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JS References

JavaScript Objects HTML DOM Objects


JavaScript typeof

The typeof Operator

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

Primitive Data Types

In JavaScript, a primitive value is a single value with no properties or methods.

JavaScript has 7 primitive data types:

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

The typeof operator returns the type of a variable or an 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
Try it Yourself »
typeof null           // Returns object
Try it Yourself »

Note:

In JavaScript, null is a primitive value. However, typeof returns "object".

This is a well-known bug in JavaScript and has historical reasons.


Complex Data Types

A complex data type can store multiple values and/or different data types together.

JavaScript has one complex data type:

  • object

All other complex types like arrays, functions, sets, and maps are just different types of objects.

The typeof operator returns only two types:

  • 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
Try it Yourself »

Note:

The typeof operator returns object for all types of objects:

  • objects
  • arrays
  • sets
  • maps

You cannot use typeof to determine if a JavaScript object is an array or a date.


How to Recognize an Array

How to know if a variable is an array?

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

Example

// Create an Array
const fruits = ["apples", "bananas", "oranges"];
Array.isArray(fruits);
Try it Yourself »

The instanceof Operator

The instanceof operator returns true if an object is an instance of a specified object type:

Examples

// Create a Date
const time = new Date();

(time instanceof Date);
Try it Yourself »
// Create an Array
const fruits = ["apples", "bananas", "oranges"];

(fruits instanceof Array);
Try it Yourself »
// Create a Map
const fruits = new Map([
  ["apples", 500],
  ["bananas", 300],
  ["oranges", 200]
]);

(fruits instanceof Map);
Try it Yourself »
// Create a Set
const fruits = new Set(["apples", "bananas", "oranges"]);

(fruits instanceof Set);
Try it Yourself »


Undefined Variables

The typeof of an undefined variable is undefined.

Example

typeof car;
Try it Yourself »

The typeof of a variable with no value is undefined. The value is also undefined.

Example

let car;
typeof car;
Try it Yourself »

Any variable can be emptied, by setting the value to undefined.

The type will also be undefined.

Example

let car = "Volvo";
car = undefined;
Try it Yourself »

Empty Values

An empty value has nothing to do with undefined.

An empty string has both a legal value and a type.

Example

let car = "";
typeof car;
Try it Yourself »

Null

In JavaScript null is "nothing". It is supposed to be something that doesn't exist.

Unfortunately, in JavaScript, the data type of null is an object.

You can empty an object by setting it to null:

Example

// Create an Object
let person = {firstName:"John", lastName:"Doe", age:50, eyeColor:"blue"};

person = null;
// Now value is null, but type is still an object
Try it Yourself »

You can also empty an object by setting it to undefined:

Example

let person = {firstName:"John", lastName:"Doe", age:50, eyeColor:"blue"};

person = undefined;
// Now both value and type is undefined
Try it Yourself »

Difference Between Undefined and Null

undefined and null are equal in value but different in type:

typeof undefined      // undefined
typeof null           // object

null === undefined    // false
null == undefined     // true
Try it Yourself »

The constructor Property

The constructor property returns the constructor function for all JavaScript variables.

Example

// Returns function Object() {[native code]}:
{name:'John',age:34}.constructor

// Returns function Array() {[native code]}:
[1,2,3,4].constructor

// Returns function Date() {[native code]}:
new Date().constructor

// Returns function Set() {[native code]}:
new Set().constructor

// Returns function Map() {[native code]}:
new Map().constructor

// Returns function Function() {[native code]}:
function () {}.constructor

Try it Yourself »

With the constructor, you can check if an object is an Array:

Example

(myArray.constructor === Array);

Try it Yourself »

With the constructor, you can check if an object is a Date:

Example

(myDate.constructor === Date);

Try it Yourself »


All Together

typeof "John"          // Returns "string"
typeof ("John"+"Doe")  // Returns "string"
typeof 3.14            // Returns "number"
typeof (33 + 66)       // Returns "number"
typeof NaN             // Returns "number"
typeof 1234n           // Returns "bigint"
typeof true            // Returns "boolean"
typeof false           // Returns "boolean"
typeof {name:'John'}   // Returns "object"
typeof [1,2,3,4]       // Returns "object"
typeof {}              // Returns "object"
typeof []              // Returns "object"
typeof new Object()    // Returns "object"
typeof new Array()     // Returns "object"
typeof new Date()      // Returns "object"
typeof new Set()       // Returns "object"
typeof new Map()       // Returns "object"
typeof function () {}  // Returns "function"
typeof x               // Returns "undefined"
typeof null            // Returns "object"
Try it Yourself »

Note:

The data type of NaN (Not a Number) is number !


The void Operator

The void operator evaluates an expression and returns undefined. This operator is often used to obtain the undefined primitive value, using "void(0)" (useful when evaluating an expression without using the return value).

Example

<a href="javascript:void(0);">
  Useless link
</a>

<a href="javascript:void(document.body.style.backgroundColor='red');">
  Click me to change the background color of body to red
</a>
Try it Yourself »

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