How to Check Data Type in JavaScript: Simple Guide

In JavaScript, you can check the data type of a value using the typeof operator for primitives and functions, Array.isArray() to detect arrays, and instanceof to check if an object is an instance of a specific class or constructor.

Syntax

The main ways to check data types in JavaScript are:

• typeof value: Returns a string describing the type of value.
• Array.isArray(value): Returns true if value is an array.
• value instanceof Constructor: Returns true if value is created by Constructor.

javascript

typeof value
Array.isArray(value)
value instanceof Constructor

Example

This example shows how to use typeof, Array.isArray(), and instanceof to check different data types.

javascript

const num = 42;
const text = "hello";
const arr = [1, 2, 3];
const obj = {a: 1};
const date = new Date();

console.log(typeof num);           // "number"
console.log(typeof text);          // "string"
console.log(Array.isArray(arr));   // true
console.log(typeof obj);           // "object"
console.log(date instanceof Date); // true
console.log(typeof date);          // "object"

Output

number
string
true
object
true
object

Common Pitfalls

Some common mistakes when checking types in JavaScript:

• typeof null returns "object", which is a known quirk.
• typeof cannot distinguish arrays from objects; use Array.isArray() instead.
• instanceof only works for objects created by constructors, not primitives.

javascript

console.log(typeof null);           // "object" (unexpected)
console.log(typeof []);             // "object" (array looks like object)
console.log(Array.isArray([]));     // true (correct way to check array)

// instanceof with arrays
console.log([] instanceof Array);   // true (works but can fail across frames)

// instanceof with primitives
console.log(42 instanceof Number);  // false
console.log(new Number(42) instanceof Number); // true

Output

object
object
true
true
false
true

Quick Reference

Method	Use Case	Example Output
typeof value	Check primitive types and functions	"string", "number", "boolean", "undefined", "object", "function"
Array.isArray(value)	Check if value is an array	true or false
value instanceof Constructor	Check if object is instance of a class	true or false

Key Takeaways

Use typeof to check basic data types like string, number, and boolean.

Use Array.isArray() to correctly identify arrays.

Use instanceof to check if an object is created by a specific constructor.

Remember that typeof null returns "object", which is a special case.

Avoid relying on typeof alone for complex types like arrays or null.