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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
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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"
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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           // undefined
typeof null                // object

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