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Text lesson

JS Typesof

The typeof Operator

The typeof operator returns the data type of a variable in JavaScript.

Primitive Data Types

In JavaScript, a primitive value is a single value that lacks properties or methods.

There are 7 primitive data types in JavaScript:

string
number
boolean
bigint
symbol
null
undefined

The typeof operator returns the type of a variable or expression.

Examples

typeof “John”         // Returns string
typeof (“John”+“Doe”) // Returns string
typeof 3.14           // Returns number
typeof 33             // Returns number
typeof (33 + 66)      // Returns number
typeof true           // Returns boolean
typeof false          // Returns boolean
typeof 1234n          // Returns bigint
typeof Symbol()       // Returns symbol
typeof x              // Returns undefined

typeof null // Returns object

Note: In JavaScript, null is considered a primitive value, but typeof returns “object.”

This is a well-known bug in JavaScript with historical roots.

Complex Data Types

A complex data type can hold multiple values and/or various data types together.

JavaScript has one complex data type:

object

All other complex types, such as arrays, functions, sets, and maps, are essentially different forms of objects.

The typeof operator returns only two types for complex data:

object
function

Example

typeof {name:‘John’}   // Returns object
typeof [1,2,3,4]       // Returns object
typeof new Map()       // Returns object
typeof new Set()       // Returns object

typeof function (){}   // Returns function

Note: The typeof operator returns “object” for all types of objects, including:

objects
arrays
sets
maps

You cannot use typeof to distinguish whether a JavaScript object is an array or a date.

How to Recognize an Array

How can you determine if a variable is an array?

ECMAScript 5 (2009) introduced a new method for this: Array.isArray().

Example

// Create an Array
const fruits = [“apples”, “bananas”, “oranges”];
Array.isArray(fruits);