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.
• You can import these snippets into your text editor of choice (VSCode, Atom, Sublime) using the files found in this repo.
• You can import these snippets into Alfred 3, using this file.
• You can find a package with all the snippets on npm. Bear in mind that most of these snippets are not production-ready.

Given a key and a set of arguments, call them when given a context. Primarily useful in composition.

Use a closure to call a stored key with stored arguments.

const call = (key, ...args) => context => context[key](...args);

Promise.resolve([1, 2, 3])
  .then(call('map', x => 2 * x))
  .then(console.log); //[ 2, 4, 6 ]
const map = call.bind(null, 'map');
Promise.resolve([1, 2, 3])
  .then(map(x => 2 * x))
  .then(console.log); //[ 2, 4, 6 ]

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Changes a function that accepts an array into a variadic function.

Given a function, return a closure that collects all inputs into an array-accepting function.

const collectInto = fn => (...args) => fn(args);

const Pall = collectInto(Promise.all.bind(Promise));
let p1 = Promise.resolve(1);
let p2 = Promise.resolve(2);
let p3 = new Promise(resolve => setTimeout(resolve, 2000, 3));
Pall(p1, p2, p3).then(console.log);

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Flip takes a function as an argument, then makes the first argument the last

Return a closure that takes variadic inputs, and splices the last argument to make it the first argument before applying the rest.

const flip = fn => (...args) => fn(args.pop(), ...args);

let a = { name: 'John Smith' };
let b = {};
const mergeFrom = flip(Object.assign);
let mergePerson = mergeFrom.bind(null, a);
mergePerson(b); // == b
b = {};
Object.assign(b, a); // == b

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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

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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

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Takes a variadic function and returns a closure that accepts an array of arguments to map to the inputs of the function.

Use closures and the spread operator (...) to map the array of arguments to the inputs of the function.

const spreadOver = fn => argsArr => fn(...argsArr);

const arrayMax = spreadOver(Math.max);
arrayMax([1, 2, 3]); // 3
arrayMax([1, 2, 4]); // 4

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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]]

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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 ]

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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

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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]

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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]

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Filters out all values from an array for which the comparator function does not return true.

Use Array.filter() and Array.find() to find the appropriate values.

const differenceWith = (arr, val, comp) => arr.filter(a => !val.find(b => comp(a, b)));

differenceWith([1, 1.2, 1.5, 3], [1.9, 3], (a, b) => Math.round(a) == Math.round(b)); // [1, 1.2]

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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]

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

Loop through the array, using Array.slice() 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 = arr.slice(1);
  return arr;
};

dropElements([1, 2, 3, 4], n => n >= 3); // [3,4]

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Returns a new array with n elements removed from the right.

Use Array.slice() to slice the remove the specified number of elements from the right.

const dropRight = (arr, n = 1) => arr.slice(0, -n);

dropRight([1, 2, 3]); // [1,2]
dropRight([1, 2, 3], 2); // [1]
dropRight([1, 2, 3], 42); // []

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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 === nth - 1);

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

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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]

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Flattens an array.

Use a new array and concatenate it with the spread input array causing a shallow denesting of any contained arrays.

const flatten = arr => [].concat(...arr);

flatten([1, [2], 3, 4]); // [1,2,3,4]

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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]); // [1,2,3,4]

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Groups the elements 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']}

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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

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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]

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Initializes a 2D array of given width and height and value.

Use Array.map() to generate h rows where each is a new array of size w initialize with value. If the value is not provided, default to null.

const initialize2DArray = (w, h, val = null) =>
  Array(h)
    .fill()
    .map(() => Array(w).fill(val));

initialize2DArray(2, 2, 0); // [[0,0], [0,0]]

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Initializes an array containing the numbers in the specified range where start and end are inclusive.

Use Array((end + 1) - 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 + 1 - start }).map((v, i) => i + start);

initializeArrayWithRange(5); // [0,1,2,3,4,5]
initializeArrayWithRange(7, 3); // [3,4,5,6,7]

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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]

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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]

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Returns 1 if the array is sorted in ascending order, -1 if it is sorted in descending order or 0 if it is not sorted.

