测试跳转

arrayMax

Returns the maximum value in an array.

Use Math.max() combined with the spread operator (...) to get the maximum value in the array.

const arrayMax = arr => Math.max(...arr);
// arrayMax([10, 1, 5]) -> 10

arrayMin

Returns the minimum value in an array.

Use Math.min() combined with the spread operator (...) to get the minimum value in the array.

const arrayMin = arr => Math.min(...arr);
// arrayMin([10, 1, 5]) -> 1

chunk

Chunks an array into smaller arrays of a specified size.

Use Array.from() to create a new array, that fits the number of chunks that will be produced. Use Array.slice() to map each element of the new array to a chunk the length of size. If the original array can't be split evenly, the final chunk will contain the remaining elements.

const chunk = (arr, size) =>
  Array.from({length: Math.ceil(arr.length / size)}, (v, i) => arr.slice(i * size, i * size + size));
// chunk([1,2,3,4,5], 2) -> [[1,2],[3,4],[5]]

compact

Removes falsey values from an array.

Use Array.filter() to filter out falsey values (false, null, 0, "", undefined, and NaN).

const compact = (arr) => arr.filter(Boolean);
// compact([0, 1, false, 2, '', 3, 'a', 'e'*23, NaN, 's', 34]) -> [ 1, 2, 3, 'a', 's', 34 ]

countOccurrences

Counts the occurrences of a value in an array.

Use Array.reduce() to increment a counter each time you encounter the specific value inside the array.

const countOccurrences = (arr, value) => arr.reduce((a, v) => v === value ? a + 1 : a + 0, 0);
// countOccurrences([1,1,2,1,2,3], 1) -> 3

deepFlatten

Deep flattens an array.

Use recursion. Use Array.concat() with an empty array ([]) and the spread operator (...) to flatten an array. Recursively flatten each element that is an array.

const deepFlatten = arr => [].concat(...arr.map(v => Array.isArray(v) ? deepFlatten(v) : v));
// deepFlatten([1,[2],[[3],4],5]) -> [1,2,3,4,5]

difference

Returns the difference between two arrays.

Create a Set from b, then use Array.filter() on a to only keep values not contained in b.

const difference = (a, b) => { const s = new Set(b); return a.filter(x => !s.has(x)); };
// difference([1,2,3], [1,2,4]) -> [3]

distinctValuesOfArray

Returns all the distinct values of an array.

Use ES6 Set and the ...rest operator to discard all duplicated values.

const distinctValuesOfArray = arr => [...new Set(arr)];
// distinctValuesOfArray([1,2,2,3,4,4,5]) -> [1,2,3,4,5]

dropElements

Removes elements in an array until the passed function returns true. Returns the remaining elements in the array.

Loop through the array, using Array.shift() to drop the first element of the array until the returned value from the function is true. Returns the remaining elements.

const dropElements = (arr, func) => {
  while (arr.length > 0 && !func(arr[0])) arr.shift();
  return arr;
};
// dropElements([1, 2, 3, 4], n => n >= 3) -> [3,4]

everyNth

Returns every nth element in an array.

Use Array.filter() to create a new array that contains every nth element of a given array.

const everyNth = (arr, nth) => arr.filter((e, i) => i % nth === 0);
// everyNth([1,2,3,4,5,6], 2) -> [ 1, 3, 5 ]

filterNonUnique

Filters out the non-unique values in an array.

Use Array.filter() for an array containing only the unique values.

const filterNonUnique = arr => arr.filter(i => arr.indexOf(i) === arr.lastIndexOf(i));
// filterNonUnique([1,2,2,3,4,4,5]) -> [1,3,5]

flatten

Flattens an array.

Use Array.reduce() to get all elements inside the array and concat() to flatten them.

const flatten = arr => arr.reduce((a, v) => a.concat(v), []);
// flatten([1,[2],3,4]) -> [1,2,3,4]

flattenDepth

Flattens an array up to the specified depth.

Use recursion, decrementing depth by 1 for each level of depth. Use Array.reduce() and Array.concat() to merge elements or arrays. Base case, for depth equal to 1 stops recursion. Omit the second element, depth to flatten only to a depth of 1 (single flatten).

const flattenDepth = (arr, depth = 1) =>
  depth != 1 ? arr.reduce((a, v) => a.concat(Array.isArray(v) ? flattenDepth(v, depth - 1) : v), [])
  : arr.reduce((a, v) => a.concat(v), []);
// flattenDepth([1,[2],[[[3],4],5]], 2) -> [1,2,[3],4,5]

groupBy

Groups the element of an array based on the given function.

Use Array.map() to map the values of an array to a function or property name. Use Array.reduce() to create an object, where the keys are produced from the mapped results.

const groupBy = (arr, func) =>
  arr.map(typeof func === 'function' ? func : val => val[func])
    .reduce((acc, val, i) => { acc[val] = (acc[val] || []).concat(arr[i]); return acc; }, {});
// groupBy([6.1, 4.2, 6.3], Math.floor) -> {4: [4.2], 6: [6.1, 6.3]}
// groupBy(['one', 'two', 'three'], 'length') -> {3: ['one', 'two'], 5: ['three']}

head

Returns the head of a list.

Use arr[0] to return the first element of the passed array.

const head = arr => arr[0];
// head([1,2,3]) -> 1

initial

Returns all the elements of an array except the last one.

