Does it mutate?

Does it mutate?

No mutation

.concat

No mutation


Description

The concat() method is used to merge two or more arrays. This method does not change the existing arrays, but instead returns a new array.

Array.prototype.concat ( [ item1 [ , item2 [ , … ] ] ] )

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/concat


Example

var array1 = ['a', 'b', 'c']; var array2 = ['d', 'e', 'f']; console.log(array1.concat(array2)); // expected output: Array ["a", "b", "c", "d", "e", "f"] 


.copyWithin()



Description

The copyWithin() method shallow copies part of an array to another location in the same array and returns it, without modifying its size.

arr.copyWithin(target) arr.copyWithin(target, start) arr.copyWithin(target, start, end) 

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/copyWithin


Example

var array1 = ['a', 'b', 'c', 'd', 'e']; // copy to index 0 the element at index 3 console.log(array1.copyWithin(0, 3, 4)); // expected output: Array ["d", "b", "c", "d", "e"] // copy to index 1 all elements from index 3 to the end console.log(array1.copyWithin(1, 3)); // expected output: Array ["d", "d", "e", "d", "e"] 

.entries()



Description

The entries() method returns a new Array Iterator object that contains the key/value pairs for each index in the array.

a.entries()

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/entries


Example

var array1 = ['a', 'b', 'c']; var iterator1 = array1.entries(); console.log(iterator1.next().value); // expected output: Array [0, "a"] console.log(iterator1.next().value); // expected output: Array [1, "b"] 

.every



Description

The every() method tests whether all elements in the array pass the test implemented by the provided function.

Array.prototype.every ( callbackfn [ , thisArg ] )

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/every


Example

function isBelowThreshold(currentValue) { return currentValue < 40; } var array1 = [1, 30, 39, 29, 10, 13]; console.log(array1.every(isBelowThreshold)); // expected output: true 

.fill()



Description

The fill() method fills all the elements of an array from a start index to an end index with a static value.

arr.fill(value) arr.fill(value, start) arr.fill(value, start, end) 

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/fill


Example

var array1 = [1, 2, 3, 4]; // fill with 0 from position 2 until position 4 console.log(array1.fill(0, 2, 4)); // expected output: [1, 2, 0, 0] // fill with 5 from position 1 console.log(array1.fill(5, 1)); // expected output: [1, 5, 5, 5] console.log(array1.fill(6)); // expected output: [6, 6, 6, 6] 

.filter



Description

The filter() method creates a new array with all elements that pass the test implemented by the provided function.

Array.prototype.filter ( callbackfn [ , thisArg ] )

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/filter


Example

var words = ['spray', 'limit', 'elite', 'exuberant', 'destruction', 'present']; const result = words.filter(word => word.length > 6); console.log(result); // expected output: Array ["exuberant", "destruction", "present"] 

.find()



Description

The find() method returns a value of the first element in the array that satisfies the provided testing function. Otherwise undefined is returned.

arr.find(callback[, thisArg])

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/find


Example

var array1 = [5, 12, 8, 130, 44]; var found = array1.find(function(element) { return element > 10; }); console.log(found); // expected output: 12 

.findIndex()



Description

The findIndex() method returns an index of the first element in the array that satisfies the provided testing function. Otherwise -1 is returned.

arr.findIndex(callback[, thisArg])

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/findIndex


Example

var array1 = [5, 12, 8, 130, 44]; function isLargeNumber(element) { return element > 13; } console.log(array1.findIndex(isLargeNumber)); // expected output: 3 

.forEach



Description

The forEach() method executes a provided function once per array element.

Array.prototype.forEach ( callbackfn [ , thisArg ] )

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/forEach


Example

var array1 = ['a', 'b', 'c']; array1.forEach(function(element) { console.log(element); }); // expected output: "a" // expected output: "b" // expected output: "c" 

.includes()



Description

The includes() method determines whether an array includes a certain element, returning true or false as appropriate.

arr.includes(searchElement) arr.includes(searchElement, fromIndex) 

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/includes


Example

var array1 = [1, 2, 3]; console.log(array1.includes(2)); // expected output: true var pets = ['cat', 'dog', 'bat']; console.log(pets.includes('cat')); // expected output: true console.log(pets.includes('at')); // expected output: false 

.indexOf



Description

The indexOf() method returns the first index at which a given element can be found in the array, or -1 if it is not present.