Calculate the ordering direction for the first two elements. Use Object.entries() to loop over array objects and compare them in pairs. Return 0 if the direction changes or the direction if the last element is reached.

const isSorted = arr => {
  const direction = arr[0] > arr[1] ? -1 : 1;
  for (let [i, val] of arr.entries())
    if (i === arr.length - 1) return direction;
    else if ((val - arr[i + 1]) * direction > 0) return 0;
};

isSorted([0, 1, 2, 3]); // 1
isSorted([0, 1, 2, 2]); // 1
isSorted([4, 3, 2]); // -1
isSorted([4, 3, 5]); // 0

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Joins all elements of an array into a string and returns this string. Uses a separator and an end separator.

Use Array.reduce() to combine elements into a string. Omit the second argument, separator, to use a default separator of ','. Omit the third argument, end, to use the same value as separator by default.

const join = (arr, separator = ',', end = separator) =>
  arr.reduce(
    (acc, val, i) =>
      i == arr.length - 2
        ? acc + val + end
        : i == arr.length - 1 ? acc + val : acc + val + separator,
    ''
  );

join(); // ''
join(['pen', 'pineapple', 'apple', 'pen'], ',', '&'); //"pen,pineapple,apple&pen"
join(['pen', 'pineapple', 'apple', 'pen'], ','); //"pen,pineapple,apple,pen"
join(['pen', 'pineapple', 'apple', 'pen']); //"pen,pineapple,apple,pen"

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Returns the last element in an array.

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

const last = arr => arr[arr.length - 1];

last([1, 2, 3]); // 3

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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 store 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 }

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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'

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

Use Array.reduce() to convert the filtered/picked keys back to an object with the corresponding key-value pair if the key exists 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 }

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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 an array by resetting it's length to zero and Array.push() to re-populate it with only the pulled values.

(For a snippet that does not mutate the original array see without)

const pull = (arr, ...args) => {
  let argState = Array.isArray(args[0]) ? args[0] : args;
  let pulled = arr.filter((v, i) => !argState.includes(v));
  arr.length = 0;
  pulled.forEach(v => arr.push(v));
};

let myArray1 = ['a', 'b', 'c', 'a', 'b', 'c'];
pull(myArray1, 'a', 'c');
console.log(myArray1); // [ 'b', 'b' ]

let myArray2 = ['a', 'b', 'c', 'a', 'b', 'c'];
pull(myArray2, ['a', 'c']);
console.log(myArray2); // [ 'b', 'b' ]

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Mutates the original array to filter out the values at the specified indexes.

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

const pullAtIndex = (arr, pullArr) => {
  let removed = [];
  let pulled = arr
    .map((v, i) => (pullArr.includes(i) ? removed.push(v) : v))
    .filter((v, i) => !pullArr.includes(i));
  arr.length = 0;
  pulled.forEach(v => arr.push(v));
  return removed;
};

let myArray = ['a', 'b', 'c', 'd'];
let pulled = pullAtIndex(myArray, [1, 3]);

console.log(myArray); // [ 'a', 'c' ]
console.log(pulled); // [ 'b', 'd' ]

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Mutates the original array to filter out the values specified. Returns the removed elements.

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

const pullAtValue = (arr, pullArr) => {
  let removed = [],
    pushToRemove = arr.forEach((v, i) => (pullArr.includes(v) ? removed.push(v) : v)),
    mutateTo = arr.filter((v, i) => !pullArr.includes(v));
  arr.length = 0;
  mutateTo.forEach(v => arr.push(v));
  return removed;
};

let myArray = ['a', 'b', 'c', 'd'];
let pulled = pullAtValue(myArray, ['b', 'd']);
console.log(myArray); // [ 'a', 'c' ]
console.log(pulled); // [ 'b', 'd' ]

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QuickSort an Array (ascending sort by default).