Use arr.slice(0,-1)to return all but the last element of the array.

const initial = arr => arr.slice(0, -1);
// initial([1,2,3]) -> [1,2]

initializeArrayWithRange

Initializes an array containing the numbers in the specified range.

Use Array(end-start) to create an array of the desired length, Array.map() to fill with the desired values in a range. You can omit start to use a default value of 0.

const initializeArrayWithRange = (end, start = 0) =>
  Array.from({ length: end - start }).map((v, i) => i + start);
// initializeArrayWithRange(5) -> [0,1,2,3,4]

initializeArrayWithValues

Initializes and fills an array with the specified values.

Use Array(n) to create an array of the desired length, fill(v) to fill it with the desired values. You can omit value to use a default value of 0.

const initializeArrayWithValues = (n, value = 0) => Array(n).fill(value);
// initializeArrayWithValues(5, 2) -> [2,2,2,2,2]

intersection

Returns a list of elements that exist in both arrays.

Create a Set from b, then use Array.filter() on a to only keep values contained in b.

const intersection = (a, b) => { const s = new Set(b); return a.filter(x => s.has(x)); };
// intersection([1,2,3], [4,3,2]) -> [2,3]

last

Returns the last element in an array.

Use arr.length - 1 to compute index of the last element of the given array and returning it.

const last = arr => arr[arr.length - 1];
// last([1,2,3]) -> 3

mapObject

Maps the values of an array to an object using a function, where the key-value pairs consist of the original value as the key and the mapped value.

Use an anonymous inner function scope to declare an undefined memory space, using closures to store a return value. Use a new Array to stor the array with a map of the function over its data set and a comma operator to return a second step, without needing to move from one context to another (due to closures and order of operations).

const mapObject = (arr, fn) => 
  (a => (a = [arr, arr.map(fn)], a[0].reduce( (acc,val,ind) => (acc[val] = a[1][ind], acc), {}) )) ( );
/*
const squareIt = arr => mapObject(arr, a => a*a)
squareIt([1,2,3]) // { 1: 1, 2: 4, 3: 9 }
*/

nthElement

Returns the nth element of an array.

Use Array.slice() to get an array containing the nth element at the first place. If the index is out of bounds, return []. Omit the second argument, n, to get the first element of the array.

const nthElement = (arr, n=0) => (n>0? arr.slice(n,n+1) : arr.slice(n))[0];
// nthElement(['a','b','c'],1) -> 'b'
// nthElement(['a','b','b'],-3) -> 'a'

pick

Picks the key-value pairs corresponding to the given keys from an object.

Use Array.reduce() to convert the filtered/picked keys back to a object with the corresponding key-value pair if the key exist in the obj.

const pick = (obj, arr) =>
  arr.reduce((acc, curr) => (curr in obj && (acc[curr] = obj[curr]), acc), {});
// pick({ 'a': 1, 'b': '2', 'c': 3 }, ['a', 'c']) -> { 'a': 1, 'c': 3 }

pull

Mutates the original array to filter out the values specified.

Use Array.filter() and Array.includes() to pull out the values that are not needed. Use Array.length = 0 to mutate the passed in array by resetting it's length to zero and Array.push() to re-populate it with only the pulled values.

const pull = (arr, ...args) => {
  let pulled = arr.filter((v, i) => !args.includes(v));
  arr.length = 0; pulled.forEach(v => arr.push(v));
};
// let myArray = ['a', 'b', 'c', 'a', 'b', 'c'];
// pull(myArray, 'a', 'c');
// console.log(myArray) -> [ 'b', 'b' ]

remove

Removes elements from an array for which the given function returns false.

Use Array.filter() to find array elements that return truthy values and Array.reduce() to remove elements using Array.splice(). The func is invoked with three arguments (value, index, array).

const remove = (arr, func) =>
  Array.isArray(arr) ? arr.filter(func).reduce((acc, val) => {
    arr.splice(arr.indexOf(val), 1); return acc.concat(val);
    }, [])
  : [];
// remove([1, 2, 3, 4], n => n % 2 == 0) -> [2, 4]

sample

Returns a random element from an array.

Use Math.random() to generate a random number, multiply it with length and round it of to the nearest whole number using Math.floor(). This method also works with strings.

const sample = arr => arr[Math.floor(Math.random() * arr.length)];
// sample([3, 7, 9, 11]) -> 9

shuffle

Randomizes the order of the values of an array.

Use Array.sort() to reorder elements, using Math.random() in the comparator.

const shuffle = arr => arr.sort(() => Math.random() - 0.5);
// shuffle([1,2,3]) -> [2,3,1]

similarity

Returns an array of elements that appear in both arrays.

Use filter() to remove values that are not part of values, determined using includes().

const similarity = (arr, values) => arr.filter(v => values.includes(v));
// similarity([1,2,3], [1,2,4]) -> [1,2]

symmetricDifference

Returns the symmetric difference between two arrays.

Create a Set from each array, then use Array.filter() on each of them to only keep values not contained in the other.

const symmetricDifference = (a, b) => {
  const sA = new Set(a), sB = new Set(b);
  return [...a.filter(x => !sB.has(x)), ...b.filter(x => !sA.has(x))];
}
// symmetricDifference([1,2,3], [1,2,4]) -> [3,4]

tail

Returns all elements in an array except for the first one.

