Curated collection of useful JavaScript snippets that you can understand in 30 seconds or less.
⚠️ NOTICE: A few of our snippets are not yet optimized for production (see disclaimers for individual snippet issues).
You can find a package with all the snippets on npm.
# With npm
npm install 30-seconds-of-code
# With yarn
yarn add 30-seconds-of-code
Browser
<script src="https://unpkg.com/30-seconds-of-code@1/dist/_30s.es5.min.js"></script>
<script>
_30s.average(1, 2, 3);
</script>
Node
// CommonJS
const _30s = require('30-seconds-of-code');
_30s.average(1, 2, 3);
// ES Modules
import _30s from '30-seconds-of-code';
_30s.average(1, 2, 3);
all
allEqual
any
arrayToCSV
bifurcate
bifurcateBy
chunk
compact
countBy
countOccurrences
deepFlatten
difference
differenceBy
differenceWith
drop
dropRight
dropRightWhile
dropWhile
everyNth
filterNonUnique
filterNonUniqueBy
findLast
findLastIndex
flatten
forEachRight
groupBy
head
indexOfAll
initial
initialize2DArray
initializeArrayWithRange
initializeArrayWithRangeRight
initializeArrayWithValues
initializeNDArray
intersection
intersectionBy
intersectionWith
isSorted
join
JSONtoCSV
last
longestItem
mapObject
maxN
minN
none
nthElement
offset
partition
permutations
pull
pullAtIndex
pullAtValue
pullBy
reducedFilter
reduceSuccessive
reduceWhich
reject
remove
sample
sampleSize
shank
shuffle
similarity
sortedIndex
sortedIndexBy
sortedLastIndex
sortedLastIndexBy
stableSort
symmetricDifference
symmetricDifferenceBy
symmetricDifferenceWith
tail
take
takeRight
takeRightWhile
takeWhile
toHash
union
unionBy
unionWith
uniqueElements
uniqueElementsBy
uniqueElementsByRight
uniqueSymmetricDifference
unzip
unzipWith
without
xProd
zip
zipObject
zipWith
arrayToHtmlList
bottomVisible
copyToClipboard
counter
createElement
createEventHub
currentURL
detectDeviceType
elementContains
elementIsVisibleInViewport
getImages
getScrollPosition
getStyle
hasClass
hashBrowser
hide
httpsRedirect
insertAfter
insertBefore
isBrowserTabFocused
nodeListToArray
observeMutations
off
on
onUserInputChange
prefix
recordAnimationFrames
redirect
runAsync
scrollToTop
setStyle
show
smoothScroll
toggleClass
triggerEvent
UUIDGeneratorBrowser
approximatelyEqual
average
averageBy
binomialCoefficient
clampNumber
degreesToRads
digitize
distance
elo
factorial
fibonacci
gcd
geometricProgression
hammingDistance
inRange
isDivisible
isEven
isNegativeZero
isPrime
lcm
luhnCheck
maxBy
median
minBy
percentile
powerset
primes
radsToDegrees
randomIntArrayInRange
randomIntegerInRange
randomNumberInRange
round
sdbm
standardDeviation
sum
sumBy
sumPower
toSafeInteger
bindAll
deepClone
deepFreeze
defaults
dig
equals
findKey
findLastKey
flattenObject
forOwn
forOwnRight
functions
get
invertKeyValues
lowercaseKeys
mapKeys
mapValues
matches
matchesWith
merge
nest
objectFromPairs
objectToPairs
omit
omitBy
orderBy
pick
pickBy
renameKeys
shallowClone
size
transform
truthCheckCollection
unflattenObject
byteSize
capitalize
capitalizeEveryWord
CSVToArray
CSVToJSON
decapitalize
escapeHTML
escapeRegExp
fromCamelCase
indentString
isAbsoluteURL
isAnagram
isLowerCase
isUpperCase
mapString
mask
pad
palindrome
pluralize
removeNonASCII
reverseString
sortCharactersInString
splitLines
stringPermutations
stripHTMLTags
toCamelCase
toKebabCase
toSnakeCase
toTitleCase
truncateString
unescapeHTML
URLJoin
words
Creates a function that accepts up to n
arguments, ignoring any additional arguments.
Call the provided function, fn
, with up to n
arguments, using Array.prototype.slice(0,n)
and the spread operator (...
