Variables & Data Types

Variables & Data Types

What it means

JavaScript variables store data values. Modern JavaScript offers three ways to declare variables: var (legacy), let (block-scoped), and const (constant).

Data types include primitives (string, number, boolean, null, undefined, symbol, bigint) and objects (including arrays, functions).

Examples

// Variable declarations
let age = 25;           // mutable
const name = "Alice";   // immutable
var legacy = "old";     // function-scoped (avoid)

// Data types
let text = "Hello";     // string
let count = 42;         // number
let price = 19.99;      // number (no separate float)
let isValid = true;     // boolean
let empty = null;       // null
let notDefined;         // undefined
let id = Symbol("id");  // symbol
let huge = 123n;        // bigint

// Objects
let user = { name: "Bob", age: 30 };
let colors = ["red", "green", "blue"];

Common mistake

Using var instead of let/const causes scope issues:

// BAD - var has function scope
for (var i = 0; i < 3; i++) {
  setTimeout(() => console.log(i), 100);
}
// Prints: 3, 3, 3

// GOOD - let has block scope
for (let i = 0; i < 3; i++) {
  setTimeout(() => console.log(i), 100);
}
// Prints: 0, 1, 2

Fix: Always use let or const instead of var.

Quick practice

1. What’s the difference between let and const?

   Answer

   `let` allows reassignment; `const` prevents reassignment (but object properties can still change).

2. What does typeof null return?

   Answer

   `"object"` (this is a known JavaScript quirk).

3. Declare a constant array and add an item to it. Does this work?

   Answer

   Yes! `const arr = []; arr.push(1);` works because `const` prevents reassignment, not mutation.