Return arr.slice(1) if the array's length is more than 1, otherwise return the whole array.

const tail = arr => arr.length > 1 ? arr.slice(1) : arr;
// tail([1,2,3]) -> [2,3]
// tail([1]) -> [1]

take

Returns an array with n elements removed from the beginning.

Use Array.slice() to create a slice of the array with n elements taken from the beginning.

const take = (arr, n = 1) => arr.slice(0, n);
// take([1, 2, 3], 5) -> [1, 2, 3]
// take([1, 2, 3], 0) -> []

takeRight

Returns an array with n elements removed from the end.

Use Array.slice() to create a slice of the array with n elements taken from the end.

const takeRight = (arr, n = 1) => arr.slice(arr.length - n, arr.length);
// takeRight([1, 2, 3], 2) -> [ 2, 3 ]
// takeRight([1, 2, 3]) -> [3]

union

Returns every element that exists in any of the two arrays once.

Create a Set with all values of a and b and convert to an array.

const union = (a, b) => Array.from(new Set([...a, ...b]));
// union([1,2,3], [4,3,2]) -> [1,2,3,4]

without

Filters out the elements of an array, that have one of the specified values.

Use Array.filter() to create an array excluding(using !Array.includes()) all given values.

const without = (arr, ...args) => arr.filter(v => !args.includes(v));
// without([2, 1, 2, 3], 1, 2) -> [3]

zip

Creates an array of elements, grouped based on the position in the original arrays.

Use Math.max.apply() to get the longest array in the arguments. Creates an array with that length as return value and use Array.from() with a map-function to create an array of grouped elements. If lengths of the argument-arrays vary, undefined is used where no value could be found.

const zip = (...arrays) => {
  const maxLength = Math.max(...arrays.map(x => x.length));
  return Array.from({length: maxLength}).map((_, i) => {
   return Array.from({length: arrays.length}, (_, k) => arrays[k][i]);
  })
}
//zip(['a', 'b'], [1, 2], [true, false]); -> [['a', 1, true], ['b', 2, false]]
//zip(['a'], [1, 2], [true, false]); -> [['a', 1, true], [undefined, 2, false]]

bottomVisible

Returns true if the bottom of the page is visible, false otherwise.

Use scrollY, scrollHeight and clientHeight to determine if the bottom of the page is visible.

const bottomVisible = () =>
  document.documentElement.clientHeight + window.scrollY >= document.documentElement.scrollHeight || document.documentElement.clientHeight;
// bottomVisible() -> true

currentURL

Returns the current URL.

Use window.location.href to get current URL.

const currentURL = () => window.location.href;
// currentUrl() -> 'https://google.com'

elementIsVisibleInViewport

Returns true if the element specified is visible in the viewport, false otherwise.

Use Element.getBoundingClientRect() and the window.inner(Width|Height) values to determine if a given element is visible in the viewport. Omit the second argument to determine if the element is entirely visible, or specify true to determine if it is partially visible.

const elementIsVisibleInViewport = (el, partiallyVisible = false) => {
  const { top, left, bottom, right } = el.getBoundingClientRect();
  return partiallyVisible
    ? ((top > 0 && top < innerHeight) || (bottom > 0 && bottom < innerHeight)) &&
      ((left > 0 && left < innerWidth) || (right > 0 && right < innerWidth))
    : top >= 0 && left >= 0 && bottom <= innerHeight && right <= innerWidth;
};
// e.g. 100x100 viewport and a 10x10px element at position {top: -1, left: 0, bottom: 9, right: 10}
// elementIsVisibleInViewport(el) -> false (not fully visible)
// elementIsVisibleInViewport(el, true) -> true (partially visible)

getScrollPosition

Returns the scroll position of the current page.

Use pageXOffset and pageYOffset if they are defined, otherwise scrollLeft and scrollTop. You can omit el to use a default value of window.

const getScrollPosition = (el = window) =>
  ({x: (el.pageXOffset !== undefined) ? el.pageXOffset : el.scrollLeft,
    y: (el.pageYOffset !== undefined) ? el.pageYOffset : el.scrollTop});
// getScrollPosition() -> {x: 0, y: 200}

getURLParameters

Returns an object containing the parameters of the current URL.

Use match() with an appropriate regular expression to get all key-value pairs, Array.reduce() to map and combine them into a single object. Pass location.search as the argument to apply to the current url.

const getURLParameters = url =>
  url.match(/([^?=&]+)(=([^&]*))/g).reduce(
    (a, v) => (a[v.slice(0, v.indexOf('='))] = v.slice(v.indexOf('=') + 1), a), {}
  );
// getURLParameters('http://url.com/page?name=Adam&surname=Smith') -> {name: 'Adam', surname: 'Smith'}

redirect

Redirects to a specified URL.

Use window.location.href or window.location.replace() to redirect to url. Pass a second argument to simulate a link click (true - default) or an HTTP redirect (false).

const redirect = (url, asLink = true) =>
  asLink ? window.location.href = url : window.location.replace(url);
// redirect('https://google.com')

scrollToTop

Smooth-scrolls to the top of the page.