).
const ary = (fn, n) => (...args) => fn(...args.slice(0, n));
const firstTwoMax = ary(Math.max, 2);
[[2, 6, 'a'], [8, 4, 6], [10]].map(x => firstTwoMax(...x)); // [6, 8, 10]
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 ]
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); // [1, 2, 3] (after about 2 seconds)
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 => (first, ...rest) => fn(...rest, first);
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
Creates a function that invokes each provided function with the arguments it receives and returns the results.
Use Array.prototype.map()
and Function.prototype.apply()
to apply each function to the given arguments.
const over = (...fns) => (...args) => fns.map(fn => fn.apply(null, args));
const minMax = over(Math.min, Math.max);
minMax(1, 2, 3, 4, 5); // [1,5]
Creates a function that invokes the provided function with its arguments transformed.
Use Array.prototype.map()
to apply transforms
to args
in combination with the spread operator (...
) to pass the transformed arguments to fn
.
const overArgs = (fn, transforms) => (...args) => fn(...args.map((val, i) => transforms[i](val)));
const square = n => n * n;
const double = n => n * 2;
const fn = overArgs((x, y) => [x, y], [square, double]);
fn(9, 3); // [81, 6]
Performs left-to-right function composition for asynchronous functions.
Use Array.prototype.reduce()
with the spread operator (...
) to perform left-to-right function composition using Promise.then()
.
The functions can return a combination of: simple values, Promise
's, or they can be defined as async
ones returning through await
.
All functions must be unary.
const pipeAsyncFunctions = (...fns) => arg => fns.reduce((p, f) => p.then(f), Promise.resolve(arg));
const sum = pipeAsyncFunctions(
x => x + 1,
x => new Promise(resolve => setTimeout(() => resolve(x + 2), 1000)),
x => x + 3,
async x => (await x) + 4
);
(async() => {
console.log(await sum(5)); // 15 (after one second)
})();
Performs left-to-right function composition.
Use Array.prototype.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
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
Creates a function that invokes the provided function with its arguments arranged according to the specified indexes.
Use Array.prototype.map()
to reorder arguments based on indexes
in combination with the spread operator (...
) to pass the transformed arguments to fn
.
const rearg = (fn, indexes) => (...args) => fn(...indexes.map(i => args[i]));
var rearged = rearg(
function(a, b, c) {
return [a, b, c];
},
[2, 0, 1]
);
rearged('b', 'c', 'a'); // ['a', 'b', 'c']
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
Creates a function that accepts up to one argument, ignoring any additional arguments.
Call the provided function, fn
, with just the first argument given.
const unary = fn => val => fn(val);
['6', '8', '10'].map(unary(parseInt)); // [6, 8, 10]
Returns true
if the provided predicate function returns true
for all elements in a collection, false
otherwise.
Use Array.prototype.every()
to test if all elements in the collection return true
based on fn
.
Omit the second argument, fn
, to use Boolean
as a default.
const all = (arr, fn = Boolean) => arr.every(fn);
all([4, 2, 3], x => x > 1); // true
all([1, 2, 3]); // true
Check if all elements in an array are equal.
Use Array.prototype.every()
to check if all the elements of the array are the same as the first one.
const allEqual = arr => arr.every(val => val === arr[0]);
allEqual([1, 2, 3, 4, 5, 6]); // false
allEqual([1, 1, 1, 1]); // true
Returns true
if the provided predicate function returns true
for at least one element in a collection, false
otherwise.
Use Array.prototype.some()
to test if any elements in the collection return true
based on fn
.
Omit the second argument, fn
, to use Boolean
as a default.
const any = (arr, fn = Boolean) => arr.some(fn);
any([0, 1, 2, 0], x => x >= 2); // true
any([0, 0, 1, 0]); // true
Converts a 2D array to a comma-separated values (CSV) string.
Use Array.prototype.map()
and Array.prototype.join(delimiter)
to combine individual 1D arrays (rows) into strings.
Use Array.prototype.join('\n')
to combine all rows into a CSV string, separating each row with a newline.
Omit the second argument, delimiter
, to use a default delimiter of ,
.
const arrayToCSV = (arr, delimiter = ',') =>
arr.map(v => v.map(x => `"${x}"`).join(delimiter)).join('\n');
arrayToCSV([['a', 'b'], ['c', 'd']]); // '"a","b"\n"c","d"'
arrayToCSV([['a', 'b'], ['c', 'd']], ';'); // '"a";"b"\n"c";"d"'
Splits values into two groups. If an element in filter
is truthy, the corresponding element in the collection belongs to the first group; otherwise, it belongs to the second group.