Array.prototype.indexOf ( searchElement [ , fromIndex ] )

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/indexOf


Example

var beasts = ['ant', 'bison', 'camel', 'duck', 'bison']; console.log(beasts.indexOf('bison')); // expected output: 1 // start from index 2 console.log(beasts.indexOf('bison', 2)); // expected output: 4 console.log(beasts.indexOf('giraffe')); // expected output: -1 

.join



Description

The join() method joins all elements of an array into a string.

Array.prototype.join (separator)

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/join


Example

var elements = ['Fire', 'Wind', 'Rain']; console.log(elements.join()); // expected output: Fire,Wind,Rain console.log(elements.join('')); // expected output: FireWindRain console.log(elements.join('-')); // expected output: Fire-Wind-Rain 

.keys()



Description

The keys() method returns a new Array Iterator that contains the keys for each index in the array.

arr.keys()

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/keys


Example

var array1 = ['a', 'b', 'c']; var iterator = array1.keys(); for (let key of iterator) { console.log(key); // expected output: 0 1 2 } 

.lastIndexOf



Description

The lastIndexOf() method returns the last index at which a given element can be found in the array, or -1 if it is not present. The array is searched backwards, starting at fromIndex.

Array.prototype.lastIndexOf ( searchElement [ , fromIndex ] )

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/lastIndexOf


Example

var animals = ['Dodo', 'Tiger', 'Penguin', 'Dodo']; console.log(animals.lastIndexOf('Dodo')); // expected output: 3 console.log(animals.lastIndexOf('Tiger')); // expected output: 1 

.map



Description

The map() method creates a new array with the results of calling a provided function on every element in this array.

Array.prototype.map ( callbackfn [ , thisArg ] )

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/map


Example

var array1 = [1, 4, 9, 16]; // pass a function to map const map1 = array1.map(x => x * 2); console.log(map1); // expected output: Array [2, 8, 18, 32] 

.pop



Description

The pop() method removes the last element from an array and returns that element. This method changes the length of the array.

Array.prototype.pop ( )

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/pop


Example

var plants = ['broccoli', 'cauliflower', 'cabbage', 'kale', 'tomato']; console.log(plants.pop()); // expected output: "tomato" console.log(plants); // expected output: Array ["broccoli", "cauliflower", "cabbage", "kale"] plants.pop(); console.log(plants); // expected output: Array ["broccoli", "cauliflower", "cabbage"] 

.push



Description

The push() method adds one or more elements to the end of an array and returns the new length of the array.

Array.prototype.push ( [ item1 [ , item2 [ , … ] ] ] )

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/push


Example

var animals = ['pigs', 'goats', 'sheep']; console.log(animals.push('cows')); // expected output: 4 console.log(animals); // expected output: Array ["pigs", "goats", "sheep", "cows"] animals.push('chickens'); console.log(animals); // expected output: Array ["pigs", "goats", "sheep", "cows", "chickens"] 

.reduce



Description

The reduce() method applies a function against an accumulator and each value of the array (from left-to-right) to reduce it to a single value.

Array.prototype.reduce ( callbackfn [ , initialValue ] )

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/Reduce


Example

const array1 = [1, 2, 3, 4]; const reducer = (accumulator, currentValue) => accumulator + currentValue; // 1 + 2 + 3 + 4 console.log(array1.reduce(reducer)); // expected output: 10 // 5 + 1 + 2 + 3 + 4 console.log(array1.reduce(reducer, 5)); // expected output: 15 

.reduceRight



Description

The reduceRight() method applies a function against an accumulator and each value of the array (from right-to-left) has to reduce it to a single value.

Array.prototype.reduceRight ( callbackfn [ , initialValue ] )

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/ReduceRight


Example

const array1 = [[0, 1], [2, 3], [4, 5]].reduceRight( (accumulator, currentValue) => accumulator.concat(currentValue) ); console.log(array1); // expected output: Array [4, 5, 2, 3, 0, 1] 

.reverse



Description

The reverse() method reverses an array in place. The first array element becomes the last, and the last array element becomes the first.

