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Array

Array concatenation

Use Array spread operators (...) to concatenate an array with any additional arrays and/or values, specified in args.

const ArrayConcat = (arr, ...args) => [...arr,...args]; 
// ArrayConcat([1], [1, 2, 3, [4]]) -> [1, 1, 2, 3, [4]]

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Array difference

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]) -> [3]

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Array includes

Use slice() to offset the array/string and indexOf() to check if the value is included. Omit the last argument, fromIndex, to check the whole array/string.

const includes = (collection, val, fromIndex=0) => collection.slice(fromIndex).indexOf(val) != -1;
// includes("30-seconds-of-code", "code") -> true
// includes([1, 2, 3, 4], [1, 2], 1) -> false

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Array intersection

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]

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Array remove

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]

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Array sample

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

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Array union

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]

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Array without

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]
// without([2, 1, 2, 3, 4, 5, 5, 5, 3, 2, 7, 7], 3, 1, 5, 2) -> [ 4, 7, 7 ]

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Array zip

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]]

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

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Chunk array

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]]

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Compact

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

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

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Count occurrences of a value in 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

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Deep flatten 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]

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Drop elements in 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]

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Fill array

Use Array.map() to map values between start (inclusive) and end (exclusive) to value. Omit start to start at the first element and/or end to finish at the last.

const fillArray = (arr, value, start = 0, end = arr.length) =>
  arr.map((v, i) => i >= start && i < end ? value : v);
// fillArray([1,2,3,4],'8',1,3) -> [1,'8','8',4]

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Filter out 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]

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Flatten array up to 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]

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Flatten 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]

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Get max value from 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

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Get min value from 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

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Group by

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']}

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Head of list

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

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

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Initial of list

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]

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

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Initialize array with 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 initializeArray = (n, value = 0) => Array(n).fill(value);
// initializeArray(5, 2) -> [2,2,2,2,2]

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Last of list

Use arr.slice(-1)[0] to get the last element of the given array.

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

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Median of 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

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Nth element of 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 nth = (arr, n=0) => (n>0? arr.slice(n,n+1) : arr.slice(n))[0];
// nth(['a','b','c'],1) -> 'b'
// nth(['a','b','b']-2) -> 'a'

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Pick

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 }
// pick(object, ['a', 'c'])['a'] -> 1

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Shuffle 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]

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Similarity between 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]

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Sum of array of numbers

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

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

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Tail of list

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]

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Take every nth element

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 ]

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Take right

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]

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Take

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) -> []

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Unique values of array

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

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

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Browser

Bottom visible

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

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Current URL

Use window.location.href to get current URL.

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

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Element is visible in viewport

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)

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Get scroll position

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

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

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Redirect to 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')

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Scroll to top

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()

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Date

Get days difference between 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

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Function

Chain 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'); }
])
*/

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Compose functions

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
*/

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Curry

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

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Pipe functions

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

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Promisify

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

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Run promises in series

Run an array of promises in series using Array.reduce() by creating a promise chain, where each promise returns the next promise when resolved.

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

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Sleep

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.');
}
*/

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Math

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

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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

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Divisible by number

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

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

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Even or odd number

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

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Factorial

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.

const factorial = n => n <= 1 ? 1 : n * factorial(n - 1);
// factorial(6) -> 720

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Fibonacci array generator

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]

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Greatest common divisor (GCD)

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

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Hamming distance

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

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Percentile

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

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Powerset

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]]

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Round number to n 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

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Standard deviation

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)

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Media

Speech synthesis (experimental)

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

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Node

Read file as array of lines

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

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Write JSON to 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'

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Object

Object from 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}

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Object to key-value pairs

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]])

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Shallow clone 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
*/

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String

Anagrams of string (with 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']

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Capitalize first letter of every word

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!'

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Capitalize first letter

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 lower case.

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

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Check for palindrome

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

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Reverse 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'

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Sort characters in string (alphabetical)

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'

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Truncate a String

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

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Utility

3-digit hexcode to 6-digit hexcode

Use Array.map(), split() and Array.join() to join the mapped array for converting a three-digit RGB notated hexadecimal colorcode to the six-digit form.

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

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Escape regular expression

Use replace() to escape special characters.

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

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Get native type of value

Returns lower-cased 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"

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Hexcode to RGB

Use Array.slice(), Array.map() and match() to convert a hexadecimal colorcode (prefixed with #) to a string with the RGB values.

const hexToRgb = hex => `rgb(${hex.slice(1).match(/.{2}/g).map(x => parseInt(x, 16)).join()})`
// hexToRgb('#27ae60') -> 'rgb(39,174,96)'

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Is 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

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Is boolean

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

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Is 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

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Is 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

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Is 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

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Is 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

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Measure time taken by function

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

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Number to array of digits

Convert the number to a string, use split() to convert build an array. Use Array.map() and parseInt() to transform each value to an integer.

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

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Ordinal suffix of 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"

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Random integer in 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

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Random number in range

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

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

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RGB to hexadecimal

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'

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Swap values of two variables

Use array destructuring to swap values between two variables.

[varA, varB] = [varB, varA];
// [x, y] = [y, x]

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URL parameters

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'}

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UUID generator

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

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

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Validate email

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

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Validate number

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

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Value or default

Returns value, or default value if passed value is falsy.

const valueOrDefault = (value, d) => value || d;
// valueOrDefault(NaN, 30) -> 30

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Credits

Icons made by Smashicons from www.flaticon.com is licensed by CC 3.0 BY.

README.md

30 seconds of code

Table of Contents

Array

Browser

Date

Function

Math

Media

Node

Object

String

Utility

Array

Array concatenation

Array difference

Array includes

Array intersection

Array remove

Array sample

Array union

Array without

Array zip

Average of array of numbers

Chunk array

Compact

Count occurrences of a value in array

Deep flatten array

Drop elements in array

Fill array

Filter out non-unique values in an array

Flatten array up to depth

Flatten array

Get max value from array

Get min value from array

Group by

Head of list

Initial of list

Initialize array with range

Initialize array with values

Last of list

Median of array of numbers

Nth element of array

Pick

Shuffle array

Similarity between arrays

Sum of array of numbers

Tail of list

Take every nth element

Take right

Take

Unique values of array

Browser

Bottom visible

Current URL

Element is visible in viewport

Get scroll position

Redirect to URL

Scroll to top

Date

Get days difference between dates

Function

Chain asynchronous functions

Compose functions

Curry

Pipe functions

Promisify

Run promises in series

Sleep

Math

Collatz algorithm

Distance between two points

Divisible by number

Even or odd number

Factorial

Fibonacci array generator

Greatest common divisor (GCD)

Hamming distance

Percentile

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