JavaScript Data Types

🚀 What You’ll Learn

In this lesson, you’ll learn:

• What are data types and how to use them in JavaScript.
• JavaScript data types (primitive & reference).
• How type conversion works in JS.

🟡 JavaScript Data Types

Primitive Types

In JavaScript, Primitive Types are the most basic data types. They represent single values (not collections) and are immutable (cannot be changed directly).

1️⃣ String

A sequence of characters enclosed in quotes.

let name = "Alice";
console.log(name); // "Alice"

2️⃣ Number

Represents both integers and floating-point numbers.

let age = 25;
let price = 99.99;
console.log(age);   // 25
console.log(price); // 99.99

3️⃣ Boolean

Represents logical values: true or false.

let isStudent = true;
let isAdmin = false;
console.log(isStudent); // true

4️⃣ Undefined

A variable declared but not assigned any value is undefined.

let city;
console.log(city); // undefined

5️⃣ Null

Represents an intentional absence of any value.

let car = null;
console.log(car); // null

6️⃣ Symbol (ES6)

Used to create unique identifiers.

let id1 = Symbol("id");
let id2 = Symbol("id");
console.log(id1 === id2); // false (always unique)

7️⃣ BigInt

Used for very large integers beyond the safe limit of Number.

let bigNum = 123456789012345678901234567890n;
console.log(bigNum);

✅ Quick Recap

• String → Textual data, e.g. "Hello".
• Number → Numeric values, e.g. 42 or 3.14.
• Boolean → Logical values: true or false.
• Null → Represents "no value" or "empty".
• Undefined → A variable that has been declared but not assigned a value.
• Symbol → A unique and immutable identifier (used for object properties).
• BigInt → For representing integers larger than 2^53 - 1.

Reference Types

• Objects → Store data as key-value pairs.
• Arrays → Store ordered lists of values.
• Functions → Reusable blocks of code that perform actions.

1️⃣ Objects

An Object in JavaScript is a collection of key-value pairs. Keys are called properties, and values can be any data type (string, number, array, function, etc.).

// Example of an object
let user = {
  name: "Alice",
  age: 25,
  isStudent: true
};

console.log(user.name); // Alice
console.log(user.age);  // 25
  

2️⃣ Arrays

An Array is an ordered list of values. Each value is called an element, and each element has an index (starting from 0).

// Example of an array
let fruits = ["apple", "banana", "cherry"];

console.log(fruits[0]); // apple
console.log(fruits[2]); // cherry
  

3️⃣ Functions

A Function is a block of code designed to perform a task. Functions can take inputs (parameters) and return outputs (values).

// Example of a function
function greet(name) {
  return "Hello, " + name + "!";
}

console.log(greet("Alice")); // Hello, Alice!
  

🔄 JavaScript Type Conversion

There are two main types of conversion:

• Implicit Conversion (Type Coercion) → JavaScript automatically converts types.
• Explicit Conversion (Type Casting) → You manually convert types using functions.
• Be careful with == because it does coercion. Prefer === for strict comparison.

1️⃣ Implicit Conversion (Type Coercion)

JavaScript automatically converts one data type to another when needed.

// String + Number → String
console.log("5" + 2);   // "52"

// String - Number → Number
console.log("5" - 2);   // 3

// Boolean + Number → Number
console.log(true + 1);  // 2

// Equality operator (==) does coercion
console.log(5 == "5");  // true
  

2️⃣ Explicit Conversion (Type Casting)

We manually convert values using built-in functions.

Convert to Number

console.log(Number("123"));     // 123
console.log(parseInt("12.34")); // 12
console.log(parseFloat("12.34"));// 12.34
console.log(+"5");              // 5 (unary plus)
  

Convert to String

console.log(String(123));       // "123"
console.log((123).toString());  // "123"
console.log(true.toString());   // "true"
  

Convert to Boolean

console.log(Boolean(1));        // true
console.log(Boolean(0));        // false
console.log(Boolean("Hello"));  // true
console.log(Boolean(""));       // false
  

3️⃣ Special Cases

// Null and Undefined
console.log(Number(null));      // 0
console.log(Number(undefined)); // NaN

// Empty string
console.log(Number(""));        // 0

// Non-numeric string
console.log(Number("abc"));     // NaN
  

👉 Next Lesson

In the next lesson, we’ll learn about Operators in JavaScript (Arithmetic, Comparison, Logical).