JS Data Types

JavaScript Data Types: A Complete Guide

A value in JavaScript is always of a certain type. For example, a string or a number. Understanding data types is fundamental to writing effective JavaScript code, as the type of a value determines what you can do with it.

JavaScript is Dynamically Typed

JavaScript is a dynamically typed language. This means that you do not have to declare the type of a variable when you create it. The data type is determined automatically at runtime.

A variable can hold a number one moment and a string the next.

// No error
let message = "hello";
console.log(typeof message); // "string"

message = 123456;
console.log(typeof message); // "number"

The Two Main Categories of Data Types

There are eight basic data types in JavaScript, which can be divided into two main categories:

1. Primitive Data Types

A primitive data type is data that has a primitive value (it has no properties or methods). There are 7 primitive data types:

• string: Represents textual data (e.g., "hello").
• number: Represents both integer and floating-point numbers.
• bigint: For integer numbers of arbitrary length.
• boolean: Represents a logical entity and can have two values: true and false.
• undefined: A variable that has been declared but not assigned a value.
• null: Represents the intentional absence of any object value.
• symbol: A unique and immutable primitive value used as the key of an Object property.

2. The Object Data Type (Non-Primitive)

The object type is a non-primitive type. Unlike primitives, objects can store collections of data and more complex entities.

• object: Represents a collection of key-value pairs. This includes standard objects, arrays, functions, dates, and more.

The typeof Operator

To find the type of a JavaScript variable, you can use the built-in typeof operator.

console.log(typeof "John");         // "string"
console.log(typeof 3.14);           // "number"
console.log(typeof NaN);            // "number"
console.log(typeof false);          // "boolean"
console.log(typeof [1,2,3,4]);      // "object"
console.log(typeof {name:'John'});  // "object"
console.log(typeof new Date());     // "object"
console.log(typeof null);           // "object" (This is a famous bug!)
console.log(typeof undefined);      // "undefined"

In the following chapters, we will explore each of these data types in greater detail.