Use Array.prototype.reduce()
and Array.prototype.push()
to add elements to groups, based on filter
.
const bifurcate = (arr, filter) =>
arr.reduce((acc, val, i) => (acc[filter[i] ? 0 : 1].push(val), acc), [[], []]);
bifurcate(['beep', 'boop', 'foo', 'bar'], [true, true, false, true]); // [ ['beep', 'boop', 'bar'], ['foo'] ]
Splits values into two groups according to a predicate function, which specifies which group an element in the input collection belongs to. If the predicate function returns a truthy value, the collection element belongs to the first group; otherwise, it belongs to the second group.
Use Array.prototype.reduce()
and Array.prototype.push()
to add elements to groups, based on the value returned by fn
for each element.
const bifurcateBy = (arr, fn) =>
arr.reduce((acc, val, i) => (acc[fn(val, i) ? 0 : 1].push(val), acc), [[], []]);
bifurcateBy(['beep', 'boop', 'foo', 'bar'], x => x[0] === 'b'); // [ ['beep', 'boop', 'bar'], ['foo'] ]
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.prototype.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]]
Removes falsey values from an array.
Use Array.prototype.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 ]
Groups the elements of an array based on the given function and returns the count of elements in each group.
Use Array.prototype.map()
to map the values of an array to a function or property name.
Use Array.prototype.reduce()
to create an object, where the keys are produced from the mapped results.
const countBy = (arr, fn) =>
arr.map(typeof fn === 'function' ? fn : val => val[fn]).reduce((acc, val) => {
acc[val] = (acc[val] || 0) + 1;
return acc;
}, {});
countBy([6.1, 4.2, 6.3], Math.floor); // {4: 1, 6: 2}
countBy(['one', 'two', 'three'], 'length'); // {3: 2, 5: 1}
Counts the occurrences of a value in an array.
Use Array.prototype.reduce()
to increment a counter each time you encounter the specific value inside the array.
const countOccurrences = (arr, val) => arr.reduce((a, v) => (v === val ? a + 1 : a), 0);
countOccurrences([1, 1, 2, 1, 2, 3], 1); // 3
Deep flattens an array.
Use recursion.
Use Array.prototype.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]
Returns the difference between two arrays.
Create a Set
from b
, then use Array.prototype.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]
Returns the difference between two arrays, after applying the provided function to each array element of both.
Create a Set
by applying fn
to each element in b
, then use Array.prototype.filter()
in combination with fn
on a
to only keep values not contained in the previously created set.
const differenceBy = (a, b, fn) => {
const s = new Set(b.map(fn));
return a.filter(x => !s.has(fn(x)));
};
differenceBy([2.1, 1.2], [2.3, 3.4], Math.floor); // [1.2]
differenceBy([{ x: 2 }, { x: 1 }], [{ x: 1 }], v => v.x); // [ { x: 2 } ]
Filters out all values from an array for which the comparator function does not return true
.
Use Array.prototype.filter()
and Array.prototype.findIndex()
to find the appropriate values.
const differenceWith = (arr, val, comp) => arr.filter(a => val.findIndex(b => comp(a, b)) === -1);
differenceWith([1, 1.2, 1.5, 3, 0], [1.9, 3, 0], (a, b) => Math.round(a) === Math.round(b)); // [1, 1.2]
Returns a new array with n
elements removed from the left.
Use Array.prototype.slice()
to slice the remove the specified number of elements from the left.
const drop = (arr, n = 1) => arr.slice(n);
drop([1, 2, 3]); // [2,3]
drop([1, 2, 3], 2); // [3]
drop([1, 2, 3], 42); // []
Returns a new array with n
elements removed from the right.
Use Array.prototype.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); // []
Removes elements from the end of an array until the passed function returns true
. Returns the remaining elements in the array.
Loop through the array, using Array.prototype.slice()
to drop the last element of the array until the returned value from the function is true
.
Returns the remaining elements.
const dropRightWhile = (arr, func) => {
while (arr.length > 0 && !func(arr[arr.length - 1])) arr = arr.slice(0, -1);
return arr;
};
dropRightWhile([1, 2, 3, 4], n => n < 3); // [1, 2]
Removes elements in an array until the passed function returns true
. Returns the remaining elements in the array.
