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JavaScript typeof


In JavaScript there are 5 different data types that can contain values:

string
number
boolean
object
function

There are 6 types of objects:

Object
Date
Array
String
Number
Boolean

And 2 data types that cannot contain values:

null
undefined


The typeof Operator
You can use the typeof operator to find the data type of a
JavaScript variable.

Example

typeof "John"                
// Returns "string"
typeof 3.14                  
// Returns "number"
typeof NaN                   
// Returns "number"
typeof false                 
// Returns "boolean"
typeof [1,2,3,4]              // Returns
"object"
typeof {name:'John', age:34} 
// Returns "object"typeof new Date()            
// Returns "object"typeof function () {}         // Returns
"function"
typeof myCar                 
// Returns "undefined" *
typeof null                  
// Returns "object"

Try it Yourself »

Please observe:

The data type of NaN is number
The data type of an array is object
The data type of a date is object
The data type of null is object
The data type of an undefined variable is undefined
*
The data type of a variable that has not been assigned a value is
also undefined *


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








Primitive Data
A primitive data value is a single simple data value with no additional
properties and methods.
The typeof operator can return one of these primitive types:

string
number
boolean
undefined


Example

typeof "John"              // Returns
"string"
typeof 3.14                // Returns
"number"
typeof true                // Returns
"boolean"typeof false               // Returns
"boolean"typeof x                  
// Returns "undefined" (if x has no value)
Try it Yourself »


Complex Data
The typeof operator can return one of two complex types:

function
object

The typeof operator returns "object" for objects, arrays, and null.
The typeof operator does not return "object" for functions.

Example

typeof {name:'John', age:34} // Returns "object"
typeof [1,2,3,4]            
// Returns "object" (not "array", see note below)
typeof null                  // Returns
"object"typeof function myFunc(){}   // Returns "function"

Try it Yourself »


The typeof operator returns "object" for arrays because in JavaScript arrays are objects.


The Data Type of typeof
The typeofoperator is not a variable. It is an operator. Operators ( + - * /
) do not have any data type.
But, the typeof operator always returns a string (containing
the type of the operand).

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

Example

"John".constructor               
// Returns function String()  {[native code]}
(3.14).constructor               
// Returns function Number()  {[native code]}
false.constructor                 // Returns
function Boolean() {[native code]}
[1,2,3,4].constructor            
// Returns function Array()   {[native code]}
{name:'John',age:34}.constructor 
// Returns function Object()  {[native code]}
new Date().constructor           
// Returns function Date()    {[native code]}
function () {}.constructor        // Returns
function Function(){[native code]}

Try it Yourself »


You can check the constructor property to find out if an object is an Array
(contains the word "Array"):

Example

function isArray(myArray) {
  return myArray.constructor.toString().indexOf("Array") > -1;
}

Try it Yourself »

Or even simpler, you can check if the object is an Array function:

Example

function isArray(myArray) {
  return myArray.constructor
=== Array;
}

Try it Yourself »


You can check the constructor property to find out if an object is a
Date (contains the word "Date"):

Example

function isDate(myDate) {
  return myDate.constructor.toString().indexOf("Date") > -1;
}

Try it Yourself »

Or even simpler, you can check if the object is a Date function:

Example

function isDate(myDate) {
  return myDate.constructor === Date;
}

Try it Yourself »


Undefined
In JavaScript, a variable without a value, has the value undefined.
The type is also undefined.

Example

let car;    // Value is undefined,
type is undefined
Try it Yourself »

Any variable can be emptied, by setting the value to undefined.
The type will also be undefined.

Example

car = undefined;    // Value is undefined,
type is 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 = "";    //
The value is
"", the typeof is "string"
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 consider it a bug in JavaScript that typeof null is an object. It should be null.

You can empty an object by setting it to null:

Example

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          
// undefinedtypeof null               
// objectnull === undefined        
// falsenull == undefined         
// true
Try it Yourself »


The instanceof Operator
The instanceof operator returns true if an object is an instance of the specified object:

Example

const cars = ["Saab", "Volvo", "BMW"];
(cars instanceof Array);
(cars instanceof Object);
(cars instanceof String);
(cars instanceof Number);

Try it Yourself »


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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Reference: https://www.w3schools.com/js/js_typeof.asp