, 8]]

const transpose = matrix => matrix[0].map((col, i) => matrix.map(row => row[i]));

// Or
const transpose = matrix => matrix[0].map((col, c) => matrix.map((row, r) => matrix[r][c]));

// Or
const transpose = matrix => matrix.reduce((prev, next) => next.map((item, i) => (prev[i] || []).concat(next[i])), []);

// Example
transpose([             // [
    [1, 2, 3],          //      [1, 4, 7],
    [4, 5, 6],          //      [2, 5, 8],
    [7, 8, 9],          //      [3, 6, 9],
]);                     //  ]

// `i` must be less than `j`
const swapItems = (a, i, j) => a[i] && a[j] && [...a.slice(0, i), a[j], ...a.slice(i + 1, j), a[i], ...a.slice(j + 1)] || a;

// Example
swapItems([1, 2, 3, 4, 5], 1, 4);   // [1, 5, 3, 4, 2] 

const unzip = arr => arr.reduce((acc, c) => (c.forEach((v, i) => acc[i].push(v)), acc), Array.from({ length: Math.max(...arr.map(a => a.length)) }, (_) => []));

// Example
unzip([['a', 1], ['b', 2], ['c', 3], ['d', 4], ['e', 5]]);  // [['a', 'b', 'c', 'd', 'e'], [1, 2, 3, 4, 5]]

/*
    a     1
     b   2
      c 3
      d 4
      e 5
*/

const zip = (...arr) => Array.from({ length: Math.max(...arr.map(a => a.length)) }, (_, i) => arr.map(a => a[i]));

// Example
zip(['a', 'b', 'c', 'd', 'e'], [1, 2, 3, 4, 5]);   // [['a', 1], ['b', 2], ['c', 3], ['d', 4], ['e', 5]]

/*
Does it look like a zipper?
        a 1
        b 2
       c   3
      d     4
     e       5
*/

const isDescendant = (child, parent) => parent.contains(child);

const hasFocus = ele => (ele === document.activeElement);

const touchSupported = () => ('ontouchstart' in window || window.DocumentTouch && document instanceof window.DocumentTouch);

const isAtBottom = () => document.documentElement.clientHeight + window.scrollY >= document.documentElement.scrollHeight;

const isIE = !!document.documentMode;

const isMacBrowser = /Mac|iPod|iPhone|iPad/.test(navigator.platform);

const siblings = ele => [].slice.call(ele.parentNode.children).filter((child) => (child !== ele));

const getPosition = ele => (r = ele.getBoundingClientRect(), { left: r.left + window.scrollX, top: r.top + window.scrollY });

// Example
getPosition(document.body);     // { left: 0, top: 0 }

const getSelectedText = () => window.getSelection().toString();

history.back();

// Or
history.go(-1);

// Pick the method that is suitable for your use case
const hide = ele => ele.style.display = 'none';

// Or
const hide = ele => ele.style.visibility = 'hidden';

// Or
const hide = ele => ele.hidden = true;

const insertAfter = (ele, anotherEle) => anotherEle.parentNode.insertBefore(ele, anotherEle.nextSibling);

// Or
const insertAfter = (ele, anotherEle) => anotherEle.insertAdjacentElement('afterend', ele);

const insertBefore = (ele, anotherEle) => anotherEle.parentNode.insertBefore(ele, anotherEle);

// Or
const insertBefore = (ele, anotherEle) => anotherEle.insertAdjacentElement('beforebegin', ele);

const insertHtmlAfter = (html, ele) => ele.insertAdjacentHTML('afterend', html);

const insertHtmlBefore = (html, ele) => ele.insertAdjacentHTML('beforebegin', html);

const goTo = url => location.href = url;

const reload = () => location.reload();

// Or
const reload = () => (location.href = location.href);

const replace = (ele, newEle) => ele.parentNode.replaceChild(newEle, ele);

const goToTop = () => window.scrollTo(0, 0);

const serialize = formEle => Array.from(new FormData(formEle)).reduce((p, [k, v]) => Object.assign({}, p, { [k]: p[k] ? (Array.isArray(p[k]) ? p[k] : [p[k]]).concat(v) : v}), {});

const show = ele => ele.style.display = '';

const stripHtml = html => (new DOMParser().parseFromString(html, 'text/html')).body.textContent || '';

const toggle = ele => (ele.style.display = (ele.style.display === 'none') ? 'block' : 'none');