Loop through the array, using Array.prototype.slice()
to drop the first element of the array until the returned value from the function is true
.
Returns the remaining elements.
const dropWhile = (arr, func) => {
while (arr.length > 0 && !func(arr[0])) arr = arr.slice(1);
return arr;
};
dropWhile([1, 2, 3, 4], n => n >= 3); // [3,4]
Returns every nth element in an array.
Use Array.prototype.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 ]
Filters out the non-unique values in an array.
Use Array.prototype.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]
Filters out the non-unique values in an array, based on a provided comparator function.
Use Array.prototype.filter()
and Array.prototype.every()
for an array containing only the unique values, based on the comparator function, fn
.
The comparator function takes four arguments: the values of the two elements being compared and their indexes.
const filterNonUniqueBy = (arr, fn) =>
arr.filter((v, i) => arr.every((x, j) => (i === j) === fn(v, x, i, j)));
filterNonUniqueBy(
[
{ id: 0, value: 'a' },
{ id: 1, value: 'b' },
{ id: 2, value: 'c' },
{ id: 1, value: 'd' },
{ id: 0, value: 'e' }
],
(a, b) => a.id == b.id
); // [ { id: 2, value: 'c' } ]
Returns the last element for which the provided function returns a truthy value.
Use Array.prototype.filter()
to remove elements for which fn
returns falsey values, Array.prototype.pop()
to get the last one.
const findLast = (arr, fn) => arr.filter(fn).pop();
findLast([1, 2, 3, 4], n => n % 2 === 1); // 3
Returns the index of the last element for which the provided function returns a truthy value.
Use Array.prototype.map()
to map each element to an array with its index and value.
Use Array.prototype.filter()
to remove elements for which fn
returns falsey values, Array.prototype.pop()
to get the last one.
const findLastIndex = (arr, fn) =>
arr
.map((val, i) => [i, val])
.filter(([i, val]) => fn(val, i, arr))
.pop()[0];
findLastIndex([1, 2, 3, 4], n => n % 2 === 1); // 2 (index of the value 3)
Flattens an array up to the specified depth.
Use recursion, decrementing depth
by 1 for each level of depth.
Use Array.prototype.reduce()
and Array.prototype.concat()
to merge elements or arrays.
Base case, for depth
equal to 1
stops recursion.
Omit the second argument, depth
to flatten only to a depth of 1
(single flatten).
const flatten = (arr, depth = 1) =>
arr.reduce((a, v) => a.concat(depth > 1 && Array.isArray(v) ? flatten(v, depth - 1) : v), []);
flatten([1, [2], 3, 4]); // [1, 2, 3, 4]
flatten([1, [2, [3, [4, 5], 6], 7], 8], 2); // [1, 2, 3, [4, 5], 6, 7, 8]
Executes a provided function once for each array element, starting from the array's last element.
Use Array.prototype.slice(0)
to clone the given array, Array.prototype.reverse()
to reverse it and Array.prototype.forEach()
to iterate over the reversed array.
const forEachRight = (arr, callback) =>
arr
.slice(0)
.reverse()
.forEach(callback);
forEachRight([1, 2, 3, 4], val => console.log(val)); // '4', '3', '2', '1'
Groups the elements of an array based on the given function.
Use Array.prototype.map()
to map the values of an array to a function or property name.
Use Array.prototype.reduce()
to create an object, where the keys are produced from the mapped results.
const groupBy = (arr, fn) =>
arr.map(typeof fn === 'function' ? fn : val => val[fn]).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']}
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
Returns all indices of val
in an array.
If val
never occurs, returns []
.
Use Array.prototype.reduce()
to loop over elements and store indices for matching elements.
Return the array of indices.
const indexOfAll = (arr, val) => arr.reduce((acc, el, i) => (el === val ? [...acc, i] : acc), []);
indexOfAll([1, 2, 3, 1, 2, 3], 1); // [0,3]
indexOfAll([1, 2, 3], 4); // []
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]
Initializes a 2D array of given width and height and value.
Use Array.prototype.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.from({ length: h }).map(() => Array.from({ length: w }).fill(val));
initialize2DArray(2, 2, 0); // [[0,0], [0,0]]
Initializes an array containing the numbers in the specified range where start
and end
are inclusive with their common difference step
.
