Javascript Essentials: Array

Array methods

The Array object is a special variable, which can hold more than one value.

Arrays are resizable and can contain a mix of different data types.

Arrays are zero-indexed: the first element of an array is at index 0, the second is at index 1.

Constructor

Array() — Creates a new Array object.

Methods

Array methods are functions built-in to JavaScript that we can apply to our arrays.

The following methods are available for arrays:

Array.map() -> new array

Returns a new array with the results of calling a provided function on every element in this array.

const list = [😫, 😫, 😫, 😫];
list.map((⚪️) => 😀); // [😀, 😀, 😀, 😀]
// Code
const list = [1, 2, 3, 4];
list.map((el) => el * 2); // [2, 4, 6, 8]

Array.filter() -> new array

Returns a new array with all elements that pass the test implemented by the provided function.

const list = [😀, 😫, 😀, 😫];
list.filter((⚪️) => ⚪️ === 😀); // [😀, 😀]
// Code
const list = [1, 2, 3, 4];
list.filter((el) => el % 2 === 0); // [2, 4]

Array.reduce() -> single value

Reduce the array to a single value. The value returned by the function is stored in an accumulator (result/total).

const list = [😀, 😫, 😀, 😫, 🤪];
list.reduce((⬜️, ⚪️) => ⬜️ + ⚪️); // 😀 + 😫 + 😀 + 😫 + 🤪
// OR
const list = [1, 2, 3, 4, 5];
list.reduce((total, item) => total + item, 0); // 15

Array.reduceRight() ->single value

Executes a reducer function (that you provide) on each element of the array resulting in a single output value(from right to left).

const list = [😀, 😫, 😀, 😫, 🤪];
list.reduceRight((⬜️, ⚪️) => ⬜️ + ⚪️); // 🤪 + 😫 + 😀 + 😫 + 😀
// Code
const list = [1, 2, 3, 4, 5];
list.reduceRight((total, item) => total + item, 0); // 15

Array.fill() -> -mutate original array

Fill the elements in an array with a static value.

const list = [😀, 😫, 😀, 😫, 🤪];
list.fill(😀); // [😀, 😀, 😀, 😀, 😀]
// Code
const list = [1, 2, 3, 4, 5];
list.fill(0); // [0, 0, 0, 0, 0]

Array.find() -> value of first element / undefined

Returns the value of the first element in the array that satisfies the provided testing function. Otherwise undefined is returned.

const list = [😀, 😫, 😀, 😫, 🤪];
list.find((⚪️) => ⚪️ === 😀); // 😀
list.find((⚪️) => ⚪️ === 😝); // undefined
// Code
const list = [1, 2, 3, 4, 5];
list.find((el) => el === 3); // 3
list.find((el) => el === 6); // undefined

Array.indexOf() -> index

Returns the first index at which a given element can be found in the array, or -1 if it is not present.

const list = [😀, 😫, 😀, 😫, 🤪];
list.indexOf(😀); // 0
list.indexOf(😡); // -1
// Code
const list = [1, 2, 3, 4, 5];
list.indexOf(3); // 2
list.indexOf(6); // -1

Array.lastIndexOf() -> last index / or -1

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.

const list = [😀, 😫, 😀, 😫, 🤪];
list.lastIndexOf(😀); // 3
list.lastIndexOf(😀, 1); // 0
// Code
const list = [1, 2, 3, 4, 5];
list.lastIndexOf(3); // 2
list.lastIndexOf(3, 1); // -1

Array.findIndex() — index / or -1

Returns the index of the first element in the array that satisfies the provided testing function. Otherwise -1 is returned.

const list = [😀, 😫, 😀, 😫, 🤪];
list.findIndex((⚪️) => ⚪️ === 😀); // 0
// You might be thinking how it's different from `indexOf` 🤔
const array = [5, 12, 8, 130, 44];
array.findIndex((element) => element > 13); // 3
// OR
const array = [{
  id: 😀
}, {
  id: 😫
}, {
  id: 🤪
}];
array.findIndex((element) => element.id === 🤪); // 2