Array.prototype.reverse ( )

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/reverse


Example

var array1 = ['one', 'two', 'three']; console.log('array1: ', array1); // expected output: Array ['one', 'two', 'three'] var reversed = array1.reverse(); console.log('reversed: ', reversed); // expected output: Array ['three', 'two', 'one'] /* Careful: reverse is destructive. It also changes the original array */ console.log('array1: ', array1); // expected output: Array ['three', 'two', 'one'] 

.shift



Description

The shift() method removes the first element from an array and returns that element. This method changes the length of the array.

Array.prototype.shift ( )

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/shift


Example

var array1 = [1, 2, 3]; var firstElement = array1.shift(); console.log(array1); // expected output: Array [2, 3] console.log(firstElement); // expected output: 1 

.slice



Description

The slice() method returns a shallow copy of a portion of an array into a new array object selected from begin to end (end not included). The original array will not be modified.

Array.prototype.slice (start, end)

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/slice


Example

var animals = ['ant', 'bison', 'camel', 'duck', 'elephant']; console.log(animals.slice(2)); // expected output: Array ["camel", "duck", "elephant"] console.log(animals.slice(2, 4)); // expected output: Array ["camel", "duck"] console.log(animals.slice(1, 5)); // expected output: Array ["bison", "camel", "duck", "elephant"] 

.some



Description

The some() method tests whether some element in the array passes the test implemented by the provided function.

Array.prototype.some ( callbackfn [ , thisArg ] )

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/some


Example

var array = [1, 2, 3, 4, 5]; var even = function(element) { // checks whether an element is even return element % 2 === 0; }; console.log(array.some(even)); // expected output: true 

.sort



Description

The sort() method sorts the elements of an array in place and returns the array. The sort is not necessarily stable. The default sort order is according to string Unicode code points.

Array.prototype.sort (comparefn)

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/sort


Example

var months = ['March', 'Jan', 'Feb', 'Dec']; months.sort(); console.log(months); // expected output: Array ["Dec", "Feb", "Jan", "March"] var array1 = [1, 30, 4, 21]; array1.sort(); console.log(array1); // expected output: Array [1, 21, 30, 4] 

.splice



Description

The splice() method changes the content of an array by removing existing elements and/or adding new elements.

Array.prototype.splice (start, deleteCount [ , item1 [ , item2 [ , … ] ] ] )

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/splice


Example

var months = ['Jan', 'March', 'April', 'June']; months.splice(1, 0, 'Feb'); // inserts at 1st index position console.log(months); // expected output: Array ['Jan', 'Feb', 'March', 'April', 'June'] months.splice(4, 1, 'May'); // replaces 1 element at 4th index console.log(months); // expected output: Array ['Jan', 'Feb', 'March', 'April', 'May'] 

.toLocaleString



Description

The toLocaleString() method returns a string representing the elements of the array. The elements are converted to Strings using their toLocaleString methods and these Strings are separated by a locale-specific String (such as a comma “,”).

Array.prototype.toLocaleString ( )

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/toLocaleString


Example

var array1 = [1, 'a', new Date('21 Dec 1997 14:12:00 UTC')]; var localeString = array1.toLocaleString('en', {timeZone: "UTC"}); console.log(localeString); // expected output: "1,a,12/21/1997, 2:12:00 PM", // This assumes "en" locale and UTC timezone - your results may vary 

.toSource()



Description

The toSource() method returns a string representing the source code of the array.

arr.toSource()

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/toSource


Example


.toString



Description

The toString() method returns a string representing the specified array and its elements.

Array.prototype.toString ( )

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/toString


Example

var array1 = [1, 2, 'a', '1a']; console.log(array1.toString()); // expected output: "1,2,a,1a" 

.unshift



Description

The unshift() method adds one or more elements to the beginning of an array and returns the new length of the array.

Array.prototype.unshift ( [ item1 [ , item2 [ , … ] ] ] )

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/unshift


Example

var array1 = [1, 2, 3]; console.log(array1.unshift(4, 5)); // expected output: 5 console.log(array1); // expected output: Array [4, 5, 1, 2, 3] 

.values()



Description

The values() method returns a new Array Iterator object that contains the values for each index in the array.

arr.values()

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/values


Example

const array1 = ['a', 'b', 'c']; const iterator = array1.values(); for (const value of iterator) { console.log(value); // expected output: "a" "b" "c" } 


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