Use recursion. Use Array.filter and spread operator (...) to create an array that all elements with values less than the pivot come before the pivot, and all elements with values greater than the pivot come after it. If the parameter desc is truthy, return array sorts in descending order.

const quickSort = ([n, ...nums], desc) =>
  isNaN(n)
    ? []
    : [
        ...quickSort(nums.filter(v => (desc ? v > n : v <= n)), desc),
        n,
        ...quickSort(nums.filter(v => (!desc ? v > n : v <= n)), desc)
      ];

quickSort([4, 1, 3, 2]); // [1,2,3,4]
quickSort([4, 1, 3, 2], true); // [4,3,2,1]

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Filter an array of objects based on a condition while also filtering out unspecified keys.

Use Array.filter() to filter the array based on the predicate fn so that it returns the objects for which the condition returned a truthy value. On the filtered array, use Array.map() to return the new object using Array.reduce() to filter out the keys which were not supplied as the keys argument.

const reducedFilter = (data, keys, fn) =>
  data.filter(fn).map(el =>
    keys.reduce((acc, key) => {
      acc[key] = el[key];
      return acc;
    }, {})
  );

const data = [
  {
    id: 1,
    name: 'john',
    age: 24
  },
  {
    id: 2,
    name: 'mike',
    age: 50
  }
];

reducedFilter(data, ['id', 'name'], item => item.age > 24); // [{ id: 2, name: 'mike'}]

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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]

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Returns a random element from an array.

Use Math.random() to generate a random number, multiply it by 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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Gets n random elements at unique keys from array up to the size of array.

Shuffle the array using the Fisher-Yates algorithm. Use Array.slice() to get the first n elements. Omit the second argument, n to get only one element at random from the array.

const sampleSize = ([...arr], n = 1) => {
  let m = arr.length;
  while (m) {
    const i = Math.floor(Math.random() * m--);
    [arr[m], arr[i]] = [arr[i], arr[m]];
  }
  return arr.slice(0, n);
};

sampleSize([1, 2, 3], 2); // [3,1]
sampleSize([1, 2, 3], 4); // [2,3,1]

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Randomizes the order of the values of an array, returning a new array.

Uses the Fisher-Yates algoritm to reorder the elements of the array, based on the Lodash implementation, but as a pure function.

const shuffle = ([...arr]) => {
  let m = arr.length;
  while (m) {
    const i = Math.floor(Math.random() * m--);
    [arr[m], arr[i]] = [arr[i], arr[m]];
  }
  return arr;
};

const foo = [1, 2, 3];
shuffle(foo); // [2,3,1]
console.log(foo); // [1,2,3]

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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]

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Returns the lowest index at which value should be inserted into array in order to maintain its sort order.

Check if the array is sorted in descending order (loosely). Use Array.findIndex() to find the appropriate index where the element should be inserted.

const sortedIndex = (arr, n) => {
  const isDescending = arr[0] > arr[arr.length - 1];
  const index = arr.findIndex(el => (isDescending ? n >= el : n <= el));
  return index === -1 ? arr.length : index;
};

sortedIndex([5, 3, 2, 1], 4); // 1
sortedIndex([30, 50], 40); // 1

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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]

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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]

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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); // []

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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]

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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]

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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.

(For a snippet that mutates the original array see pull)

const without = (arr, ...args) => arr.filter(v => !args.includes(v));

without([2, 1, 2, 3], 1, 2); // [3]

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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]]

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Given an array of valid property identifiers and an array of values, return an object associating the properties to the values.

Since an object can have undefined values but not undefined property pointers, the array of properties is used to decide the structure of the resulting object using Array.reduce().

const zipObject = (props, values) =>
  props.reduce((obj, prop, index) => ((obj[prop] = values[index]), obj), {});

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

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Converts the given array elements into <li> tags and appends them to the list of the given id.

Use Array.map() and document.querySelector() to create a list of html tags.

const arrayToHtmlList = (arr, listID) =>
  arr.map(item => (document.querySelector('#' + listID).innerHTML += `<li>${item}</li>`));

arrayToHtmlList(['item 1', 'item 2'], 'myListID');

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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

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Copy a string to the clipboard. Only works as a result of user action (i.e. inside a click event listener).

