Variables and Data Types

JavaScript has three ways to declare variables: var, let, and const.

let and const (ES6+)

let name = "John";     // Can be reassigned
const age = 25;        // Cannot be reassigned

Data Types

JavaScript has 7 primitive types:

// String
let greeting = "Hello, World!";
// Number
let count = 42;
let price = 19.99;
// Boolean
let isActive = true;
// undefined
let notDefined;
// null
let empty = null;
// Symbol (ES6)
let sym = Symbol('unique');
// BigInt (ES2020)
let bigNumber = 9007199254740991n;

typeof Operator

typeof "hello"     // "string"
typeof 42          // "number"
typeof true        // "boolean"
typeof undefined   // "undefined"
typeof null        // "object" (historical bug)
typeof {}          // "object"
typeof []          // "object"
typeof function(){} // "function"

Best Practices