Variables

Variables are containers for storing data. JavaScript has three ways to declare variables: var, let, and const.

var, let, const

var name = 'Alice'    // old way (avoid in modern JS)
let age = 25          // block-scoped, can be reassigned
const PI = 3.14159    // block-scoped, cannot be reassigned

Naming Rules

• Use camelCase for variable names: myVariable, firstName
• Names can contain letters, digits, _, $
• Names cannot start with a digit
• Names are case-sensitive: myVar ≠ myvar

Reserved Words (28)

These words cannot be used as variable names:

break, case, catch, class, const, continue, debugger,
default, delete, do, else, export, extends, finally,
for, function, if, import, in, instanceof, let, new,
return, super, switch, this, throw, try, typeof, var,
void, while, with, yield

Examples

let firstName = 'John'
let lastName = 'Doe'
let fullName = firstName + ' ' + lastName
console.log(fullName) // John Doe

const MAX_SIZE = 100
// MAX_SIZE = 200  // Error! Cannot reassign const