What are the data types in JavaScript?

Praveen Kumar
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🔍 Introduction: Why Data Types Matter in JavaScript

JavaScript is a dynamically typed language, meaning you don’t need to declare variable types explicitly. While this flexibility makes JS beginner-friendly, it also introduces quirks that can trip up even experienced developers.

Understanding JavaScript data types helps you:

Write better, bug-free code
Avoid confusing type coercion behavior
Debug more efficiently
Communicate data clearly between APIs, databases, and the browser

Let’s dive deep into data types in JavaScript and how they really work!

🧱 JavaScript Data Types: The Two Main Categories

JavaScript has two categories of data types:

1. Primitive Types (Immutable)

These are basic types with no properties or methods. They are stored directly in memory.

Data Type	Example
String	'Hello', "World"
Number	42, 3.14, -7
Boolean	true, false
Null	null
Undefined	undefined
Symbol (ES6)	Symbol('id')
BigInt (ES11)	12345678901234567890n

2. Non-Primitive (Reference) Types

These are objects and are stored as references.

Object { name: "John" }
Array [1, 2, 3]
Function function() {}
Date, RegExp, Set, Map, etc.

🔢 Primitive Types in Detail

📝 String

Used to represent textual data.

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

🔢 Number

Includes integers, floats, and special values like NaN, Infinity.

let count = 10;
let price = 19.99;
console.log(typeof count); // "number"

🔁 Boolean

Represents a logical entity: true or false.

let isAvailable = false;
console.log(typeof isAvailable); // "boolean"

🚫 Null

Represents intentional absence of any object value.

let data = null;
console.log(typeof data); // "object" (this is a long-standing JavaScript bug!)

📦 Undefined

A variable declared but not assigned a value.

let user;
console.log(typeof user); // "undefined"

🔐 Symbol (ES6+)

Used for unique identifiers.

let id = Symbol('id');
console.log(typeof id); // "symbol"

🔢 BigInt (ES2020+)

For arbitrarily large integers.

let big = 123456789012345678901234567890n;
console.log(typeof big); // "bigint"

📦 Object (Reference) Types Explained

Objects in JavaScript can contain key-value pairs and methods.

let person = {
  name: "Bob",
  age: 30
};
console.log(typeof person); // "object"

🧩 Array

Technically, an object with indexed values.

let fruits = ["apple", "banana"];
console.log(typeof fruits); // "object"

⚙️ Function

Also an object, but callable.

function greet() {
  return "Hello!";
}
console.log(typeof greet); // "function"

🔁 JavaScript is Dynamically Typed

You can assign different types to the same variable at different times:

let data = 10;       // number
data = "Ten";        // string
data = true;         // boolean

This dynamic nature is powerful but dangerous. Type-related bugs are common if you’re not careful.

🧪 The typeof Operator

Use typeof to check a variable’s type:

console.log(typeof 123);          // "number"
console.log(typeof "hi");         // "string"
console.log(typeof true);         // "boolean"
console.log(typeof {});           // "object"
console.log(typeof []);           // "object"
console.log(typeof function(){}); // "function"

❗ Caution: typeof null returns "object" — a known bug in JavaScript.

🤯 Common Pitfalls to Avoid

typeof null === “object” – historical bug, don’t rely on it
NaN is a number – typeof NaN === "number"
Arrays return "object" – use Array.isArray() to check
Loose vs strict equality – == allows type coercion, === doesn’t

0 == "0";     // true
0 === "0";    // false

✅ Best Practices

Prefer === over == to avoid type coercion issues
Use let and const instead of var
Always initialize variables
Avoid mixing types in the same variable
Use Array.isArray() for array checks
Use TypeScript if strict typing is needed

🧠 Summary

JavaScript offers a lot of flexibility with its types, but that comes with responsibility. Understanding the difference between primitive and reference types, using typeof wisely, and writing type-safe code can help you avoid most runtime surprises.

❓ Your Turn

What’s the weirdest behavior you’ve encountered with JavaScript data types? Share it in the comments and let’s debug together! 🐛💬