Does it mutate 😱
Array.prototype.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"]
Array.prototype.copyWithin()
Mutates
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"]
Array.prototype.entries()
No mutation
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"]
Array.prototype.every
No mutation
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
Array.prototype.fill()
Mutates
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]
Array.prototype.filter
No mutation
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"]
Array.prototype.find()
No mutation
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
Array.prototype.findIndex()
No mutation
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
Array.prototype.flat
No mutation
Description
The flat() method creates a new array with all sub-array elements concatenated into it recursively up to the specified depth.
var newArray = arr.flat([depth]);https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/flat
Example
var arr1 = [1, 2, [3, 4]];
arr1.flat();
// [1, 2, 3, 4]
var arr2 = [1, 2, [3, 4, [5, 6]]];
arr2.flat();
// [1, 2, 3, 4, [5, 6]]
var arr3 = [1, 2, [3, 4, [5, 6]]];
arr3.flat(2);
// [1, 2, 3, 4, 5, 6]
var arr4 = [1, 2, [3, 4, [5, 6, [7, 8, [9, 10]]]]];
arr4.flat(Infinity);
// [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
Array.prototype.forEach
No mutation
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"
Array.prototype.includes()
No mutation
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
Array.prototype.indexOf
No mutation
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
Array.prototype.join
No mutation
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', 'Air', 'Water'];
console.log(elements.join());
// expected output: "Fire,Air,Water"
console.log(elements.join(''));
// expected output: "FireAirWater"
console.log(elements.join('-'));
// expected output: "Fire-Air-Water"
Array.prototype.keys()
No mutation
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
}
Array.prototype.lastIndexOf
No mutation
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
Array.prototype.map
No mutation
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]
Array.prototype.pop
Mutates
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"]
Array.prototype.push
Mutates
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"]
Array.prototype.reduce
No mutation
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
Array.prototype.reduceRight
No mutation
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]
Array.prototype.reverse
Mutates
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']
Array.prototype.shift
Mutates
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
Array.prototype.slice
No mutation
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"]
Array.prototype.some
No mutation
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
Array.prototype.sort
Mutates
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, 100000];
array1.sort();
console.log(array1);
// expected output: Array [1, 100000, 21, 30, 4]
Array.prototype.splice
Mutates
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']
Array.prototype.toLocaleString
No mutation
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 ( )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
Array.prototype.toSource()
No mutation
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
var alpha = new Array('a', 'b', 'c');
alpha.toSource();
//returns ['a', 'b', 'c']Array.prototype.toString
No mutation
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"
Array.prototype.unshift
Mutates
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]
Array.prototype.values()
No mutation
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"
}