Arrays in JavaScript are a special type of object. They resemble Python Lists (an ordered and changeable collection, allowing duplicate members and members of different types).
const numbers = [1, 2, 2, "three", 4];
console.log(numbers); // 1,2,2,three,4
console.log(typeof numbers); // objectYou can use the bracket notation to access and modify the array elements.
const numbers = [1, 2, 3, 4];
for (let i = 1; i < numbers.length; i++) {
numbers[i] = numbers[i] + numbers[i - 1];
}
console.log(numbers); // 1,3,6,10You can create an empty array and then add values to it:
const numbers = []; // []
numbers[0] = 10; // [10]
numbers[1] = 11; // [10, 11]
numbers.push(12); // [10, 11, 12]
numbers.pop(); // [10, 11]
numbers.push(13); // [10, 11, 13]
console.log(numbers); // 10,11,13You can make an empty array using array constructor syntax:
const numbers = new Array();But it is more common to use the array literal syntax instead:
const numbers = [];Madness: In an array, you can leave some elements undefined!
const numbers = [, 2, , 4, ]; // [undefined, 2, undefined, 4]
for (let i = 0; i < numbers.length; i++) {
console.log(numbers[i]);
}Note how the trailing comma does not denote an undefined element!
Allowing undefined elements leads to more annoying behaviors.
const numbers = [];
numbers[99] = "hundred";
console.log(numbers.length); // 100
console.log(numbers[0]); // undefined
console.log(numbers[99]); // hundred
console.log(numbers[100]); // undefinedlength is calculated as one more than the highest index!
And the craziest thing is that you can overwrite the length property!
const numbers = [1, 2, 3, 4];
numbers.length = 0;
console.log(numbers[0]); // undefined
console.log(numbers[1]); // undefinedConvenient syntax for fetching multiple elements:
const numbers = [10, 20, 30, 40];
// let first = numbers[0];
// let second = numbers[1];
let [first, second] = numbers;
console.log(first, second); // 10 20You can even do this:
const numbers = [10, 20, 30, 40];
let [first, second, ...others] = numbers;
console.log(first, second, others); // 10 20 30,40Use arrays of arrays for multi-dimensional arrays:
const magicSquare = [
[16, 3, 2, 13],
[5, 10, 11, 8],
[9, 6, 7, 12],
[4, 15, 14, 1]
];
// Use two bracket pairs to access an element:
console.log(magicSquare[1][2]); // 11Array object comes with many useful built-in operations. We will explore some of these in future lessons. For the impatient, please visit Array on MDN web docs.