Get distance from top using document.documentElement.scrollTop or document.body.scrollTop. Scroll by a fraction of the distance from top. Use window.requestAnimationFrame() to animate the scrolling.

const scrollToTop = () => {
  const c = document.documentElement.scrollTop || document.body.scrollTop;
  if (c > 0) {
    window.requestAnimationFrame(scrollToTop);
    window.scrollTo(0, c - c / 8);
  }
};
// scrollToTop()

getDaysDiffBetweenDates

Returns the difference (in days) between two dates.

Calculate the difference (in days) between to Date objects.

const getDaysDiffBetweenDates = (dateInitial, dateFinal) => (dateFinal - dateInitial) / (1000 * 3600 * 24);
// getDaysDiffBetweenDates(new Date("2017-12-13"), new Date("2017-12-22")) -> 9

JSONToDate

Converts a JSON object to a date.

Use Date(), to convert dates in JSON format to readable format (dd/mm/yyyy).

const JSONToDate = arr => {
  const dt = new Date(parseInt(arr.toString().substr(6)));
  return `${ dt.getDate() }/${ dt.getMonth() + 1 }/${ dt.getFullYear() }`
};
// JSONToDate(/Date(1489525200000)/) -> "14/3/2017"

toEnglishDate

Converts a date from American format to English format.

Use Date.toISOString(), split('T') and replace() to convert a date from American format to English format. Throws an error if the passed time cannot be converted to a date.

const toEnglishDate  = (time) =>
  {try{return new Date(time).toISOString().split('T')[0].replace(/-/g, '/')}catch(e){return}};
// toEnglishDate('09/21/2010') -> '21/09/2010'

chainAsync

Chains asynchronous functions.

Loop through an array of functions containing asynchronous events, calling next when each asynchronous event has completed.

const chainAsync = fns => { let curr = 0; const next = () => fns[curr++](next); next(); };
/*
chainAsync([
  next => { console.log('0 seconds'); setTimeout(next, 1000); },
  next => { console.log('1 second');  setTimeout(next, 1000); },
  next => { console.log('2 seconds'); }
])
*/

compose

Performs right-to-left function composition.

Use Array.reduce() to perform right-to-left function composition. The last (rightmost) function can accept one or more arguments; the remaining functions must be unary.

const compose = (...fns) => fns.reduce((f, g) => (...args) => f(g(...args)));
/*
const add5 = x => x + 5
const multiply = (x, y) => x * y
const multiplyAndAdd5 = compose(add5, multiply)
multiplyAndAdd5(5, 2) -> 15
*/

curry

Curries a function.

Use recursion. If the number of provided arguments (args) is sufficient, call the passed function f. Otherwise return a curried function f that expects the rest of the arguments. If you want to curry a function that accepts a variable number of arguments (a variadic function, e.g. Math.min()), you can optionally pass the number of arguments to the second parameter arity.

const curry = (fn, arity = fn.length, ...args) =>
  arity <= args.length
    ? fn(...args)
    : curry.bind(null, fn, arity, ...args);
// curry(Math.pow)(2)(10) -> 1024
// curry(Math.min, 3)(10)(50)(2) -> 2

functionName

Logs the name of a function.

Use console.debug() and the name property of the passed method to log the method's name to the debug channel of the console.

const functionName = fn => (console.debug(fn.name), fn);
// functionName(Math.max) -> max (logged in debug channel of console)

pipe

Performs left-to-right function composition.

Use Array.reduce() with the spread operator (...) to perform left-to-right function composition. The first (leftmost) function can accept one or more arguments; the remaining functions must be unary.

const pipeFunctions = (...fns) => fns.reduce((f, g) => (...args) => g(f(...args)));
/*
const add5 = x => x + 5
const multiply = (x, y) => x * y
const multiplyAndAdd5 = pipeFunctions(multiply, add5)
multiplyAndAdd5(5, 2) -> 15
*/

promisify

Converts an asynchronous function to return a promise.

Use currying to return a function returning a Promise that calls the original function. Use the ...rest operator to pass in all the parameters.

In Node 8+, you can use util.promisify

const promisify = func =>
  (...args) =>
    new Promise((resolve, reject) =>
      func(...args, (err, result) =>
        err ? reject(err) : resolve(result))
    );
// const delay = promisify((d, cb) => setTimeout(cb, d))
// delay(2000).then(() => console.log('Hi!')) -> Promise resolves after 2s

runPromisesInSeries

Runs an array of promises in series.

Use Array.reduce() to create a promise chain, where each promise returns the next promise when resolved.

const runPromisesInSeries = ps => ps.reduce((p, next) => p.then(next), Promise.resolve());
// const delay = (d) => new Promise(r => setTimeout(r, d))
// runPromisesInSeries([() => delay(1000), () => delay(2000)]) -> executes each promise sequentially, taking a total of 3 seconds to complete

sleep

Delays the execution of an asynchronous function.

Delay executing part of an async function, by putting it to sleep, returning a Promise.

const sleep = ms => new Promise(resolve => setTimeout(resolve, ms));
/*
async function sleepyWork() {
  console.log('I\'m going to sleep for 1 second.');
  await sleep(1000);
  console.log('I woke up after 1 second.');
}
*/

arrayAverage

Returns the average of an array of numbers.

Use Array.reduce() to add each value to an accumulator, initialized with a value of 0, divide by the length of the array.

const arrayAverage = arr => arr.reduce((acc, val) => acc + val, 0) / arr.length;
// arrayAverage([1,2,3]) -> 2

arraySum

Returns the sum of an array of numbers.

