JavaScript in keyword

JavaScript “in” keyword is used to check if a given property exists in an object. It returns true if the property exists, and false if it doesn’t.

property in object

Note: the in keyword only checks for the existence of a property, not its value. To check for the value of a property, you can use other operators such as === or !==.

JavaScript in keyword example

A simple example code uses the in keyword to check if the name property exists in the person object.

<!DOCTYPE html>
<html>
<body>
<script>
    let person = {
        name: 'John',
        age: 30,
        isStudent: false
    };

    if ('name' in person) {
        console.log('Name property exists in object');
    } else {
        console.log('Name property does not exist in object');
    }
</script>
</body>
</html>

Output:

We can also use the in keyword to check if a property exists in an array. But you have to use the index number of the element you want to check for existence in Array.

let fruits = ['apple', 'banana', 'orange'];

if (1 in fruits) {
  console.log('Second element exists in array');
} else {
  console.log('Second element does not exist in array');
}

Comment if you have any doubts or suggestions on this Js keyword topic.

Note: The All JS Examples codes are tested on the Firefox browser and the Chrome browser.
OS: Windows 10
Code: HTML 5 Version

JavaScript in keyword

JavaScript in keyword example

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