Array.includes() / true/false

Returns true if the given element is present in the array.

const list = [😀, 😫, 😀, 😫, 🤪];
list.includes(😀); // true
// Code
const list = [1, 2, 3, 4, 5];
list.includes(3); // true
list.includes(6); // false

Array.pop() -mutate original array

Removes the last element from an array and returns that element.

const list = [😀, 😫, 😀, 😫, 🤪];
list.pop(); // 🤪
list; // [😀, 😫, 😀, 😫]
// Code
const list = [1, 2, 3, 4, 5];
list.pop(); // 5
list; // [1, 2, 3, 4]

Array.push() -mutate original array

Appends new elements to the end of an array, and returns the new length.

const list = [😀, 😫, 😀, 😫, 🤪];
list.push(😡); // 5
list; // [😀, 😫, 😀, 😫, 🤪, 😡]
// Code
const list = [1, 2, 3, 4, 5];
list.push(6); // 6
list; // [1, 2, 3, 4, 5, 6]

Array.shift() -mutate original array

Removes the first element from an array and returns that element.

const list = [😀, 😫, 😀, 😫, 🤪];
list.shift(); // 😀
list; // [😫, 😀, 😫, 🤪]
// Code
const list = [1, 2, 3, 4, 5];
list.shift(); // 1
list; // [2, 3, 4, 5]

Array.unshift() -mutate original array

Adds new elements to the beginning of an array, and returns the new length.

const list = [😀, 😫, 😀, 😫, 🤪];
list.unshift(😡); // 6
list; // [😡, 😀, 😫, 😀, 😫, 🤪]
// Code
const list = [1, 2, 3, 4, 5];
list.unshift(0); // 6
list; // [0, 1, 2, 3, 4, 5]

Array.splice() -mutate original array

Changes the contents of an array by removing or replacing existing elements and/or adding new elements in place.

const list = [😀, 😫, 😀, 😫, 🤪];
list.splice(1, 2); // [😀, 😫]
list; // [😀, 😫, 🤪]
// Code
const list = [1, 2, 3, 4, 5];
list.splice(1, 2); // [2, 3]
list; // [1, 4, 5]

Array.slice() -> copy of array

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.

const list = [😀, 😫, 😀, 😫, 🤪];
list.slice(1, 3); // [😫, 😀]
list; // [😀, 😫, 😀, 😫, 🤪]
// Code
const list = [1, 2, 3, 4, 5];
list.slice(1, 3); // [2, 3]
list; // [1, 2, 3, 4, 5]

Array.join() → join elements into a string

Joins all elements of an array into a string.

const list = [😀, 😫, 😀, 😫, 🤪];
list.join('〰️'); // "😀〰️😫〰️😀〰️😫〰️🤪"
// Code
const list = [1, 2, 3, 4, 5];
list.join(', '); // "1, 2, 3, 4, 5"

Array.reverse() -> mutate original array

Reverses the order of the elements in an array.

const list = [😀, 😫, 😀, 😫, 🤪];
list.reverse(); // [🤪, 😫, 😀, 😫, 😀]
list; // [🤪, 😫, 😀, 😫, 😀]
// Code
const list = [1, 2, 3, 4, 5];
list.reverse(); // [5, 4, 3, 2, 1]
list; // [5, 4, 3, 2, 1]

Array.sort()->mutate original array

Sorts the elements of an array in place and returns the array. The default sort order is according to string Unicode code points.

const list = [😀, 😫, 😀, 😫, 🤪];
list.sort(); // [😀, 😀, 😫, 😫, 🤪]
// This make more sense 🤔
const array = ['D', 'B', 'A', 'C'];
array.sort(); // 😀 ['A', 'B', 'C', 'D']
// OR
const array = [4, 1, 3, 2, 10];
array.sort(); // 😧 [1, 10, 2, 3, 4]
array.sort((a, b) => a - b); // 😀 [1, 2, 3, 4, 10]