// Or
const toggle = ele => ele.hidden = !ele.hidden;

// `h` is an hour number between 0 and 23
const suffixAmPm = h => `${h % 12 === 0 ? 12 : h % 12}${h < 12 ? 'am' : 'pm'}`;

// Examples
suffixAmPm(0);      // '12am'
suffixAmPm(5);      // '5am'
suffixAmPm(12);     // '12pm'
suffixAmPm(15);     // '3pm'
suffixAmPm(23);     // '11pm'

const diffDays = (date, otherDate) => Math.ceil(Math.abs(date - otherDate) / (1000 * 60 * 60 * 24));

// Example
diffDays(new Date('2014-12-19'), new Date('2020-01-01'));   // 1839

const monthDiff = (startDate, endDate) => Math.max(0, (endDate.getFullYear() - startDate.getFullYear()) * 12 - startDate.getMonth() + endDate.getMonth());

// Example
monthDiff(new Date('2020-01-01'), new Date('2021-01-01'));  // 12

// `a` and `b` are `Date` instances
const compare = (a, b) => a.getTime() > b.getTime();

// Example
compare(new Date('2020-03-30'), new Date('2020-01-01'));    // true

// `date` is a `Date` object
const formatYmd = date => date.toISOString().slice(0, 10);

// Example
formatYmd(new Date());      // 2020-05-06

// `s` is number of seconds
const formatSeconds = s => new Date(s * 1000).toISOString().substr(11, 8);

// Or
const formatSeconds = s => (new Date(s * 1000)).toUTCString().match(/(\d\d:\d\d:\d\d)/)[0];

// Or
const formatSeconds = s => [parseInt(s / 60 / 60), parseInt(s / 60 % 60), parseInt(s % 60)].join(':').replace(/\b(\d)\b/g, '0$1');

// Examples
formatSeconds(200);     // 00:03:20
formatSeconds(500);     // 00:08:20

// `date` is a `Date` object
const extract = date => date.toISOString().split(/[^0-9]/).slice(0, -1);

// `extract` is an array of [year, month, day, hour, minute, second, millisecond]

// `date` is a `Date` object
// `locale` is a locale (en-US, pt-BR, for example)
const format = (date, locale) => new Intl.DateTimeFormat(locale).format(date);

// Example
format(new Date(), 'pt-BR');    // 06/05/2020

const getQuarter = (d = new Date()) => Math.ceil((d.getMonth() + 1) / 3);

const ts = () => Math.floor(new Date().getTime() / 1000);

// `date` is a Date object
const dayOfYear = date => Math.floor((date - new Date(date.getFullYear(), 0, 0)) / (1000 * 60 * 60 * 24));

// Example
dayOfYear(new Date(2020, 04, 16));      // 137

const getFirstDate = (d = new Date()) => new Date(d.getFullYear(), d.getMonth(), 1);

const getLastDate = (d = new Date()) => new Date(d.getFullYear(), d.getMonth() + 1, 0);

// `date` is a Date object
const getMonthName = date => ['January', 'February', 'March', 'April', 'May', 'June', 'July', 'August', 'September', 'October',' November', 'December'][date.getMonth()];

// `month` is zero-based index
const daysInMonth = (month, year) => new Date(year, month, 0).getDate();

const getTimezone = () => Intl.DateTimeFormat().resolvedOptions().timeZone;

// Example
getTimezone();      // 'Asia/Saigon'

const tomorrow = (d => new Date(d.setDate(d.getDate() + 1)))(new Date);

// Or
const tomorrow = new Date((new Date()).valueOf() + 1000 * 60 * 60 * 24);

const numberOfDays = year => (((year % 4 === 0) && (year % 100 !== 0)) || (year % 400 === 0)) ? 366 : 365;

// Or
const numberOfDays = year => new Date(year, 1, 29).getDate() === 29 ? 366 : 365;

// `date` is a Date object
const getWeekday = date => ['Sunday', 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday'][date.getDay()];

const yesterday = (d => new Date(d.setDate(d.getDate() - 1)))(new Date);

// Or
const yesterday = new Date((new Date()).valueOf() - 1000 * 60 * 60 * 24);

const midnightOfToday = () => new Date(new Date().setHours(0, 0, 0, 0));

