JavaScript Data Types

Data types describe the different kinds of data that we can work with and store in variables. In JavaScript, there are eight basic data types, which are divided into two categories: Primitive and Object (Reference) types.

The Concept of Dynamic Typing

JavaScript is a "dynamically typed" language. This means you don't have to declare the type of data a variable will hold, and the same variable can hold different types of data at different times.

let x;           // x is currently undefined
x = 5;           // x is now a number
x = "Mim";       // x is now a string 

1. Primitive Data Types

Primitive values are single pieces of data. They are immutable (cannot be changed). There are 7 primitive types in JavaScript:

2. Non-Primitive (Reference) Type

There is only one non-primitive type in JavaScript: The Object.

Objects can store collections of data and more complex entities. Common objects you will use include:

• Regular Objects — { name: "Mim", age: 25 }
• Arrays — [1, 2, 3, 4]
• Dates — new Date()

Why Knowing Data Types is Important

When you use operators like +, JavaScript behaves differently based on the data type:

let x = 5 + 5;       // Result is 10 (Addition)
let y = "5" + 5;     // Result is "55" (Concatenation)
let z = "5" + "5";   // Result is "55" (Concatenation)

Key Points to Remember

• JavaScript variables can hold any data type (dynamic typing)
• There are 7 primitive types: string, number, bigint, boolean, undefined, null, symbol
• There is 1 non-primitive type: Object
• Arrays and Dates are specific types of objects
• Types matter when performing calculations and joining data

Common Mistakes to Avoid

Try It: typeof Quirks

Run this in your browser console to see the well-known type quirks for yourself:

console.log(typeof null);        // "object"  (the famous quirk)
console.log(typeof undefined);   // "undefined"
console.log(typeof NaN);         // "number"  (NaN is technically a Number)
console.log(NaN === NaN);        // false
console.log(Number.isNaN(NaN));  // true

Frequently Asked Questions