JavaScript Variables & Data Types

Variables store data values. JavaScript has multiple ways to declare variables and many data types.

Declaring Variables

var — Old Way (Avoid)

var name = "John";
var age = 25;

Function-scoped, can be redeclared, and hoisted. Use let or const instead.

let — When Reassigning

let name = "John";
name = "Jane"; // OK

Block-scoped and can be reassigned.

const — Default Choice

const PI = 3.14159;
// PI = 3; // Error!

Block-scoped, cannot be reassigned. Must initialize immediately.

Rule: Use const by default. Use let only when reassignment is needed.

Naming Rules

let userName = "John";  // camelCase
let _private = true;    // underscore OK
let $element = "div";   // dollar sign OK

// let 123abc = "bad";  // Error: can't start with number
// let my-name = "bad"; // Error: no hyphens

Convention: Use camelCase for variables and functions.

Primitive Data Types

let name = "John";          // String
let age = 25;               // Number
let isActive = true;        // Boolean
let x;                      // undefined
let data = null;            // null
let id = Symbol('id');      // Symbol
let big = 9007199254740991n; // BigInt

Remember: null is intentional emptiness. undefined means no value assigned.

Checking Types

typeof "John"     // "string"
typeof 25         // "number"
typeof true       // "boolean"
typeof undefined  // "undefined"
typeof null       // "object" (known JS bug)
typeof {}         // "object"
typeof []         // "object"

Type Conversion

// String → Number
Number("123")      // 123
parseInt("123abc") // 123
+"123"             // 123

// Number → String
String(123)        // "123"
(123).toString()   // "123"
`${123}`           // "123"

// To Boolean
Boolean(0)         // false
Boolean("")        // false
Boolean(null)      // false
Boolean("hello")   // true
Boolean({})        // true
Boolean([])        // true

Best Practices

1. Use const by default — let only when needed
2. Use descriptive names — userName not x
3. Use camelCase — myVariable not my_variable
4. Initialize variables — Avoid undefined
5. Use template literals — Instead of string concatenation

Common Mistakes

1. Using var — Use let/const instead
2. Redeclaring with let — Not allowed
3. Confusing null and undefined — Different meanings
4. Using typeof null — Returns "object" (bug in JS)
5. Confusing = and === — Assignment vs comparison