JavaScript(JS) object method - preventExtensions

The preventExtensions() method is a built-in method of the JavaScript Object constructor. It prevents new properties from being added to an object, i.e., makes the object non-extensible. However, existing properties can still be modified or deleted.

Here's the syntax:

Object.preventExtensions(obj)

where obj is the object to make non-extensible. This method takes only one argument, which is the object to make non-extensible.

Here's an example that shows how to use the preventExtensions() method:

const myObj = { prop1: "value1", prop2: "value2" };
console.log(Object.isExtensible(myObj)); // true

Object.preventExtensions(myObj);
console.log(Object.isExtensible(myObj)); // false

myObj.prop1 = "new value";
console.log(myObj); // { prop1: "new value", prop2: "value2" }

myObj.prop3 = "value3";
console.log(myObj); // { prop1: "new value", prop2: "value2" }

In this example, we create an object myObj with two properties prop1 and prop2. We use the isExtensible() method to check whether the object is extensible. Since the object is extensible, isExtensible() returns true.

Next, we use the Object.preventExtensions() method to make the object non-extensible. We then use the isExtensible() method again to check whether the object is extensible. Since we have made the object non-extensible, isExtensible() now returns false.

Finally, we try to modify an existing property prop1 and add a new property prop3 to the object. The modification of prop1 is successful, but the addition of prop3 is not allowed since the object is non-extensible. When we log the value of myObj, we see that it still contains both properties, but the prop3 property is not added.

The preventExtensions() method is useful for making an object non-extensible. If an object is non-extensible, you cannot add new properties to it, but you can still modify or delete its existing properties.