30 seconds of code

Curated collection of useful Javascript snippets that you can understand in 30 seconds or less.

• Use Ctrl + F or command + F to search for a snippet.
• Contributions welcome, please read the contribution guide.
• Snippets are written in ES6, use the Babel transpiler to ensure backwards-compatibility.

Contents

• Anagrams of string (with duplicates)
• Average of array of numbers
• Bottom visible
• Capitalize first letter of every word
• Capitalize first letter
• Chain asynchronous functions
• Check for palindrome
• Chunk array
• Count occurrences of a value in array
• Current URL
• Curry
• Deep flatten array
• Difference between arrays
• Distance between two points
• Divisible by number
• Escape regular expression
• Even or odd number
• Factorial
• Fibonacci array generator
• Filter out non unique values in an array
• Flatten array
• Get max value from array
• Get min value from array
• Get native type of value
• Get scroll position
• Greatest common divisor (GCD)
• Hamming distance
• Head of list
• Initial of list
• Initialize array with range
• Initialize array with values
• Last of list
• Measure time taken by function
• Median of array of numbers
• Object from key value pairs
• Percentile
• Pipe
• Powerset
• Promisify
• Random integer in range
• Random number in range
• Randomize order of array
• Redirect to url
• Reverse a string
• RGB to hexadecimal
• Run promises in series
• Scroll to top
• Shuffle array values
• Similarity between arrays
• Sort characters in string (alphabetical)
• Sum of array of numbers
• Swap values of two variables
• Tail of list
• Truncate a string
• Unique values of array
• URL parameters
• UUID generator
• Validate number
• Value or default

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']

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

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

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!'

Capitalize first letter

Use slice(0,1) and toUpperCase() to capitalize first letter, slice(1) to get the rest of the string. Omit the lowerRest parameter to keep the rest of the string intact, or set it to true to convert to lower case.

const capitalize = (str, lowerRest = false) =>
  str.slice(0, 1).toUpperCase() + (lowerRest? str.slice(1).toLowerCase() : str.slice(1));
// capitalize('myName', true) -> 'Myname'

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'); }
])
*/

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

Chunk array

Use Array.apply() to create a new array, that fits the number of chunks that will be produced. Use Array.map() 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.apply(null, {length: Math.ceil(arr.length/size)}).map((v, i) => arr.slice(i*size, i*size+size));
// chunk([1,2,3,4,5], 2) -> [[1,2],[3,4],5]

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

Current URL

Use window.location.href to get current URL.

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

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 = (f, arity = f.length, next) =>
  (next = prevArgs =>
    nextArg => {
      const args = [ ...prevArgs, nextArg ];
      return args.length >= arity ? f(...args) : next(args);
    }
  )([]);
// curry(Math.pow)(2)(10) -> 1024
// curry(Math.min, 3)(10)(50)(2) -> 2

Deep flatten array

Use recursion. Use Array.reduce() to get all elements that are not arrays, flatten each element that is an array.

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

Difference between arrays

Use filter() to remove values that are part of values, determined using includes().

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

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

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

Escape regular expression

Use replace() to escape special characters.

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

Even or odd number

Use Math.abs() to extend logic to negative numbers, check using the modulo (%) operator. Return true if the number is even, false if the number is odd.

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

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

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]

Filter out non-unique values in an array

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

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

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]

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

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

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"

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}

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

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

Head of list

Return arr[0].

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

Initial of list

Return arr.slice(0,-1).

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

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.apply(null, Array(end-start)).map( (v,i) => i + start );
// initializeArrayRange(5) -> [0,1,2,3,4]

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]

Last of list

Return arr.slice(-1)[0].

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

Measure time taken by function

Use performance.now() to get start and end time for the function, console.log() the time taken. Pass a callback function as the argument.

const timeTaken = callback => {
  const t0 = performance.now(), r = callback();
  console.log(performance.now() - t0);
  return r;
}
// timeTaken(() => Math.pow(2, 10)) -> 1024 (0.010000000009313226 logged in console)

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

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}

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

Pipe

Use Array.reduce() to pass value through functions.

const pipe = (...funcs) => arg => funcs.reduce((acc, func) => func(acc), arg);
// pipe(btoa, x => x.toUpperCase())("Test") -> "VGVZDA=="

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]]

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

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

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

Randomize order of array

Use Array.sort() to reorder elements, utilizing Math.random() to randomize the sorting.

const randomizeOrder = arr => arr.sort( (a,b) => Math.random() >= 0.5 ? -1 : 1);
// randomizeOrder([1,2,3]) -> [1,3,2]

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')

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'

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'

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

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()

Shuffle array values

Create an array of random values by using Array.map() and Math.random(). Use Array.sort() to sort the elements of the original array based on the random values.

const shuffle = arr => {
  let r = arr.map(Math.random);
  return arr.sort((a,b) => r[a] - r[b]);
}
// shuffle([1,2,3]) -> [2, 1, 3]

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]

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'

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

Swap values of two variables

Use array destructuring to swap values between two variables.

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

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]

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...'

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]

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('=')), a), {}
  );
// getUrlParameters('http://url.com/page?name=Adam&surname=Smith') -> {name: 'Adam', surname: 'Smith'}

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'

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.

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

Value or default

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

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

Credits

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