Use Array.reduce() to add each value to an accumulator, initialized with a value of 0.

const arraySum = arr => arr.reduce((acc, val) => acc + val, 0);
// arraySum([1,2,3,4]) -> 10

collatz

Applies the Collatz algorithm.

If n is even, return n/2. Otherwise return 3n+1.

const collatz = n => (n % 2 == 0) ? (n / 2) : (3 * n + 1);
// collatz(8) --> 4
// collatz(5) --> 16

digitize

Converts a number to an array of digits.

Convert the number to a string, using spread operators in ES6([...string]) build an array. Use Array.map() and parseInt() to transform each value to an integer.

const digitize = n => [...''+n].map(i => parseInt(i));
// digitize(2334) -> [2, 3, 3, 4]

distance

Returns the distance between two points.

Use Math.hypot() to calculate the Euclidean distance between two points.

const distance = (x0, y0, x1, y1) => Math.hypot(x1 - x0, y1 - y0);
// distance(1,1, 2,3) -> 2.23606797749979

factorial

Calculates the factorial of a number.

Use recursion. If n is less than or equal to 1, return 1. Otherwise, return the product of n and the factorial of n - 1. Throws an exception if n is a negative number.

const factorial = n =>
  n < 0 ? (() => { throw new TypeError('Negative numbers are not allowed!') })()
  : n <= 1 ? 1 : n * factorial(n - 1);
// factorial(6) -> 720

fibonacci

Generates an array, containing the Fibonacci sequence, up until the nth term.

Create an empty array of the specific length, initializing the first two values (0 and 1). Use Array.reduce() to add values into the array, using the sum of the last two values, except for the first two.

const fibonacci = n =>
  Array(n).fill(0).reduce((acc, val, i) => acc.concat(i > 1 ? acc[i - 1] + acc[i - 2] : i), []);
// fibonacci(5) -> [0,1,1,2,3]

gcd

Calculates the greatest common divisor between two numbers.

Use recursion. Base case is when y equals 0. In this case, return x. Otherwise, return the GCD of y and the remainder of the division x/y.

const gcd = (x, y) => !y ? x : gcd(y, x % y);
// gcd (8, 36) -> 4

hammingDistance

Calculates the Hamming distance between two values.

Use XOR operator (^) to find the bit difference between the two numbers, convert to binary string using toString(2). Count and return the number of 1s in the string, using match(/1/g).

const hammingDistance = (num1, num2) =>
  ((num1 ^ num2).toString(2).match(/1/g) || '').length;
// hammingDistance(2,3) -> 1

isDivisible

Checks if the first numeric argument is divisible by the second one.

Use the modulo operator (%) to check if the remainder is equal to 0.

const isDivisible = (dividend, divisor) => dividend % divisor === 0;
// isDivisible(6,3) -> true

isEven

Returns true if the given number is even, false otherwise.

Checks whether a number is odd or even using the modulo (%) operator. Returns true if the number is even, false if the number is odd.

const isEven = num => num % 2 === 0;
// isEven(3) -> false

lcm

Returns the least common multiple of two numbers.

Use the greatest common divisor (GCD) formula and Math.abs() to determine the least common multiple. The GCD formula uses recursion.

const lcm = (x,y) => {
  const gcd = (x, y) => !y ? x : gcd(y, x % y);
  return Math.abs(x*y)/(gcd(x,y));
};
// lcm(12,7) -> 84

median

Returns the median of an array of numbers.

Find the middle of the array, use Array.sort() to sort the values. Return the number at the midpoint if length is odd, otherwise the average of the two middle numbers.

const median = arr => {
  const mid = Math.floor(arr.length / 2), nums = arr.sort((a, b) => a - b);
  return arr.length % 2 !== 0 ? nums[mid] : (nums[mid - 1] + nums[mid]) / 2;
};
// median([5,6,50,1,-5]) -> 5
// median([0,10,-2,7]) -> 3.5

palindrome

Returns true if the given string is a palindrome, false otherwise.

Convert string toLowerCase() and use replace() to remove non-alphanumeric characters from it. Then, split('') into individual characters, reverse(), join('') and compare to the original, unreversed string, after converting it tolowerCase().

const palindrome = str => {
  const s = str.toLowerCase().replace(/[\W_]/g,'');
  return s === s.split('').reverse().join('');
}
// palindrome('taco cat') -> true

percentile

Uses the percentile formula to calculate how many numbers in the given array are less or equal to the given value.

Use Array.reduce() to calculate how many numbers are below the value and how many are the same value and apply the percentile formula.

const percentile = (arr, val) =>
  100 * arr.reduce((acc,v) => acc + (v < val ? 1 : 0) + (v === val ? 0.5 : 0), 0) / arr.length;
// percentile([1,2,3,4,5,6,7,8,9,10], 6) -> 55

powerset

Returns the powerset of a given array of numbers.

Use Array.reduce() combined with Array.map() to iterate over elements and combine into an array containing all combinations.

const powerset = arr =>
  arr.reduce((a, v) => a.concat(a.map(r => [v].concat(r))), [[]]);
// powerset([1,2]) -> [[], [1], [2], [2,1]]

randomIntegerInRange

Returns a random integer in the specified range.

Use Math.random() to generate a random number and map it to the desired range, using Math.floor() to make it an integer.

const randomIntegerInRange = (min, max) => Math.floor(Math.random() * (max - min + 1)) + min;
// randomIntegerInRange(0, 5) -> 2

randomNumberInRange

Returns a random number in the specified range.

