Arrays

I. What is array?

Arrays are lists that store data in .

II. Create an array

1. Use array constructure

We use the new keyword to construct an array.

const numbers = new Array(1, 2, 3);

console.log(numbers); // [1, 2, 3]

2. Use square brackets

We can create an array by wrapping items in square brackets []. Array is 0 based.

const names = ["John", "Ana", "Peter"];

console.log(names[1]); // Ana

Variables that contain arrays can be declared with let or const.

Although variables declared with the const keyword cannot be reassigned, elements in an array declared with const remain mutable.

Meaning that we can change the contents of a const array, but cannot reassign a new array or a different value.

const students = ["John", "Ana", "Peter", "Noah"];

students[3] = "Mira";

console.log(students); // Output: ['John', 'Ana', 'Mira', 'Noah']

III. Copy an array

To copy arrays, we use spread operator.

const itemsCopy = [...items];

Example:

const arr1 = [1, 2, 3, 4, 5];
let arr2;
(function () {
  arr2 = [...arr1];
  arr1[0] = 10;
})();
console.log(arr2); // [ 1, 2, 3, 4, 5 ]

IV. Access an array

Each element in an array has a numbered position known as its index, starting at 0.

We can access one item in an array using its index, with syntax myArray[0].

In the example below, the console will display e because the index is 1.

const hello = "Hello World";
console.log(hello[1]);
// Output: e

V. Update elements in array

We can also change an item in an array using its index, with syntax

myArray[0] = 'new string';

VI. Nested arrays

Arrays can be nested inside other arrays. To access the nested arrays we can use bracket notation with the index value

const nestedArray = [[5], [8, 10]];

console.log(nestedArray[1]); // Output: [8, 10]

VII. Check length of array

We can check how many elements are in an array with the .length property.

console.log(array.length);
const students = ["John", "Ana", "Peter", "Noah"];

console.log(students.length);
// Output: 4