Declaring arrays

Learn to declare and initialize arrays with [].

Declaring arrays

To hold several values in a single variable you use an array. In JavaScript an array is declared with comma-separated values inside square brackets []: const colors = ["red", "blue", "green"];.

Each value in an array is called an element. An array may hold zero, one or more elements. An empty array is created with []; you can add elements later with push. Strings, numbers, booleans and even objects can all live in the same array; but good practice keeps elements of a similar type.

The length property

The .length property tells how many elements an array has. [10, 20, 30].length evaluates to 3. length is a property, not a method; there is no parenthesis at the end.

length is especially useful for loop bounds: with for (let i = 0; i < arr.length; i++) you walk all elements. An empty array has length 0. Although length can be set directly, it is uncommon; to resize an array you use methods like push, pop or splice.

Exercise Declare and print the array

Declare a variable colors and set its value to ["red", "blue", "green"]. Then print colors.length with console.log. The output on a single line must be 3.

Show example solution

const colors = ["red", "blue", "green"];
console.log(colors.length);

Test yourself

Which is a valid array declaration?
```
const arr = ...;
```
- const arr = [1, 2, 3];
- const arr = {1, 2, 3};
- const arr = (1, 2, 3);
- const arr = <1, 2, 3>;
Show answer

const arr = [1, 2, 3]; — Arrays are declared inside square brackets [].

Write the array literal

const colors = ____;

Show answer

const colors = ["red", "blue"];

Match each expression to its meaning

[]

Empty array
[1, 2, 3]

Three-element array
.length

Element count
["a"]

Single-element array

Next lesson: Array access

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