Use Math.random() to generate a random value, map it to the desired range using multiplication.

const randomNumberInRange = (min, max) => Math.random() * (max - min) + min;
// randomNumberInRange(2,10) -> 6.0211363285087005

round

Rounds a number to a specified amount of digits.

Use Math.round() and template literals to round the number to the specified number of digits. Omit the second argument, decimals to round to an integer.

const round = (n, decimals=0) => Number(`${Math.round(`${n}e${decimals}`)}e-${decimals}`);
// round(1.005, 2) -> 1.01

standardDeviation

Returns the standard deviation of an array of numbers.

Use Array.reduce() to calculate the mean, variance and the sum of the variance of the values, the variance of the values, then determine the standard deviation. You can omit the second argument to get the sample standard deviation or set it to true to get the population standard deviation.

const standardDeviation = (arr, usePopulation = false) => {
  const mean = arr.reduce((acc, val) => acc + val, 0) / arr.length;
  return Math.sqrt(
    arr.reduce((acc, val) => acc.concat(Math.pow(val - mean, 2)), [])
       .reduce((acc, val) => acc + val, 0) / (arr.length - (usePopulation ? 0 : 1))
  );
};
// standardDeviation([10,2,38,23,38,23,21]) -> 13.284434142114991 (sample)
// standardDeviation([10,2,38,23,38,23,21], true) -> 12.29899614287479 (population)

speechSynthesis

Performs speech synthesis (experimental).

Use SpeechSynthesisUtterance.voice and window.speechSynthesis.getVoices() to convert a message to speech. Use window.speechSynthesis.speak() to play the message.

Learn more about the SpeechSynthesisUtterance interface of the Web Speech API
.

const speechSynthesis = message => {
  const msg = new SpeechSynthesisUtterance(message);
  msg.voice = window.speechSynthesis.getVoices()[0];
  window.speechSynthesis.speak(msg);
};
// speechSynthesis('Hello, World') -> plays the message

JSONToFile

Writes a JSON object to a file.

Use fs.writeFile(), template literals and JSON.stringify() to write a json object to a .json file.

const fs = require('fs');
const JSONToFile = (obj, filename) => fs.writeFile(`${filename}.json`, JSON.stringify(obj, null, 2))
// JSONToFile({test: "is passed"}, 'testJsonFile') -> writes the object to 'testJsonFile.json'

readFileLines

Returns an array of lines from the specified file.

Use readFileSync function in fs node package to create a Buffer from a file. convert buffer to string using toString(encoding) function. creating an array from contents of file by spliting file content line by line (each \n).

const fs = require('fs');
const readFileLines = filename => fs.readFileSync(filename).toString('UTF8').split('\n');
/*
contents of test.txt :
  line1
  line2
  line3
  ___________________________
let arr = readFileLines('test.txt')
console.log(arr) // -> ['line1', 'line2', 'line3']
*/

cleanObj

Removes any properties except the ones specified from a JSON object.

Use Object.keys() method to loop over given json object and deleting keys that are not included in given array. Also if you give it a special key (childIndicator) it will search deeply inside it to apply function to inner objects too.

const cleanObj = (obj, keysToKeep = [], childIndicator) => {
  Object.keys(obj).forEach(key => {
    if (key === childIndicator) {
      cleanObj(obj[key], keysToKeep, childIndicator);
    } else if (!keysToKeep.includes(key)) {
      delete obj[key];
    }
  })
}
/*
  const testObj = {a: 1, b: 2, children: {a: 1, b: 2}}
  cleanObj(testObj, ["a"],"children")
  console.log(testObj)// { a: 1, children : { a: 1}}
*/

objectFromPairs

Creates an object from the given key-value pairs.

Use Array.reduce() to create and combine key-value pairs.

const objectFromPairs = arr => arr.reduce((a, v) => (a[v[0]] = v[1], a), {});
// objectFromPairs([['a',1],['b',2]]) -> {a: 1, b: 2}

objectToPairs

Creates an array of key-value pair arrays from an object.

Use Object.keys() and Array.map() to iterate over the object's keys and produce an array with key-value pairs.

const objectToPairs = obj => Object.keys(obj).map(k => [k, obj[k]]);
// objectToPairs({a: 1, b: 2}) -> [['a',1],['b',2]])

shallowClone

Creates a shallow clone of an object.

Use Object.assign() and an empty object ({}) to create a shallow clone of the original.

const shallowClone = obj => Object.assign({}, obj);
/*
const a = { x: true, y: 1 };
const b = shallowClone(a);
a === b -> false
*/

truthCheckCollection

Checks if the predicate (second argument) is truthy on all elements of a collection (first argument).

Use Array.every() to check if each passed object has the specified property and if it returns a truthy value.

truthCheckCollection = (collection, pre) => (collection.every(obj => obj[pre]));
// truthCheckCollection([{"user": "Tinky-Winky", "sex": "male"}, {"user": "Dipsy", "sex": "male"}], "sex") -> true

anagrams

Generates all anagrams of a string (contains duplicates).

