Javascript: Preventing Object Modification

All objects you create are extensible by default, meaning new properties can be added to the object at any time. By setting Extensible attribute on an object to false, you can prevent new properties from being added to an object. There are three different ways to accomplish this.

Preventing Extensions

One way to create a nonextensible object is with Object.preventExtensions(). In the code below, as person1 is nonextensible, the sayName() method is never added to it.

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 can use the Object.seal() method on an object to seal it. A sealed object is nonextensible, and all of its properties are nonconfigurable. That means not only can you not add new properties to the object, but you also can’t remove properties or change their type.

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

If an object is frozen, you can’t add or remove properties, you can’t change properties’ types, and you can’t write to any data properties. In essence, a frozen object is a sealed object where data properties are also read-only. Frozen objects can’t become unfrozen, so they remain in the state they were in when they became frozen.

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