JavaScript - in operator

The JavaScript in operator returns true if the specified property is in the specified object. The syntax for using this operator is given below:

Syntax

propertyName in objectName

Parameters

`propertyName`	Specify a string, numeric, or symbol expression representing a property name or array index.
`objectName`	Specify the name of an object to check if it (or its prototype chain) contains the property with specified name (propertyName).

Example: using in operator with an Array

The example below shows the usage of in operator when used with an array.

var Numbers = [10, 20, 30, 40, 50]

//returns false
document.write("60 in Numbers : ", (60 in Numbers), "<br>");
//returns false
document.write("'abc' in Numbers : ", ('abc' in Numbers), "<br>");
//returns false - specify the index number 
//not the value at that index
document.write("10 in Numbers : ", (10 in Numbers), "<br>");
//returns true
document.write("0 in Numbers : ", (0 in Numbers), "<br>");
//returns true (length is an Array property)
document.write("length in Numbers : ", (length in Numbers), "<br>");

The output of the above code will be:

60 in Numbers : false
'abc' in Numbers : false
10 in Numbers : false
0 in Numbers : true
length in Numbers : true

Example: using in operator with built-in objects

The example below shows the usage of in operator when used with built-in objects.

var MyString = new String('Hello');

//returns true
document.write("'PI' in Math : ", ('PI' in Math), "<br>");
//returns true (length is a String property)
document.write("length in MyString : ", (length in MyString), "<br>");

The output of the above code will be:

'PI' in Math : true
length in MyString : true

Example: using in operator with custom objects

The example below shows the usage of in operator when used with custom objects.

var Person = { name: 'John', age: 25, city: 'London' }

//returns true
document.write("'name' in Person : ", ('name' in Person), "<br>");
//returns false
document.write("'hobby' in Person : ", ('hobby' in Person), "<br>");

The output of the above code will be:

'name' in Person : true
'hobby' in Person : false

❮ JavaScript - Operators