Variables & Types

Declaring variables

In JavaScript, you can declare a variable with var, let, or const.

Keyword	Description
var	Old way of declaring variables (still works but should be avoided because of confusing scope).
let	A modifiable variable, limited to the block { } where it is declared.
const	A constant variable, cannot be reassigned (but its content can still change if it’s an object or an array).

let name = "Baptiste";
const age = 26;

⚠️ With const, if the value is an object or an array, you can still modify its contents:

const fruits = ["apple", "banana"];
fruits.push("pear");
console.log(fruits); // ["apple", "banana", "pear"]

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Data types in JavaScript

In JavaScript, there are two main categories:

Primitive types

Primitives are immutable (their value cannot change once created) and are stored directly in memory.

• String: text
• Number: number
• Boolean: true/false
• Null: empty value
• Undefined: undefined value
• Symbol: unique identifier (rare, advanced)
• BigInt: very large integers

let text = "Hello";     // string
let number = 42;        // number
let active = true;      // boolean
let empty = null;       // null
let unknown;            // undefined
let bigNumber = 123456789012345678901234567890n; // BigInt

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Reference types

References point to objects in memory.

• Object: key/value pairs
• Array: ordered list
• Function: also an object in JavaScript

let person = { name: "Alice", age: 30 }; // object
let fruits = ["apple", "banana"];        // array
function greet() { return "Hello"; }     // function

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Checking a variable’s type

Using typeof:

console.log(typeof "Hello");   // string
console.log(typeof 42);        // number
console.log(typeof true);      // boolean
console.log(typeof null);      // object (⚠️ historical bug in the language)
console.log(typeof []);        // object
console.log(typeof function(){}); // function

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Practical example

let x = 10;
let y = 5;
let sum = x + y;
console.log("The sum is:", sum); // The sum is: 15

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Modern best practices

• Use const by default.
• Use let only when the value needs to change.
• Avoid var, except when reading old code.
• Use typeof to check the type when necessary.