JavaScript in and instanceof operators
Last Updated :
14 Nov, 2025
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In JavaScript, the in and instanceof operators are used to check relationships within objects and classes.
- The in operator checks whether a property exists in an object or an index exists in an array.
- The instanceof operator checks whether an object is an instance of a specific class or constructor.
Both operators return a Boolean value (true or false) based on the result of the check.
JavaScript in Operator
The in-operator in JavaScript checks if a specified property exists in an object or if an element exists in an array. It returns a Boolean value.
let languages = ["HTML", "CSS", "JavaScript"];
// true (index 1 exists in the array)
console.log(1 in languages);
// false (index 3 doesn't exist in the array)
console.log(3 in languages);
Output
true false
Example-Using in with Objects:
const Data = {
name: "Rahul",
age: 21,
city: "Noida"
};
// true ("name" property exists in the object)
console.log("name" in Data);
// false ("gender" property doesn't exist in the object)
console.log("address" in Data);
Output
true false
JavaScript instanceof Operator
The instanceof operator in JavaScript tests if an object is an instance of a particular class or constructor, returning a Boolean value.
let languages = ["HTML", "CSS", "JavaScript"];
console.log(languages instanceof Array);
console.log(languages instanceof Object);
console.log(languages instanceof String);
console.log(languages instanceof Number);
Output
true true false false
Example–Using in with Objects:
let myString = new String();
let myDate = new Date();
console.log(myString instanceof Object);
console.log(myString instanceof Date);
console.log(myString instanceof String);
console.log(myDate instanceof Date);
console.log(myDate instanceof Object);
console.log(myDate instanceof String);
Output
true false true true true false
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