Complete JavaScript Cheat Sheet: Arrays

JavaScript arrays are versatile structures that allow you to store and manipulate collections of data. This cheat sheet provides a detailed guide to array properties, methods, and examples for each to help you get started or refresh your knowledge.

Array Properties

`constructor`

Returns the constructor function for an array.
Example 1:

let arr = [1, 2, 3]; console.log(arr.constructor); // Output: function Array() { [native code] }

Example 2

let str = "Hello"; console.log(str.constructor); // Output: function String() { [native code] }

`length`

Sets or returns the number of elements in an array.
Example 1:

let arr = [1, 2, 3, 4]; console.log(arr.length); // Output: 4

Example 2:

arr.length = 2; console.log(arr); // Output: [1, 2]

`prototype`

Allows you to add new properties and methods to arrays.

Example 1:

Array.prototype.sum = function() { return this.reduce((acc, val) => acc + val, 0); }; console.log([1, 2, 3].sum()); // Output: 6

Example 2

Array.prototype.last = function() { return this[this.length - 1]; }; console.log([1, 2, 3].last()); // Output: 3

Array Methods

`concat()`

Joins two or more arrays and returns a new array.
Example 1

let arr1 = [1, 2]; let arr2 = [3, 4]; console.log(arr1.concat(arr2)); // Output: [1, 2, 3, 4]

Example 2:

let arr3 = ['a', 'b']; let arr4 = ['c', 'd']; console.log(arr3.concat(arr4, [5, 6])); // Output: ['a', 'b', 'c', 'd', 5, 6]

`copyWithin()`

Copies array elements to another position in the array.
Example 1

let arr = [1, 2, 3, 4, 5]; console.log(arr.copyWithin(0, 3)); // Output: [4, 5, 3, 4, 5]

Example 2

let arr = ['a', 'b', 'c', 'd']; console.log(arr.copyWithin(1, 2)); // Output: ['a', 'c', 'd', 'd']

`entries()`

Returns an Array Iterator object with key/value pairs.

Example 1

let arr = ['a', 'b', 'c']; for (let [index, value] of arr.entries()) { console.log(index, value); } // Output: // 0 'a' // 1 'b' // 2 'c'

Example 2

let iterator = arr.entries(); console.log(iterator.next().value); // Output: [0, 'a']

`every()`

Tests whether all elements in an array pass a test.
Example 1

let arr = [1, 2, 3, 4]; console.log(arr.every(val => val < 5)); // Output: true

Example 2

let arr = [1, 2, 3, 4]; console.log(arr.every(val => val % 2 === 0)); // Output: false

`fill()`

Fills elements in an array with a static value.
Example 1

let arr = [1, 2, 3, 4]; console.log(arr.fill(0, 2, 4)); // Output: [1, 2, 0, 0]

Example 2:

let arr = Array(4); console.log(arr.fill('a')); // Output: ['a', 'a', 'a', 'a']

`filter()`

Creates an array filled with elements that pass a test.

Example 1

let arr = [1, 2, 3, 4]; console.log(arr.filter(val => val > 2)); // Output: [3, 4]

Example 2

let arr = ['apple', 'banana', 'cherry']; console.log(arr.filter(val => val.includes('a'))); // Output: ['apple', 'banana']

`findIndex()`

Returns the index of the first element in an array that passes a test.
Example 1

let arr = [1, 2, 3, 4]; console.log(arr.findIndex(val => val > 2)); // Output: 2

Example 2

let arr = ['cat', 'dog', 'fish']; console.log(arr.findIndex(val => val.length > 3)); // Output: 2

`forEach()`

Calls a function once for each element in an array, in order.
Example 1:

let arr = [1, 2, 3]; arr.forEach((value, index) => { console.log(Index: ${index}, Value: ${value}); }); // Output: // Index: 0, Value: 1 // Index: 1, Value: 2 // Index: 2, Value: 3

Example 2

let fruits = ['apple', 'banana', 'cherry']; fruits.forEach(fruit => console.log(fruit)); // Output: apple, banana, cherry

`includes()`

Determines whether an array contains a specified element.

Example 1:

let arr = [1, 2, 3]; console.log(arr.includes(2)); // Output: true

Example 2

let names = ['John', 'Jane', 'Joe']; console.log(names.includes('Jack')); // Output: false

`join()`

Converts all the elements of an array into a string.
Example 1:

let arr = ['a', 'b', 'c']; console.log(arr.join('-')); // Output: "a-b-c"

Example 2

let numbers = [1, 2, 3]; console.log(numbers.join(', ')); // Output: "1, 2, 3"

`lastIndexOf()`

Searches the array for the specified item and returns its position (last occurrence).
Example 1

let arr = [1, 2, 3, 2, 1]; console.log(arr.lastIndexOf(2)); // Output: 3

Example 2

let chars = ['a', 'b', 'c', 'b', 'a']; console.log(chars.lastIndexOf('b')); // Output: 3

`map()`

Creates a new array by calling a function on every element in the array.

Example 1

let arr = [1, 2, 3]; let doubled = arr.map(x => x * 2); console.log(doubled); // Output: [2, 4, 6]

Example 2

let names = ['John', 'Jane']; let greetings = names.map(name => Hello, ${name}!); console.log(greetings); // Output: ["Hello, John!", "Hello, Jane!"]

`pop()`

Removes the last element of an array and returns that element.
Example 1:

let arr = [1, 2, 3]; let last = arr.pop(); console.log(last); // Output: 3 console.log(arr); // Output: [1, 2]

Example 2

let fruits = ['apple', 'banana', 'cherry']; console.log(fruits.pop()); // Output: cherry

`push()`

Adds new items to the end of an array and returns the new length.
Example 1

let arr = [1, 2, 3]; console.log(arr.push(4)); // Output: 4 console.log(arr); // Output: [1, 2, 3, 4]

Example 2

let fruits = ['apple']; fruits.push('banana', 'cherry'); console.log(fruits); // Output: ['apple', 'banana', 'cherry']

`isArray()`

Determines whether an object is an array.

Example 1

let arr = [1, 2, 3]; console.log(Array.isArray(arr)); // Output: true

Example 2

let notArray = "hello"; console.log(Array.isArray(notArray)); // Output: false

reduce()

Reduces the array to a single value by executing a function for each value (from left to right).

Example 1

let arr = [1, 2, 3]; let sum = arr.reduce((acc, val) => acc + val, 0); console.log(sum); // Output: 6

Example 2

let numbers = [5, 10, 15]; let product = numbers.reduce((acc, val) => acc * val, 1); console.log(product); // Output: 750

`reduceRight()`

Reduces the array to a single value by executing a function for each value (from right to left).
Example 1

let arr = [1, 2, 3]; let sum = arr.reduceRight((acc, val) => acc + val, 0); console.log(sum); // Output: 6

Example 2

let words = ["right", "from", "reduce"]; let sentence = words.reduceRight((acc, word) => acc + " " + word); console.log(sentence); // Output: "reduce from right"