Create a new <textarea> element, fill it with the supplied data and add it to the HTML document. Use Selection.getRangeAt()to store the selected range (if any). Use document.execCommand('copy') to copy to the clipboard. Remove the <textarea> element from the HTML document. Finally, use Selection().addRange() to recover the original selected range (if any).

const copyToClipboard = str => {
  const el = document.createElement('textarea');
  el.value = str;
  el.setAttribute('readonly', '');
  el.style.position = 'absolute';
  el.style.left = '-9999px';
  document.body.appendChild(el);
  const selected =
    document.getSelection().rangeCount > 0 ? document.getSelection().getRangeAt(0) : false;
  el.select();
  document.execCommand('copy');
  document.body.removeChild(el);
  if (selected) {
    document.getSelection().removeAllRanges();
    document.getSelection().addRange(selected);
  }
};

copyToClipboard('Lorem ipsum'); // 'Lorem ipsum' copied to clipboard.

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Returns the current URL.

Use window.location.href to get current URL.

const currentURL = () => window.location.href;

currentURL(); // 'https://google.com'

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Detects wether the website is being opened in a mobile device or a desktop/laptop.

Use a regular expression to test the navigator.userAgent property to figure out if the device is a mobile device or a desktop/laptop.

const detectDeviceType = () =>
  /Android|webOS|iPhone|iPad|iPod|BlackBerry|IEMobile|Opera Mini/i.test(navigator.userAgent)
    ? 'Mobile'
    : 'Desktop';

detectDeviceType(); // "Mobile"
detectDeviceType(); // "Desktop"

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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();
  const { innerHeight, innerWidth } = window;
  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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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}

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Returns the value of a CSS rule for the specified element.

Use Window.getComputedStyle() to get the value of the CSS rule for the specified element.

const getStyle = (el, ruleName) => getComputedStyle(el)[ruleName];

getStyle(document.querySelector('p'), 'font-size'); // '16px'

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Returns true if the element has the specified class, false otherwise.

Use element.classList.contains() to check if the element has the specified class.

const hasClass = (el, className) => el.classList.contains(className);

hasClass(document.querySelector('p.special'), 'special'); // true

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Hides all the elements specified.

Use the spread operator (...) and Array.forEach() to apply display: none to each element specified.

const hide = (...el) => [...el].forEach(e => (e.style.display = 'none'));

hide(document.querySelectorAll('img')); // Hides all <img> elements on the page

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Redirects the page to HTTPS if its currently in HTTP. Also, pressing the back button doesn't take it back to the HTTP page as its replaced in the history.

Use location.protocol to get the protocol currently being used. If it's not HTTPS, use location.replace() to replace the existing page with the HTTPS version of the page. Use location.href to get the full address, split it with String.split() and remove the protocol part of the URL.

const httpsRedirect = () => {
  if (location.protocol !== 'https:') location.replace('https://' + location.href.split('//')[1]);
};

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Run the callback whenever the user input type changes (mouse or touch). Useful for enabling/disabling code depending on the input device. This process is dynamic and works with hybrid devices (e.g. touchscreen laptops).

Use two event listeners. Assume mouse input initially and bind a touchstart event listener to the document. On touchstart, add a mousemove event listener to listen for two consecutive mousemove events firing within 20ms, using performance.now(). Run the callback with the input type as an argument in either of these situations.

const onUserInputChange = callback => {
  let type = 'mouse',
    lastTime = 0;
  const mousemoveHandler = () => {
    const now = performance.now();
    if (now - lastTime < 20)
      (type = 'mouse'), callback(type), document.removeEventListener('mousemove', mousemoveHandler);
    lastTime = now;
  };
  document.addEventListener('touchstart', () => {
    if (type === 'touch') return;
    (type = 'touch'), callback(type), document.addEventListener('mousemove', mousemoveHandler);
  });
};

onUserInputChange(type => {
  console.log('The user is now using', type, 'as an input method.');
});

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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');

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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 the 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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Sets the value of a CSS rule for the specified element.

Use element.style to set the value of the CSS rule for the specified element to value.

const setStyle = (el, ruleName, value) => (el.style[ruleName] = value);

setStyle(document.querySelector('p'), 'font-size', '20px'); // The first <p> element on the page will have a font-size of 20px

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Shows all the elements specified.

Use the spread operator (...) and Array.forEach() to clear the display property for each element specified.

const show = (...el) => [...el].forEach(e => (e.style.display = ''));

show(document.querySelectorAll('img')); // Shows all <img> elements on the page

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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

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Toggle a class for an element.