Use Array.from()
to create an array of the desired length, (end - start + 1)/step
, and a map function to fill it with the desired values in the given range.
You can omit start
to use a default value of 0
.
You can omit step
to use a default value of 1
.
const initializeArrayWithRange = (end, start = 0, step = 1) =>
Array.from({ length: Math.ceil((end - start + 1) / step) }, (v, i) => i * step + start);
initializeArrayWithRange(5); // [0,1,2,3,4,5]
initializeArrayWithRange(7, 3); // [3,4,5,6,7]
initializeArrayWithRange(9, 0, 2); // [0,2,4,6,8]
Initializes an array containing the numbers in the specified range (in reverse) where start
and end
are inclusive with their common difference step
.
Use Array.from(Math.ceil((end+1-start)/step))
to create an array of the desired length(the amounts of elements is equal to (end-start)/step
or (end+1-start)/step
for inclusive end), Array.prototype.map()
to fill with the desired values in a range.
You can omit start
to use a default value of 0
.
You can omit step
to use a default value of 1
.
const initializeArrayWithRangeRight = (end, start = 0, step = 1) =>
Array.from({ length: Math.ceil((end + 1 - start) / step) }).map(
(v, i, arr) => (arr.length - i - 1) * step + start
);
initializeArrayWithRangeRight(5); // [5,4,3,2,1,0]
initializeArrayWithRangeRight(7, 3); // [7,6,5,4,3]
initializeArrayWithRangeRight(9, 0, 2); // [8,6,4,2,0]
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 val
to use a default value of 0
.
const initializeArrayWithValues = (n, val = 0) => Array(n).fill(val);
initializeArrayWithValues(5, 2); // [2, 2, 2, 2, 2]
Create a n-dimensional array with given value.
Use recursion.
Use Array.prototype.map()
to generate rows where each is a new array initialized using initializeNDArray
.
const initializeNDArray = (val, ...args) =>
args.length === 0
? val
: Array.from({ length: args[0] }).map(() => initializeNDArray(val, ...args.slice(1)));
initializeNDArray(1, 3); // [1,1,1]
initializeNDArray(5, 2, 2, 2); // [[[5,5],[5,5]],[[5,5],[5,5]]]
Returns a list of elements that exist in both arrays.
Create a Set
from b
, then use Array.prototype.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]
Returns a list of elements that exist in both arrays, after applying the provided function to each array element of both.
Create a Set
by applying fn
to all elements in b
, then use Array.prototype.filter()
on a
to only keep elements, which produce values contained in b
when fn
is applied to them.
const intersectionBy = (a, b, fn) => {
const s = new Set(b.map(fn));
return a.filter(x => s.has(fn(x)));
};
intersectionBy([2.1, 1.2], [2.3, 3.4], Math.floor); // [2.1]
Returns a list of elements that exist in both arrays, using a provided comparator function.
Use Array.prototype.filter()
and Array.prototype.findIndex()
in combination with the provided comparator to determine intersecting values.
const intersectionWith = (a, b, comp) => a.filter(x => b.findIndex(y => comp(x, y)) !== -1);
intersectionWith([1, 1.2, 1.5, 3, 0], [1.9, 3, 0, 3.9], (a, b) => Math.round(a) === Math.round(b)); // [1.5, 3, 0]
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 => {
let direction = -(arr[0] - arr[1]);
for (let [i, val] of arr.entries()) {
direction = !direction ? -(arr[i - 1] - arr[i]) : direction;
if (i === arr.length - 1) return !direction ? 0 : direction;
else if ((val - arr[i + 1]) * direction > 0) return 0;
}
};
isSorted([0, 1, 2, 2]); // 1
isSorted([4, 3, 2]); // -1
isSorted([4, 3, 5]); // 0
Joins all elements of an array into a string and returns this string. Uses a separator and an end separator.
Use Array.prototype.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(['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"
Converts an array of objects to a comma-separated values (CSV) string that contains only the columns
specified.
Use Array.prototype.join(delimiter)
to combine all the names in columns
to create the first row.
Use Array.prototype.map()
and Array.prototype.reduce()
to create a row for each object, substituting non-existent values with empty strings and only mapping values in columns
.
Use Array.prototype.join('\n')
to combine all rows into a string.
