Array.includes() Method: In JavaScript, includes() method is used to determine that a particular element is present in an array or not. It returns true if the element is present and false when it is absent.
Syntax:
array_name.includes(searchElement, ?fromIndex)
Parameters:
- searchElement: The element to be search in the array.
- fromIndex: The index from which the element is to be searched. This is an optional parameter.
Example:
HTML
<!DOCTYPE html> <html lang="en"> <body> <h2> Checking if the countries array contains Japan ---> <span id="ans"></span> </h2> <h2> Checking for Japan in the countries array from index 2 ---> <span id="ans2"></span> </h2> <script> let countries = ["India", "Japan", "Canada", "Germany", "Australia"]; // 1st Output let ans = document.querySelector("#ans"); let output = countries.includes("Japan"); ans.append(output); // 2nd Output let ans2 = document.querySelector("#ans2"); let output2 = countries.includes("Japan", 2); ans2.append(output2); </script> </body> </html> |
Output: 
Checking for a property in an Object: To check that an Object contains a particular property or not, we have 3 methods to do this-
1. Using in operator: It returns true if the property exists in the object and false if it doesn’t exists. It checks for both the own and inherited properties of the object.
Syntax:
'property_name' in object_name
Example:
Javascript
<script> let Person = { name: "durgesh", age: 16 } // Output: true console.log('name' in Person) // Returns true for an inherited // property // Output: true console.log('toString' in Person) // Output: false console.log('gender' in Person) </script> |
Output:
true true false
Note: The toString() method used in above example as an inherited property from prototype object. The ‘in’ operator returns true for prototype inherited properties.
Using hasOwnProperty() Method: It returns true if the property exists in the object and false if it doesn’t exists. It checks only for ‘own’ properties(The properties that are defined inside the object) of the object.
Syntax:
object_name.hasOwnProperty('property_name')
Example:
Javascript
<script> let Person = { name: 'Durgesh', age: 16 }; // Output: true console.log(Person.hasOwnProperty('name')) /* hasOwnProperty() doesn't checks for inherited properties of the object. */ /* toString() is an inherited property. */ // Output: false console.log(Person.hasOwnProperty('toString')); // Output: false console.log(Person.hasOwnProperty('gender')); </script> |
Output:
true false false
Comparing with undefined: Evaluating a property that doesn’t exists in an object results in undefined. So we can compare the result with undefined to know that a property is present or absent.
Example:
Javascript
let Person = { name: 'Durgesh', age: 16 }; // Returns true if the property is present // Output: true console.log(Person.name !== undefined) // Returns true for inherited property // Output: true console.log(Person.toString !== undefined) // Output: false console.log(Person.gender !== undefined) |
Output:
true true false
Note: This is an unpleasant approach compared to the above two because, if a property is defined as undefined in the object then this method evaluates it to false. It is advisable to use above two methods if there is a possibility that your object’s property value can be undefined.
Javascript
let Person = { // Setting name to undefined name: undefined, age: 16 }; /* This evaluates to false despite the fact that name property exists */// Output: false console.log(Person.name!==undefined) |
Output:
false