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Step Up Your JS: A Comprehensive Guide to Intermediate JavaScript

Object.preventExtensions, seal, and freeze

Explore how to use Object.preventExtensions, seal, and freeze to make JavaScript objects less mutable. This lesson covers how these methods restrict property additions, deletions, and modifications to help enforce functional programming principles and maintain object integrity.

We'll cover the following...

In this lesson, we’ll cover three functions that allow us to enforce functional programming. Each of these functions makes our object inflexible to varying degrees. We can prevent property addition, or we can freeze an object entirely.

Object.preventExtensions

Preventing Extensions

This is a function that helps us approach immutability. Once it’s called on an object, we can’t add anything to it.

Deletions and changes are still allowed. Only property addition is forbidden.

Node.js
const obj = {
    prop: 'value',
};
Object.preventExtensions(obj);
obj.nextProp = 8; // fails
console.log(obj); // -> { prop: 'value' }
obj.prop = 4; // succeeds
console.log(obj); // -> { prop: 4 }
delete obj.prop; // succeeds
console.log(obj); // -> {}
obj.prop = 17; // since the property no longer exists, fails
console.log(obj); // -> {}

Checking Extensibility

We have a utility method that tells us if an object is extensible or not: Object.isExtensible.

Node.js
const obj = {};
console.log(Object.isExtensible(obj)); // -> true
Object.preventExtensions(obj);
console.log(Object.isExtensible(obj)); // -> false

Locked Prototype

Object.preventExtensions permanently locks an object’s prototype. Attempts to use Object.setPrototypeOf or to change the __proto__ property directly result in an error.

Node.js
const obj = {};
Object.preventExtensions(obj);
Object.setPrototypeOf(obj, {});
//-> TypeError: #<Object> is not extensible

Configuration

Property attributes (configurable, writable, ...