Omit the third argument, delimiter
, to use a default delimiter of ,
.
const JSONtoCSV = (arr, columns, delimiter = ',') =>
[
columns.join(delimiter),
...arr.map(obj =>
columns.reduce(
(acc, key) => `${acc}${!acc.length ? '' : delimiter}"${!obj[key] ? '' : obj[key]}"`,
''
)
)
].join('\n');
JSONtoCSV([{ a: 1, b: 2 }, { a: 3, b: 4, c: 5 }, { a: 6 }, { b: 7 }], ['a', 'b']); // 'a,b\n"1","2"\n"3","4"\n"6",""\n"","7"'
JSONtoCSV([{ a: 1, b: 2 }, { a: 3, b: 4, c: 5 }, { a: 6 }, { b: 7 }], ['a', 'b'], ';'); // 'a;b\n"1";"2"\n"3";"4"\n"6";""\n"";"7"'
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
Takes any number of iterable objects or objects with a length
property and returns the longest one.
If multiple objects have the same length, the first one will be returned.
Returns undefined
if no arguments are provided.
Use Array.prototype.reduce()
, comparing the length
of objects to find the longest one.
const longestItem = (...vals) => vals.reduce((a, x) => (x.length > a.length ? x : a));
longestItem('this', 'is', 'a', 'testcase'); // 'testcase'
longestItem(...['a', 'ab', 'abc']); // 'abc'
longestItem(...['a', 'ab', 'abc'], 'abcd'); // 'abcd'
longestItem([1, 2, 3], [1, 2], [1, 2, 3, 4, 5]); // [1, 2, 3, 4, 5]
longestItem([1, 2, 3], 'foobar'); // 'foobar'
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 }
Returns the n
maximum elements from the provided array.
If n
is greater than or equal to the provided array's length, then return the original array (sorted in descending order).
Use Array.prototype.sort()
combined with the spread operator (...
) to create a shallow clone of the array and sort it in descending order.
Use Array.prototype.slice()
to get the specified number of elements.
Omit the second argument, n
, to get a one-element array.
const maxN = (arr, n = 1) => [...arr].sort((a, b) => b - a).slice(0, n);
maxN([1, 2, 3]); // [3]
maxN([1, 2, 3], 2); // [3,2]
Returns the n
minimum elements from the provided array.
If n
is greater than or equal to the provided array's length, then return the original array (sorted in ascending order).
Use Array.prototype.sort()
combined with the spread operator (...
) to create a shallow clone of the array and sort it in ascending order.
Use Array.prototype.slice()
to get the specified number of elements.
Omit the second argument, n
, to get a one-element array.
const minN = (arr, n = 1) => [...arr].sort((a, b) => a - b).slice(0, n);
minN([1, 2, 3]); // [1]
minN([1, 2, 3], 2); // [1,2]
Returns true
if the provided predicate function returns false
for all elements in a collection, false
otherwise.
Use Array.prototype.some()
to test if any elements in the collection return true
based on fn
.
Omit the second argument, fn
, to use Boolean
as a default.
const none = (arr, fn = Boolean) => !arr.some(fn);
none([0, 1, 3, 0], x => x == 2); // true
none([0, 0, 0]); // true
Returns the nth element of an array.
Use Array.prototype.slice()
to get an array containing the nth element at the first place.
If the index is out of bounds, return undefined
.
Omit the second argument, n
, to get the first element of the array.
const nthElement = (arr, n = 0) => (n === -1 ? arr.slice(n) : arr.slice(n, n + 1))[0];
nthElement(['a', 'b', 'c'], 1); // 'b'
nthElement(['a', 'b', 'b'], -3); // 'a'
Moves the specified amount of elements to the end of the array.
Use Array.prototype.slice()
twice to get the elements after the specified index and the elements before that.
Use the spread operator(...
) to combine the two into one array.
If offset
is negative, the elements will be moved from end to start.
const offset = (arr, offset) => [...arr.slice(offset), ...arr.slice(0, offset)];
offset([1, 2, 3, 4, 5], 2); // [3, 4, 5, 1, 2]
offset([1, 2, 3, 4, 5], -2); // [4, 5, 1, 2, 3]
Groups the elements into two arrays, depending on the provided function's truthiness for each element.
Use Array.prototype.reduce()
to create an array of two arrays.
