Understanding Variables and Data Types in JavaScript

// var (older style — function scoped)
var name = "Madhukar";
console.log(name); // Madhukar

// let (block scoped, reassignable)
let age = 20;
console.log(age); // 20
age = 21;
console.log(age); // 21

// const (block scoped, not reassignable)
const isStudent = true;
console.log(isStudent); // true
// isStudent = false; // Error: Assignment to constant variable.

Primitive data types (simple definitions + examples)

1. String — text.
Example: “Alice”, “hello”

2. Number — integers or decimals.
Example: 42, 3.14

3. Boolean — true or false.
Example: true, false

4. Null — deliberate absence of a value.
Example:
let x = null;

5. Undefined — variable declared but not given a value.
Example:
let y;

6. BigInt — represents integers larger than JavaScript’s safe integer limit (`Number.MAX_SAFE_INTEGER`).
Example:
let bigNumber = 9007199254740993n;

7. Symbol — represents a unique and immutable value, commonly used as a unique object property key.
Example:
const uniqueId = Symbol(“id”);

let name = "Alice";               // string
let score = 95.5;                 // number
let passed = true;                // boolean
let nothing = null;               // null
let notSet;                       // undefined
let bigNumber = 9007199254740993n; // bigint
let uniqueId = Symbol("id");      // symbol

console.log(typeof name);       // "string"
console.log(typeof score);      // "number"
console.log(typeof passed);     // "boolean"
console.log(nothing);           // null
console.log(notSet);            // undefined
console.log(typeof bigNumber);  // "bigint"
console.log(typeof uniqueId);   // "symbol"

What is scope — a beginner-friendly explanation

Scope defines where a variable is visible in the code.

Global scope — accessible anywhere in the script.

Module scope — variables declared at the top level of an ES module are accessible throughout that module but are not automatically available in other modules unless exported.

Function scope — visible only inside the function.

Block scope — visible only inside { … } (for let and const).

Think: each {} is a room. If you declare a variable inside a room, people outside the room can't see it (unless it's global).

Examples:

// global
let globalVar = "I am global";

function example() {
  // function scope
  var functionVar = "Only in function";
  let blockVar = "Only in block";
  if (true) {
    let insideBlock = "Visible only inside this if-block";
    console.log(insideBlock); // works
  }
  // console.log(insideBlock); // Error: not visible here
}

console.log(globalVar); // visible
// console.log(functionVar); // Error: not visible here (outside function)

Block-scope example showing let/const vs var:

if (true) {
  var oldStyle = "var here";
  let modern = "let here";
}
console.log(oldStyle); // "var here" — visible outside the block
// console.log(modern); // Error — modern is block-scoped

Module scope example

// math.js
const pi = 3.14159;

export function area(radius) {
  return pi * radius * radius;
}

How values can change (and when they can’t)

• let variables can be reassigned:
• let age = 20;
  age = age + 1; // now 21
• const cannot be reassigned — but if it holds an object or array, you can still change the contents:
• const person = { name: “Alice” };
  person.name = “Bob”; // allowed — the object is mutated
  // person = {}; // not allowed — cannot reassign the variable

const Name = "Madhukar";
let Age = 20;
const IsStudent = true;

console.log(Name, Age, IsStudent); // Madhukar 20 true

Age = 21;
console.log(Age); // 21

// IsStudent = false; // Uncommenting this will raise an error