How to Check If an Object Property is Undefined in JavaScript

Use the strict equality operator (===)

You can use the typeof operator in combination with the strict equality operator (===) to check or detect if a JavaScript object property is undefined.

Let's take a look at the following example to understand how it actually works:

<script>
    var person = {
        name: "Clark",
        age: 32,
        country: "United States"
    };
    
    // Access an existing object property
    console.log(person.age); // Prints: 32
    
    // Detect if object property is undefined
    if(typeof person.gender === "undefined"){
        console.log("This property is not defined.");
    }
</script>