Use Array.prototype.push()
to add elements for which fn
returns true
to the first array and elements for which fn
returns false
to the second one.
const partition = (arr, fn) =>
arr.reduce(
(acc, val, i, arr) => {
acc[fn(val, i, arr) ? 0 : 1].push(val);
return acc;
},
[[], []]
);
const users = [{ user: 'barney', age: 36, active: false }, { user: 'fred', age: 40, active: true }];
partition(users, o => o.active); // [[{ 'user': 'fred', 'age': 40, 'active': true }],[{ 'user': 'barney', 'age': 36, 'active': false }]]
⚠️ WARNING: This function's execution time increases exponentially with each array element. Anything more than 8 to 10 entries will cause your browser to hang as it tries to solve all the different combinations.
Generates all permutations of an array's elements (contains duplicates).
Use recursion.
For each element in the given array, create all the partial permutations for the rest of its elements.
Use Array.prototype.map()
to combine the element with each partial permutation, then Array.prototype.reduce()
to combine all permutations in one array.
Base cases are for array length
equal to 2
or 1
.
const permutations = arr => {
if (arr.length <= 2) return arr.length === 2 ? [arr, [arr[1], arr[0]]] : arr;
return arr.reduce(
(acc, item, i) =>
acc.concat(
permutations([...arr.slice(0, i), ...arr.slice(i + 1)]).map(val => [item, ...val])
),
[]
);
};
permutations([1, 33, 5]); // [ [ 1, 33, 5 ], [ 1, 5, 33 ], [ 33, 1, 5 ], [ 33, 5, 1 ], [ 5, 1, 33 ], [ 5, 33, 1 ] ]
Mutates the original array to filter out the values specified.
Use Array.prototype.filter()
and Array.prototype.includes()
to pull out the values that are not needed.
Use Array.prototype.length = 0
to mutate the passed in an array by resetting it's length to zero and Array.prototype.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 myArray = ['a', 'b', 'c', 'a', 'b', 'c'];
pull(myArray, 'a', 'c'); // myArray = [ 'b', 'b' ]
Mutates the original array to filter out the values at the specified indexes.
Use Array.prototype.filter()
and Array.prototype.includes()
to pull out the values that are not needed.
Use Array.prototype.length = 0
to mutate the passed in an array by resetting it's length to zero and Array.prototype.push()
to re-populate it with only the pulled values.
Use Array.prototype.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]); // myArray = [ 'a', 'c' ] , pulled = [ 'b', 'd' ]
Mutates the original array to filter out the values specified. Returns the removed elements.
Use Array.prototype.filter()
and Array.prototype.includes()
to pull out the values that are not needed.
Use Array.prototype.length = 0
to mutate the passed in an array by resetting it's length to zero and Array.prototype.push()
to re-populate it with only the pulled values.
Use Array.prototype.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']); // myArray = [ 'a', 'c' ] , pulled = [ 'b', 'd' ]
Mutates the original array to filter out the values specified, based on a given iterator function.
Check if the last argument provided in a function.
Use Array.prototype.map()
to apply the iterator function fn
to all array elements.
Use Array.prototype.filter()
and Array.prototype.includes()
to pull out the values that are not needed.
Use Array.prototype.length = 0
to mutate the passed in an array by resetting it's length to zero and Array.prototype.push()
to re-populate it with only the pulled values.
const pullBy = (arr, ...args) => {
const length = args.length;
let fn = length > 1 ? args[length - 1] : undefined;
fn = typeof fn == 'function' ? (args.pop(), fn) : undefined;
let argState = (Array.isArray(args[0]) ? args[0] : args).map(val => fn(val));
let pulled = arr.filter((v, i) => !argState.includes(fn(v)));
arr.length = 0;
pulled.forEach(v => arr.push(v));
};
var myArray = [{ x: 1 }, { x: 2 }, { x: 3 }, { x: 1 }];
pullBy(myArray, [{ x: 1 }, { x: 3 }], o => o.x); // myArray = [{ x: 2 }]
Filter an array of objects based on a condition while also filtering out unspecified keys.
Use Array.prototype.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.prototype.map()
to return the new object using Array.prototype.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'}]
Applies a function against an accumulator and each element in the array (from left to right), returning an array of successively reduced values.
Use Array.prototype.reduce()
to apply the given function to the given array, storing each new result.
const reduceSuccessive = (arr, fn, acc) =>
arr.reduce((res, val, i, arr) => (res.push(fn(res.slice(-1)[0], val, i, arr)), res), [acc]);
reduceSuccessive([1, 2, 3, 4, 5, 6], (acc, val) => acc + val, 0); // [0, 1, 3, 6, 10, 15, 21]
Returns the minimum/maximum value of an array, after applying the provided function to set comparing rule.