Use recursion. For each letter in the given string, create all the partial anagrams for the rest of its letters. Use Array.map() to combine the letter with each partial anagram, then Array.reduce() to combine all anagrams in one array. Base cases are for string length equal to 2 or 1.

const anagrams = str => {
  if (str.length <= 2) return str.length === 2 ? [str, str[1] + str[0]] : [str];
  return str.split('').reduce((acc, letter, i) =>
    acc.concat(anagrams(str.slice(0, i) + str.slice(i + 1)).map(val => letter + val)), []);
};
// anagrams('abc') -> ['abc','acb','bac','bca','cab','cba']

Capitalize

Capitalizes the first letter of a string.

Use destructuring and toUpperCase() to capitalize first letter, ...rest to get array of characters after first letter and then Array.join('') to make it a string again. Omit the lowerRest parameter to keep the rest of the string intact, or set it to true to convert to lowercase.

const capitalize = ([first,...rest], lowerRest = false) =>
  first.toUpperCase() + (lowerRest ? rest.join('').toLowerCase() : rest.join(''));
// capitalize('myName') -> 'MyName'
// capitalize('myName', true) -> 'Myname'

capitalizeEveryWord

Capitalizes the first letter of every word in a string.

Use replace() to match the first character of each word and toUpperCase() to capitalize it.

const capitalizeEveryWord = str => str.replace(/\b[a-z]/g, char => char.toUpperCase());
// capitalizeEveryWord('hello world!') -> 'Hello World!'

escapeRegExp

Escapes a string to use in a regular expression.

Use replace() to escape special characters.

const escapeRegExp = str => str.replace(/[.*+?^${}()|[\]\\]/g, '\\$&');
// escapeRegExp('(test)') -> \\(test\\)

fromCamelCase

Converts a string from camelcase.

Use replace() to remove underscores, hyphens and spaces and convert words to camelcase. Omit the second argument to use a default separator of _.

const fromCamelCase = (str, separator = '_') =>
  str.replace(/([a-z\d])([A-Z])/g, '$1' + separator + '$2')
    .replace(/([A-Z]+)([A-Z][a-z\d]+)/g, '$1' + separator + '$2').toLowerCase();
// fromCamelCase('someDatabaseFieldName', ' ') -> 'some database field name'
// fromCamelCase('someLabelThatNeedsToBeCamelized', '-') -> 'some-label-that-needs-to-be-camelized'
// fromCamelCase('someJavascriptProperty', '_') -> 'some_javascript_property'

reverseString

Reverses a string.

Use array destructuring and Array.reverse() to reverse the order of the characters in the string. Combine characters to get a string using join('').

const reverseString = str => [...str].reverse().join('');
// reverseString('foobar') -> 'raboof'

sortCharactersInString

Alphabetically sorts the characters in a string.

Split the string using split(''), Array.sort() utilizing localeCompare(), recombine using join('').

const sortCharactersInString = str =>
  str.split('').sort((a, b) => a.localeCompare(b)).join('');
// sortCharactersInString('cabbage') -> 'aabbceg'

toCamelCase

Converts a string to camelcase.

Use replace() to remove underscores, hyphens and spaces and convert words to camelcase.

const toCamelCase = str =>
  str.replace(/^([A-Z])|[\s-_]+(\w)/g, (match, p1, p2, offset) =>  p2 ? p2.toUpperCase() : p1.toLowerCase());
// toCamelCase("some_database_field_name") -> 'someDatabaseFieldName'
// toCamelCase("Some label that needs to be camelized") -> 'someLabelThatNeedsToBeCamelized'
// toCamelCase("some-javascript-property") -> 'someJavascriptProperty'
// toCamelCase("some-mixed_string with spaces_underscores-and-hyphens") -> 'someMixedStringWithSpacesUnderscoresAndHyphens'

truncateString

Truncates a string up to a specified length.

Determine if the string's length is greater than num. Return the string truncated to the desired length, with ... appended to the end or the original string.

const truncateString = (str, num) =>
  str.length > num ? str.slice(0, num > 3 ? num - 3 : num) + '...' : str;
// truncateString('boomerang', 7) -> 'boom...'

coalesce

Returns the first non-null/undefined argument.

Use Array.find() to return the first non null/undefined argument.

const coalesce = (...args) => args.find(_ => ![undefined, null].includes(_))
// coalesce(null,undefined,"",NaN, "Waldo") -> ""

coalesceFactory

Returns a customized coalesce function that returns the first argument that returns true from the provided argument validation function.

Use Array.find() to return the first argument that returns true from the provided argument validation function.

const coalesceFactory = valid => (...args) => args.find(valid);
// const customCoalesce = coalesceFactory(_ => ![null, undefined, "", NaN].includes(_))
// customCoalesce(undefined, null, NaN, "", "Waldo") //-> "Waldo"

extendHex

Extends a 3-digit color code to a 6-digit color code.

Use Array.map(), split() and Array.join() to join the mapped array for converting a 3-digit RGB notated hexadecimal color-code to the 6-digit form. Array.slice() is used to remove # from string start since it's added once.

const extendHex = shortHex =>
  '#' + shortHex.slice(shortHex.startsWith('#') ? 1 : 0).split('').map(x => x+x).join('')
// extendHex('#03f') -> '#0033ff'
// extendHex('05a') -> '#0055aa'

getType

Returns the native type of a value.

