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
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typeof null
// Returns object
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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
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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);
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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);
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// Create an Array
const fruits = ["apples", "bananas", "oranges"];
(fruits instanceof Array);
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// Create a Map
const fruits = new Map([
["apples", 500],
["bananas", 300],
["oranges", 200]
]);
(fruits instanceof Map);
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// Create a Set
const fruits = new Set(["apples", "bananas", "oranges"]);
(fruits instanceof Set);
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Undefined Variables
The typeof of an undefined variable is
undefined.
The typeof of a variable with no value is
undefined. The value is also undefined.
Any variable can be emptied, by setting the value to undefined.
The type will also be undefined.
Empty Values
An empty value has nothing to do with undefined.
An empty string has both a legal value and a type.
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
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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
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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
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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
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With the constructor, you can check if an object is an Array:
With the constructor, you can check if an object is a Date:
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"
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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>
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