// `arr` is an array of `Date` items
const sortDescending = arr => arr.sort((a, b) => a.getTime() > b.getTime());
const sortAscending = arr => arr.sort((a, b) => a.getTime() < b.getTime());

const boxHandler = x => ({ next: f => boxHandler(f(x)), done: f => f(x) });

// Example 1
const getPercentNumber = str => boxHandler(str).next(str => str.replace(/\%/, '')).next(str => parseFloat(str)).done(res => res * 0.01);

getPercentNumber('50%');    // 0.5

// Example 2
const getMoney = (price) => Number.parseFloat(price.replace(/\$/, ''));
const getPercent = (percent) => Number.parseFloat(percent.replace(/\%/)) * 0.01;

const getDiscountPrice = (price, discount) => boxHandler(getMoney(price)).done(cents => boxHandler(getPercent(discount)).next(save => cents - (cents * save))).done(res => res);

getDiscountPrice('$6.00', '20%');    // 4.8

const isFunction = v => ['[object Function]', '[object GeneratorFunction]', '[object AsyncFunction]', '[object Promise]'].includes(Object.prototype.toString.call(v));

// Examples
isFunction(function() {});          // true
isFunction(function*() {});         // true
isFunction(async function() {});    // true

const isGeneratorFunction = v => Object.prototype.toString.call(v) === '[object GeneratorFunction]';

// Examples
isGeneratorFunction(function() {});     // false
isGeneratorFunction(function*() {});    // true

const isAsyncFunction = v => Object.prototype.toString.call(v) === '[object AsyncFunction]';

// Examples
isAsyncFunction(function() {});         // false
isAsyncFunction(function*() {});        // false
isAsyncFunction(async function() {});   // true

// Compose functions from right to left
const compose = (...fns) => x => fns.reduceRight((y, f) => f(y), x);

// Example
const lowercase = str => str.toLowerCase();
const capitalize = str => `${str.charAt(0).toUpperCase()}${str.slice(1)}`;
const reverse = str => str.split('').reverse().join('');

const fn = compose(reverse, capitalize, lowercase);

// We will execute `lowercase`, `capitalize` and `reverse` in order
fn('Hello World') === 'dlrow olleH';

// Compose functions from left to right
const pipe = (...fns) => x => fns.reduce((y, f) => f(y), x);

// Example
const lowercase = str => str.toLowerCase();
const capitalize = str => `${str.charAt(0).toUpperCase()}${str.slice(1)}`;
const reverse = str => str.split('').reverse().join('');

const fn = pipe(lowercase, capitalize, reverse);

// We will execute `lowercase`, `capitalize` and `reverse` in order
fn('Hello World') === 'dlrow olleH';

const unary = fn => arg => fn(arg);

// Example
['1', '2', '3', '4', '5'].map(unary(parseInt));     // [1, 2, 3, 4, 5]

const noop = () => {};

// Or
const noop = Function();
// calling Function() might be detected as using eval by some security tools

const curry = (fn, ...args) => fn.length <= args.length ? fn(...args) : curry.bind(null, fn, ...args);

// Example
const sum = (a, b, c) => a + b + c;
curry(sum)(1)(2)(3);    // 6
curry(sum)(1, 2, 3);    // 6
curry(sum, 1)(2, 3);    // 6
curry(sum, 1)(2)(3);    // 6
curry(sum, 1, 2)(3);    // 6
curry(sum, 1, 2, 3);    // 6

// returns a new version of `fn` that returns values as lazy evaluable
const thunkfy = fn => (...args) => () => fn(...args);

// Example
const heavyComputation = x => doStuff(x);
const unnecessarySlow = manyThings.map(heavyComputation)
    .find(result => result.criteria);
const probablyFaster = manyThings.map(thunkfy(heavyComputation))
    .find(thunk => thunk().criteria);

const once = fn => ((ran = false) => () => ran ? fn : (ran = !ran, fn = fn()))();

// Example
let n = 0;
const incOnce = once(() => ++n);
incOnce();      // n = 1
incOnce();      // n = 1
incOnce();      // n = 1

// Reverse the order of arguments
const flip = fn => (...args) => fn(...args.reverse());

// For binary functions
const flip = fn => (b, a) => fn(a, b);

