What are the data types in JavaScript? How do you identify a variable's data type?

What are the data types in JavaScript?

JavaScript data types are divided into two main categories:

Primitive Types

There are 7 primitive types:

• string: Text values, e.g. 'hello'
• number: Numeric values, e.g. 42, 3.14
• bigint: Large integers, e.g. 9007199254740991n
• boolean: true / false
• undefined: A variable that has been declared but not assigned a value
• null: Represents "no value" or "empty"
• symbol: Unique and immutable values (introduced in ES6)

Object Types

• object: Includes plain objects, arrays, functions, etc.

---

How do you identify a variable's data type?

1. typeof Operator

The most common approach, but has some quirks:

typeof 'hello'        // 'string'
typeof 42             // 'number'
typeof true           // 'boolean'
typeof undefined      // 'undefined'
typeof Symbol()       // 'symbol'
typeof 42n            // 'bigint'
typeof {}             // 'object'
typeof []             // 'object' (arrays are also 'object'!)
typeof null           // 'object' (a well-known historical bug)
typeof function(){}   // 'function'

2. instanceof Operator

Checks whether an object is an instance of a particular constructor:

[] instanceof Array       // true
{} instanceof Object      // true
/regex/ instanceof RegExp // true

3. Object.prototype.toString.call()

The most accurate method — distinguishes null, arrays, functions, etc.:

Object.prototype.toString.call('hello')      // '[object String]'
Object.prototype.toString.call(42)           // '[object Number]'
Object.prototype.toString.call(null)         // '[object Null]'
Object.prototype.toString.call([])           // '[object Array]'
Object.prototype.toString.call({})           // '[object Object]'
Object.prototype.toString.call(function(){}) // '[object Function]'

4. Array.isArray()

The recommended way to check specifically for arrays:

Array.isArray([])  // true
Array.isArray({})  // false

---

Summary

Method	Best For	Caveats
typeof	Primitive types	Returns 'object' for null and arrays
instanceof	Checking object instances	Doesn't work for primitives
Object.prototype.toString.call()	Precise type checking for all types	More verbose
Array.isArray()	Checking for arrays	Recommended for array detection