Variables and Types

Three ways to declare a variable

JavaScript gives you let, const, and the legacy var:

let score = 0;          // reassignable
const maxPlayers = 4;   // cannot be reassigned
var legacy = "avoid";   // function-scoped — avoid in modern JS

Default to const. Switch to let only when you need to reassign. Never use var.

Primitive types

JavaScript has 7 primitive types:

• string — text: "hello"
• number — integers and floats: 42, 3.14
• boolean — true / false
• null — intentionally empty
• undefined — not yet assigned
• symbol — unique identifiers
• bigint — large integers: 9007199254740991n

Type coercion

JavaScript silently converts between types, which can surprise you:

"5" + 3    // "53"  (string concatenation)
"5" - 3    // 2     (numeric subtraction)
0 == ""    // true  (loose equality coerces)
0 === ""   // false (strict equality does not coerce)

Always use === (strict equality) to avoid hidden type coercion bugs.

Checking types

typeof "hello"    // "string"
typeof 42         // "number"
typeof undefined  // "undefined"
typeof null       // "object"  (historical bug — use === null)