Array.some() — true / false

Returns true if at least one element in the array passes the test implemented by the provided function.

const list = [😀, 😫, 😀, 😫, 🤪];
list.some((⚪️) => ⚪️ === 😀); // true
list.some((⚪️) => ⚪️ === 😡); // false// Code
const list = [1, 2, 3, 4, 5];
list.some((el) => el === 3); // true
list.some((el) => el === 6); // false

Array.every() -> true/false

Returns true if all elements in the array pass the test implemented by the provided function.

const list = [😀, 😫, 😀, 😫, 🤪];
list.every((⚪️) => ⚪️ === 😀); // false
const list = [😀, 😀, 😀, 😀, 😀];
list.every((⚪️) => ⚪️ === 😀); // true
// Code
const list = [1, 2, 3, 4, 5];
list.every((el) => el === 3); // false
const list = [2, 4, 6, 8, 10];
list.every((el) => el%2 === 0); // true

Array.from() -> array

Creates a new array from an array-like or iterable object.

const list = 😀😫😀😫🤪;
Array.from(list); // [😀, 😫, 😀, 😫, 🤪]
const set = new Set(['😀', '😫', '😀', '😫', '🤪']);
Array.from(set); // [😀, 😫, 🤪]
const range = (n) => Array.from({ length: n }, (_, i) => i + 1);
console.log(range(10)); // [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

Array.of() -> array

Creates a new array with a variable number of arguments, regardless of number or type of the arguments.

const list = Array.of(😀, 😫, 😀, 😫, 🤪);
list; // [😀, 😫, 😀, 😫, 🤪]
// Code
const list = Array.of(1, 2, 3, 4, 5);
list; // [1, 2, 3, 4, 5]

Array.isArray() -> true/false

Returns true if the given value is an array.

Array.isArray([😀, 😫, 😀, 😫, 🤪]); // true
Array.isArray(🤪); // false
// Code
Array.isArray([1, 2, 3, 4, 5]); // true
Array.isArray(5); // false

Array.at() -> index

Returns a value at the specified index.

const list = [😀, 😫, 😀, 😫, 🤪];
list.at(1); // 😫
// Return from last 🤔
list.at(-1); // 🤪
list.at(-2); // 😫
// Code
const list = [1, 2, 3, 4, 5];
list.at(1); // 2
list.at(-1); // 5
list.at(-2); // 4

Array.copyWithin() -> array

Copies array elements within the array. Returns the modified array.

const list = [😀, 😫, 😀, 😫, 🤪];
list.copyWithin(1, 3); // [😀, 😀, 🤪, 😫, 🤪]

const list = [😀, 😫, 😀, 😫, 🤪];
list.copyWithin(0, 3, 4); // [😫, 😫, 😀, 😫, 🤪]

// Code
const list = [1, 2, 3, 4, 5];
list.copyWithin(0, 3, 4); // [4, 2, 3, 4, 5]

NOTE: 🤔

• first argument is the target at which to start copying elements from.
• second argument is the index at which to start copying elements from.
• third argument is the index at which to stop copying elements from.

Array.flat()-> new array

Returns a new array with all sub-array elements concatenated into it recursively up to the specified depth.

const list = [😀, 😫, [😀, 😫, 🤪]];
list.flat(Infinity); // [😀, 😫, 😀, 😫, 🤪]
// Code
const list = [1, 2, [3, 4, [5, 6]]];
list.flat(Infinity); // [1, 2, 3, 4, 5, 6]

Array.flatMap()-> new array

Returns a new array formed by applying a given callback function to each element of the array,

const list = [😀, 😫, [😀, 😫, 🤪]];
list.flatMap((⚪️) => [⚪️, ⚪️ + ⚪️ ]); // [😀, 😀😀, 😫, 😫😫, 😀, 😀😀, 😫, 😫😫, 🤪, 🤪🤪]
// Code
const list = [1, 2, 3];
list.flatMap((el) => [el, el * el]); // [1, 1, 2, 4, 3, 9]