JavaScript Object Properties —— Preventing Object Modification

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Preventing Object Modification

When you want to lock down an object’s properties in some way, there are three different ways to do so.

 

Preventing Extensions

If you use Object.preventExtensions(), objects will no longer allow properties to be added.

var person1 = {
    name: "Nicholas"
};

console.log(Object.isExtensible(person1)); // true
Object.preventExtensions(person1);

console.log(Object.isExtensible(person1)); // false

person1.sayName = function () {
    console.log(this.name);
};
console.log("sayName" in person1);          // false

 

Sealing Objects

You could also create a sealed object with the Object.seal() method, which makes that object non-extensible and makes its properties nonconfigurable.

var person1 = {
    name: "Nicholas"
};

console.log(Object.isExtensible(person1));  // true
console.log(Object.isSealed(person1));      // false

Object.seal(person1);
console.log(Object.isExtensible(person1));  // false
console.log(Object.isSealed(person1));      // true

person1.sayName = function() {
    console.log(this.name);
};

console.log("sayName" in person1);          // false

person1.name = "Greg";
console.log(person1.name);                  // "Greg"

delete person1.name;
console.log("name" in person1);             // true
console.log(person1.name);                  // "Greg"

var descriptor = Object.getOwnPropertyDescriptor(person1, "name");
console.log(descriptor.configurable);       // false

 

Freezing Objects

The Object.freeze() method creates a frozen object, which is a sealed object with nonwritable data properties.

var person1 = {
    name: "Nicholas"
};

console.log(Object.isExtensible(person1));  // true
console.log(Object.isSealed(person1));      // false
console.log(Object.isFrozen(person1));      // false
Object.freeze(person1);
console.log(Object.isExtensible(person1));  // false
console.log(Object.isSealed(person1));      // true
console.log(Object.isFrozen(person1));      // true

person1.sayName = function() {
    console.log(this.name);
};

console.log("sayName" in person1);          // false
person1.name = "Greg";
console.log(person1.name);                  // "Nicholas"

delete person1.name;
console.log("name" in person1);             // true
console.log(person1.name);                  // "Nicholas"

var descriptor = Object.getOwnPropertyDescriptor(person1, "name");
console.log(descriptor.configurable);       // false
console.log(descriptor.writable);           // false

 

Be careful with nonextensible objects, and always use strict mode so that attempts to access the objects incorrectly will throw an error.

 

Reference

Leanpub.Principles.of.Object-Oriented.Programming.in.JavaScript.Jun.2014