Use element.classList.toggle() to toggle the specified class for the element.

const toggleClass = (el, className) => el.classList.toggle(className);

toggleClass(document.querySelector('p.special'), 'special'); // The paragraph will not have the 'special' class anymore

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Generates a UUID in a browser.

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

const UUIDGeneratorBrowser = () =>
  ([1e7] + -1e3 + -4e3 + -8e3 + -1e11).replace(/[018]/g, c =>
    (c ^ (crypto.getRandomValues(new Uint8Array(1))[0] & (15 >> (c / 4)))).toString(16)
  );

UUIDGeneratorBrowser(); // '7982fcfe-5721-4632-bede-6000885be57d'

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Returns the difference (in days) between two dates.

Calculate the difference (in days) between two 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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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"

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Converts a date from American format to English format.

Use Date.toISOString(), split('T') and replace() to convert a date from American format to the 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) {}
};

toEnglishDate('09/21/2010'); // '21/09/2010'

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Results in a string representation of tomorrow's date. Use new Date() to get today's date, adding 86400000 of seconds to it(24 hours), using toISOString to convert Date object to string.

const tomorrow = () => new Date(new Date().getTime() + 86400000).toISOString().split('T')[0];

tomorrow(); // 2017-12-27 (if current date is 2017-12-26)

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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');
  }
]);

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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

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Curries a function.

Use recursion. If the number of provided arguments (args) is sufficient, call the passed function fn. Otherwise, return a curried function fn 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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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)

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Returns the memoized (cached) function.

Use Object.create(null) to create an empty object without Object.prototype (so that those properties are not resolved if the input value is something like 'hasOwnProperty'). Return a function which takes a single argument to be supplied to the memoized function by first checking if the function's output for that specific input value is already cached, or store and return it if not.

const memoize = fn => {
  const cache = Object.create(null);
  return value => cache[value] || (cache[value] = fn(value));
};

// See the `anagrams` snippet.
const anagramsCached = memoize(anagrams);
anagramsCached('javascript'); // takes a long time
anagramsCached('javascript'); // returns virtually instantly since it's now cached

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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

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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.');
}

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Negates a predicate function.

Take a predicate function and apply not to it with its arguments.

const negate = func => (...args) => !func(...args);

filter([1, 2, 3, 4, 5, 6], negate(isEven)); // [1, 3, 5]
negate(isOdd)(1); // false

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Returns the average of an of two or more numbers/arrays.

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) => {
  const nums = [].concat(...arr);
  return nums.reduce((acc, val) => acc + val, 0) / nums.length;
};

average([1, 2, 3]); // 2
average(1, 2, 3); // 2

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Clamps num within the inclusive range specified by the boundary values a and b.

If num falls within the range, return num. Otherwise, return the nearest number in the range.

const clampNumber = (num, a, b) => Math.max(Math.min(num, Math.max(a, b)), Math.min(a, b));

clampNumber(2, 3, 5); // 3
clampNumber(1, -1, -5); // -1
clampNumber(3, 2, 4); // 3

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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

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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(123); // [1, 2, 3]

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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

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Computes the new ratings between two opponents using the Elo rating system. It takes an array of two pre-ratings and returns an array containing two post-ratings. The winner's rating is the first element of the array.

Use the exponent ** operator and math operators to compute the expected score (chance of winning) of each opponent and compute the new rating for each. Omit the second argument to use the default K-factor of 32, or supply a custom K-factor value.

const elo = ([a, b], kFactor = 32) => {
  const expectedScore = (self, opponent) => 1 / (1 + 10 ** ((opponent - self) / 400));
  const newRating = (rating, i) => rating + kFactor * (i - expectedScore(i ? a : b, i ? b : a));
  return [newRating(a, 1), newRating(b, 0)];
};

elo([1200, 1200]); // [1216, 1184]
elo([1000, 2000]); // [1031.8991261061358, 1968.1008738938642]
elo([1500, 1000]); // [1501.7036868864648, 998.2963131135352]
elo([1200, 1200], 64); // [1232, 1168]