JavaScript Mastery: Lock Down Objects with preventExtensions()

Unlocking the Power of Non-Extensible Objects in JavaScript

When working with objects in JavaScript, it’s essential to understand how to control their behavior and properties. One crucial aspect of this is making objects non-extensible, which means preventing new properties from being added to them.

What is a Non-Extensible Object?

A non-extensible object is an object that cannot have new properties added to it. However, this doesn’t mean that its existing properties cannot be deleted or modified. It’s essential to note that properties can still be added to the prototype of a non-extensible object.

The Object.preventExtensions() Method

The Object.preventExtensions() method is a static method that makes an object non-extensible. Its syntax is straightforward:

Object.preventExtensions(obj)

The preventExtensions() method takes one parameter, obj, which is the object that should be made non-extensible. The method returns the obj itself, which is now non-extensible.

Example: Preventing New Properties

Let’s create an empty object obj and add a property name to it using the defineProperty() method. Then, we’ll make obj non-extensible using preventExtensions() and try to add a new property age. As expected, this will result in a TypeError.

“`javascript
let obj = {};

Object.defineProperty(obj, ‘name’, { value: ‘Smith’ });
console.log(obj.name); // Output: Smith

Object.preventExtensions(obj);

try {
Object.defineProperty(obj, ‘age’, { value: 30 });
} catch (e) {
console.error(e); // Output: TypeError: Cannot add property age, object is not extensible
}
“`

Key Takeaways

• Non-extensible objects cannot have new properties added to them.
• Existing properties of a non-extensible object can still be deleted or modified.
• Properties can still be added to the prototype of a non-extensible object.
• Attempting to add new properties to a non-extensible object will fail, either silently or by throwing a TypeError in strict mode.

By mastering the Object.preventExtensions() method, you’ll gain more control over your objects and ensure that they behave as expected in your JavaScript applications.