JavaScript: Prevent Adding Property

What's Extensible?

Objects have internal slot “IsExtensible” that indicates if properties can added to the object.

(Note, in ES5.1 (year 2011), this was called “Extensible”. It's renamed to “IsExtensible” in ES2015.)

ECMAScript® 2016 Language Specification#table-5

Object's properties have attribute writable that determines if a property can be changed, and the attribute configurable determines if a property can be deleted,

〔►see JavaScript: Property Attributes〕

Object's extensible attribute and property attributes together determine if property can be added to the object, deleted, or if their values can be modified.

Note: if a object is not extensible, but its parents may be, so people can add properties to the parent object, and your object may still get unexpected properties, because of inheritance.

What Objects Are Extensible?

• User-defined objects are extensible, by default.
• Standard objects (For example, Object, Array, Function, Date, …) are extensible, by default.
• Host objects (For example, windows.document) may or may not be extensible.

console.log ( Object.isExtensible ( Object ) ); // true
console.log ( Object.isExtensible ( Array ) ); // true
console.log ( Object.isExtensible ( Function ) ); // true
console.log ( Object.isExtensible ( String ) ); // true
console.log ( Object.isExtensible ( Date ) ); // true
console.log ( Object.isExtensible ( RegExp ) ); // true
// ...

〔►see JavaScript: What's Object?〕

Check If Object is Extensible

Object.isExtensible(obj)

〔►see JavaScript: Object.isExtensible〕

Prevent Adding Properties

Object.preventExtensions(obj)

〔►see JavaScript: Object.preventExtensions〕

Prevent Adding/Deleting Properties

Object.seal(obj)

〔►see JavaScript: Object.seal〕

Object.isSealed(obj)

〔►see JavaScript: Object.isSealed〕

Prevent Adding/Deleting/Writing Properties

Object.freeze(obj)

〔►see JavaScript: Object.freeze〕

Object.isFrozen(obj)

〔►see JavaScript: Object.isFrozen〕