Use Array.prototype.reduce()
in combination with the comparator
function to get the appropriate element in the array.
You can omit the second parameter, comparator
, to use the default one that returns the minimum element in the array.
const reduceWhich = (arr, comparator = (a, b) => a - b) =>
arr.reduce((a, b) => (comparator(a, b) >= 0 ? b : a));
reduceWhich([1, 3, 2]); // 1
reduceWhich([1, 3, 2], (a, b) => b - a); // 3
reduceWhich(
[{ name: 'Tom', age: 12 }, { name: 'Jack', age: 18 }, { name: 'Lucy', age: 9 }],
(a, b) => a.age - b.age
); // {name: "Lucy", age: 9}
Takes a predicate and array, like Array.prototype.filter()
, but only keeps x
if pred(x) === false
.
const reject = (pred, array) => array.filter((...args) => !pred(...args));
reject(x => x % 2 === 0, [1, 2, 3, 4, 5]); // [1, 3, 5]
reject(word => word.length > 4, ['Apple', 'Pear', 'Kiwi', 'Banana']); // ['Pear', 'Kiwi']
Removes elements from an array for which the given function returns false
.
Use Array.prototype.filter()
to find array elements that return truthy values and Array.prototype.reduce()
to remove elements using Array.prototype.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]
Returns a random element from an array.
Use Math.random()
to generate a random number, multiply it by length
and round it off 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
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.prototype.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]
Has the same functionality as Array.prototype.splice()
, but returning a new array instead of mutating the original array.
Use Array.prototype.slice()
and Array.prototype.concat()
to get a new array with the new contents after removing existing elements and/or adding new elements.
Omit the second argument, index
, to start at 0
.
Omit the third argument, delCount
, to remove 0
elements.
Omit the fourth argument, elements
, in order to not add any new elements.
const shank = (arr, index = 0, delCount = 0, ...elements) =>
arr
.slice(0, index)
.concat(elements)
.concat(arr.slice(index + delCount));
const names = ['alpha', 'bravo', 'charlie'];
const namesAndDelta = shank(names, 1, 0, 'delta'); // [ 'alpha', 'delta', 'bravo', 'charlie' ]
const namesNoBravo = shank(names, 1, 1); // [ 'alpha', 'charlie' ]
console.log(names); // ['alpha', 'bravo', 'charlie']
Randomizes the order of the values of an array, returning a new array.
Uses the Fisher-Yates algorithm to reorder the elements of the array.
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], foo = [1, 2, 3]
Returns an array of elements that appear in both arrays.
Use Array.prototype.filter()
to remove values that are not part of values
, determined using Array.prototype.includes()
.
const similarity = (arr, values) => arr.filter(v => values.includes(v));
similarity([1, 2, 3], [1, 2, 4]); // [1, 2]
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.prototype.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
Returns the lowest index at which value should be inserted into array in order to maintain its sort order, based on a provided iterator function.
Check if the array is sorted in descending order (loosely).
Use Array.prototype.findIndex()
to find the appropriate index where the element should be inserted, based on the iterator function fn
.
const sortedIndexBy = (arr, n, fn) => {
const isDescending = fn(arr[0]) > fn(arr[arr.length - 1]);
const val = fn(n);
const index = arr.findIndex(el => (isDescending ? val >= fn(el) : val <= fn(el)));
return index === -1 ? arr.length : index;
};
sortedIndexBy([{ x: 4 }, { x: 5 }], { x: 4 }, o => o.x); // 0
Returns the highest 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.prototype.reverse()
and Array.prototype.findIndex()
to find the appropriate last index where the element should be inserted.
const sortedLastIndex = (arr, n) => {
const isDescending = arr[0] > arr[arr.length - 1];
const index = arr.reverse().findIndex(el => (isDescending ? n <= el : n >= el));
return index === -1 ? 0 : arr.length - index;
};
sortedLastIndex([10, 20, 30, 30, 40], 30); // 4
Version | Tag | Published |
---|---|---|
1.2.3 | 4yrs ago | |
1.2.2 | 4yrs ago | |
1.2.1 | 4yrs ago | |
1.2.0 | 4yrs ago |