Returns lowercased constructor name of value, "undefined" or "null" if value is undefined or null

const getType = v =>
  v === undefined ? 'undefined' : v === null ? 'null' : v.constructor.name.toLowerCase();
// getType(new Set([1,2,3])) -> "set"

hexToRGB

Converts a colorcode to a rgb() string.

Use bitwise right-shift operator and mask bits with & (and) operator to convert a hexadecimal color code (prefixed with #) to a string with the RGB values. In case it's a 3-digit-colorcode, do the same with the 6-digit-colorcode extended by the extendHex() function (ref. extendHex snippet)

const hexToRgb = hex => {
  const extendHex = shortHex =>
    '#' + shortHex.slice(shortHex.startsWith('#') ? 1 : 0).split('').map(x => x+x).join('');
  const extendedHex = hex.slice(hex.startsWith('#') ? 1 : 0).length === 3 ? extendHex(hex) : hex;
  return `rgb(${parseInt(extendedHex.slice(1), 16) >> 16}, ${(parseInt(extendedHex.slice(1), 16) & 0x00ff00) >> 8}, ${parseInt(extendedHex.slice(1), 16) & 0x0000ff})`;
}
// hexToRgb('#27ae60') -> 'rgb(39, 174, 96)'
// hexToRgb('#acd') -> 'rgb(170, 204, 221)'

isArray

Checks if the given argument is an array.

Use Array.isArray() to check if a value is classified as an array.

const isArray = val => !!val && Array.isArray(val);
// isArray(null) -> false
// isArray([1]) -> true

isBoolean

Checks if the given argument is a native boolean element.

Use typeof to check if a value is classified as a boolean primitive.

const isBoolean = val => typeof val === 'boolean';
// isBoolean(null) -> false
// isBoolean(false) -> true

isFunction

Checks if the given argument is a function.

Use typeof to check if a value is classified as a function primitive.

const isFunction = val => val && typeof val === 'function';
// isFunction('x') -> false
// isFunction(x => x) -> true

isNumber

Checks if the given argument is a number.

Use typeof to check if a value is classified as a number primitive.

const isNumber = val => typeof val === 'number';
// isNumber('1') -> false
// isNumber(1) -> true

isString

Checks if the given argument is a string.

Use typeof to check if a value is classified as a string primitive.

const isString = val => typeof val === 'string';
// isString(10) -> false
// isString('10') -> true

isSymbol

Checks if the given argument is a symbol.

Use typeof to check if a value is classified as a symbol primitive.

const isSymbol = val => typeof val === 'symbol';
// isSymbol('x') -> false
// isSymbol(Symbol('x')) -> true

RGBToHex

Converts the values of RGB components to a colorcode.

Convert given RGB parameters to hexadecimal string using bitwise left-shift operator (<<) and toString(16), then padStart(6,'0') to get a 6-digit hexadecimal value.

const RGBToHex = (r, g, b) => ((r << 16) + (g << 8) + b).toString(16).padStart(6, '0');
// RGBToHex(255, 165, 1) -> 'ffa501'

timeTaken

Measures the time taken by a function to execute.

Use console.time() and console.timeEnd() to measure the difference between the start and end times to determine how long the callback took to execute.

const timeTaken = callback => {
  console.time('timeTaken');  const r = callback();
  console.timeEnd('timeTaken');  return r;
};
// timeTaken(() => Math.pow(2, 10)) -> 1024
// (logged): timeTaken: 0.02099609375ms

toOrdinalSuffix

Adds an ordinal suffix to a number.

Use the modulo operator (%) to find values of single and tens digits. Find which ordinal pattern digits match. If digit is found in teens pattern, use teens ordinal.

const toOrdinalSuffix = num => {
  const int = parseInt(num), digits = [(int % 10), (int % 100)],
    ordinals = ['st', 'nd', 'rd', 'th'], oPattern = [1, 2, 3, 4],
    tPattern = [11, 12, 13, 14, 15, 16, 17, 18, 19];
  return oPattern.includes(digits[0]) && !tPattern.includes(digits[1]) ? int + ordinals[digits[0] - 1] : int + ordinals[3];
};
// toOrdinalSuffix("123") -> "123rd"

UUIDGenerator

Generates a UUID.

Use crypto API to generate a UUID, compliant with RFC4122
version 4.

const UUIDGenerator = () =>
  ([1e7] + -1e3 + -4e3 + -8e3 + -1e11).replace(/[018]/g, c =>
    (c ^ crypto.getRandomValues(new Uint8Array(1))[0] & 15 >> c / 4).toString(16)
  );
// UUIDGenerator() -> '7982fcfe-5721-4632-bede-6000885be57d'

validateEmail

Returns true if the given string is a valid email, false otherwise.

Use a regular expression to check if the email is valid. Returns true if email is valid, false if not.

const validateEmail = str =>
  /^(([^<>()\[\]\\.,;:\s@"]+(\.[^<>()\[\]\\.,;:\s@"]+)*)|(".+"))@((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\])|(([a-zA-Z\-0-9]+\.)+[a-zA-Z]{2,}))$/.test(str);
// validateEmail(mymail@gmail.com) -> true

validateNumber

Returns true if the given value is a number, false otherwise.

Use !isNaN in combination with parseFloat() to check if the argument is a number. Use isFinite() to check if the number is finite. Use Number() to check if the coercion holds.

const validateNumber = n => !isNaN(parseFloat(n)) && isFinite(n) && Number(n) == n;
// validateNumber('10') -> true