// Or for curried functions
const flip = fn => b => a => fn(a)(b);

// Example
const isParent = (parent, child) => parent.children.includes(child);
const isChild = flip(isParent);

const identity = x => x;

// returns `true` if one of the arguments is truthy and the other is falsy

const xor = (a, b) => (a && !b) || (!a && b);

// Or
const xor = (a, b) => !(!a && !b) && !(a && b);

// Or
const xor = (a, b) => Boolean(!a ^ !b);

// Examples
xor(true, true);        // false
xor(false, false);      // false
xor(true, false);       // true
xor(false, true);       // true

const memoize = fn => ((cache = {}) => arg => cache[arg] || (cache[arg] = fn(arg)))();

// Example
// Calculate Fibonacci numbers
const fibo = memoize(n => n <= 2 ? 1 : fibo(n - 1) + fibo(n - 2));

fibo(1);    // 1
fibo(2);    // 1
fibo(3);    // 2
fibo(4);    // 3
fibo(5);    // 5
fibo(6);    // 8

const partial = (fn, ...a) => (...b) => fn(...a, ...b);

// Example
const sum = (x, y) => x + y;
const inc = partial(sum, 1);
inc(9);     // 10

// `fn` is a curried function
// `n` is the depth of parameters
const uncurry = (fn, n = 1) => (...args) => (acc => args => args.reduce((x, y) => x(y), acc))(fn)(args.slice(0, n));

// Example
const sum = a => b => c => a + b + c;
uncurry(sum, 1)(1)(2)(3);       // 6
uncurry(sum, 2)(1, 2)(3);       // 6
uncurry(sum, 3)(1, 2, 3);       // 6

// In radians
const radiansAngle = (p1, p2) => Math.atan2(p2.y - p1.y, p2.x - p1.x);

// In degrees
const degreesAngle = (p1, p2) => Math.atan2(p2.y - p1.y, p2.x - p1.x) * 180 / Math.PI;

const distance = (p1, p2) => Math.sqrt(Math.pow(p2.x - p1.x, 2) + Math.pow(p2.y - p1.y, 2));

const lerp = (a, b, amount) => (1 - amount) * a + amount * b;

const midpoint = (p1, p2) => [(p1.x + p2.x) / 2, (p1.y + p2.y) / 2];

const isInside = (point, rect) => point.x > rect.left && point.x < rect.right && point.y > rect.top && point.y < rect.bottom;

// Returns true if `a` contains `b`
// (x1, y1) and (x2, y2) are top-left and bottom-right corners
const contains = (a, b) => (a.x1 <= b.x1 && a.y1 <= b.y1 && a.x2 >= b.x2 && a.y2 >= b.y2);

// Returns true if `a` overlaps `b`
// (x1, y1) and (x2, y2) are top-left and bottom-right corners
const overlaps = (a, b) => (a.x1 < b.x2 && b.x1 < a.x2) || (a.y1 < b.y2 && b.y1 < a.y2);

const degsToRads = deg => deg * Math.PI / 180.0;

const radsToDegs = rad => rad * 180 / Math.PI;

const normalizeRatio = (value, min, max) => (value - min) / (max - min);

const roundNearest = (value, nearest) => Math.round(value / nearest) * nearest;

// Examples
roundNearest(100, 30);      // 90
roundNearest(200, 30);      // 210
roundNearest(200, 40);      // 200

const isNode = typeof process !== 'undefined' && process.versions != null && process.versions.node != null;

const isBrowser = typeof window === 'object' && typeof document === 'object';

const clearCookies = document.cookie.split(';').forEach(c => document.cookie = c.replace(/^ +/, '').replace(/=.*/, `=;expires=${new Date().toUTCString()};path=/`));

const toFullHexColor = color => `#${(color.startsWith('#') ? color.slice(1) : color).split('').map(c => `${c}${c}`).join('')}`;

// Example
toFullHexColor('123');      // '#112233'
toFullHexColor('#123');     // '#112233'
toFullHexColor('#abc');     // '#aabbcc'

const celsiusToFahrenheit = celsius => celsius * 9/5 + 32;

// Examples
celsiusToFahrenheit(15);    // 59
celsiusToFahrenheit(0);     // 32
celsiusToFahrenheit(-20);   // -4