JS: Prevent Adding Property

By Xah Lee. Date: . Last updated: .

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 JS: Property Attributes, writable, enumerable, configurable]

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?

const arr = [ Object, Array, Function , String , Date, RegExp ];

console.log (
 arr . every (x => Object.isExtensible(x) )
); // true

[see JS: Object Type]

Check If Object is Extensible


[see JS: Object.isExtensible]

Prevent Adding Properties


[see JS: Object.preventExtensions]

Prevent Adding/Deleting Properties


[see JS: Object.seal]


[see JS: Object.isSealed]

Prevent Adding/Deleting/Writing Properties


[see JS: Object.freeze]


[see JS: Object.isFrozen]

JS Object and Inheritance

  1. Object Overview
  2. Object Type
  3. Prototype and Inheritance
  4. Create Object
  5. Object Literal Expression
  6. Create Object with Parent X
  7. Get/Set Prototype
  8. Prevent Adding Property
  9. Determine Type of Object
  10. Clone Object
  11. Test Object Equality
  12. Add Method to Prototype
  13. Object Object
  14. Object.prototype
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