JavaScript typeof Operator

The typeof operator in JavaScript returns the data type of a variable or an expression. It is commonly used to determine the type of a given value.

Primitive Data Types

In JavaScript, primitive values are single, immutable data types without properties or methods. JavaScript has seven primitive data types:

• string
• number
• boolean
• bigint
• symbol
• null
• undefined

Example

console.log(typeof "John");           // "string"
console.log(typeof ("John" + "Doe")); // "string"
console.log(typeof 3.14);             // "number"
console.log(typeof 33);               // "number"
console.log(typeof (33 + 66));        // "number"
console.log(typeof true);             // "boolean"
console.log(typeof false);            // "boolean"
console.log(typeof 1234n);            // "bigint"
console.log(typeof Symbol());         // "symbol"
console.log(typeof x);                // "undefined"
console.log(typeof null);             // "object"

Note: In JavaScript, null is a primitive value. However, typeof null returns object due to a historical bug.

Complex Data Types

JavaScript has one primary complex data type:

• object

Example: Objects include various structures like arrays, functions, maps, and sets.

console.log(typeof {name: "John"});   // "object"
console.log(typeof [1, 2, 3, 4]);     // "object"
console.log(typeof new Map());        // "object"
console.log(typeof new Set());        // "object"
console.log(typeof function(){});     // "function"

How to Identify an Array?

Example:  To check if a variable is an array, use Array.isArray():

const fruits = ["apples", "bananas", "oranges"];
console.log(Array.isArray(fruits)); // true

The instanceof Operator

Example: The instanceof operator checks whether an object belongs to a specific class or type.

const time = new Date();
console.log(time instanceof Date); // true
const fruits = ["apples", "bananas", "oranges"];
console.log(fruits instanceof Array); // true
const fruitMap = new Map([
 ["apples", 500],
 ["bananas", 300],
]);
console.log(fruitMap instanceof Map); // true

const fruitSet = new Set(["apples", "bananas", "oranges"]);
console.log(fruitSet instanceof Set); // true

Undefined Variables

Example: When a variable is declared but not assigned a value, its type is undefined.

let car;
console.log(typeof car); // "undefined"

Example: Setting a variable to undefined explicitly also results in the same type.

let vehicle = "Toyota";
vehicle = undefined;

console.log(typeof vehicle); // "undefined"

Empty Values vs. Undefined

Example:  An empty string has a valid type (string) and is different from undefined.

let car = "";
console.log(typeof car); // "string"

Null and Objects

Example: In JavaScript, null represents an intentional absence of value. However, its type is object.

let person = { firstName: "John", lastName: "Doe" };
person = null;

console.log(typeof person); // "object"

Example: To set an object to an undefined state

let person = { firstName: "John", lastName: "Doe" };
person = undefined;

console.log(typeof person); // "undefined"

Example:  Difference Between undefined and null

console.log(typeof undefined); // "undefined"
console.log(typeof null);      // "object"

console.log(null === undefined); // false
console.log(null == undefined);  // true

The constructor Property

Example: The constructor property identifies the constructor function of an object.

console.log({name:"John", age:34}.constructor); // function Object()
console.log([1,2,3,4].constructor);            // function Array()
console.log(new Date().constructor);           // function Date()
console.log(new Set().constructor);            // function Set()
console.log(new Map().constructor);            // function Map()
console.log(function() {}.constructor);        // function Function()

Example: Using the constructor to check for data types:

console.log(myArray.constructor === Array); // true
console.log(myDate.constructor === Date);   // true

Note: The data type of NaN (Not a Number) is number.