How to use array that include and check an object against a property of an object ?

Last Updated : 17 Feb, 2023

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.