JavaScript Reflect preventExtensions() Method

JavaScript Reflect.preventExtensions() method in Javascript is used to prevent future extensions to the object which means preventing from adding new properties to the object.

Syntax:

Reflect.preventExtensions( obj )

Parameters: This method accepts a single parameter as mentioned above and described below:

Obj: This parameter holds the target object and it is used to prevent extensions.

Return value: This method returns the Boolean value. It returns true for the successful prevention of the extensions. Otherwise, returns false.

Exceptions: A TypeError is an exception given as the result when the target is not an Object.

The below example illustrates the Reflect.preventExtensions() method in JavaScript:

Example 1: In this example, we will prevent the object to get new properties using the Reflect.preventExtensions() method in JavaScript:

javascript

const object1 = {};
 
console.log(Reflect.isExtensible(object1));
Reflect.preventExtensions(object1);
console.log(Reflect.isExtensible(object1));
 
const obj = {};
Reflect.preventExtensions(obj);
console.log(
    Reflect.isExtensible(obj)
);
obj.val = 3;
console.log(
    obj.hasOwnProperty("val")
);

Output

true
false
false
false

Example 2: In this example, we will prevent the object to get new properties using the Reflect.preventExtensions() method in JavaScript:

javascript

let empty = {}
console.log(Reflect.isExtensible(empty));
console.log(Reflect.preventExtensions(empty));
console.log(Reflect.isExtensible(empty));
 
const obj = { "val1": 3, "val2": 4 };
Reflect.preventExtensions(obj);
console.log(obj.hasOwnProperty("val1"));
delete obj.val1;
console.log(obj.hasOwnProperty("val2"));