JS: Reflect.preventExtensions

By Xah Lee. Date: 2017-02-12. Last updated: 2022-09-01.

New in JS2015.

Reflect.preventExtensions(obj)

Make object not extensible.
Return true if success, else false.
If obj is not object, throw a TypeError exception.

const jj = {};
console.log(Reflect.isExtensible(jj));
Reflect.preventExtensions(jj);
console.log(Reflect.isExtensible(jj) === false);

Non-extensible object, cannot revert, property can still be deleted, parent object may add properties

Once a object is not extensible, you cannot revert it.
Property can still be deleted for Non-Extensible object
if a object is not extensible, but its parents may be, so you can add properties to the parent object, and your object may still get unexpected properties, because of inheritance.

// property can still be deleted for non-extensible object
const jj = { "p": 3 };
Reflect.preventExtensions(jj);
console.log(jj.hasOwnProperty("p"));
delete jj.p;
console.log(jj.hasOwnProperty("p") === false);

〔see Property Attributes〕

JS: Reflect.preventExtensions

Non-extensible object, cannot revert, property can still be deleted, parent object may add properties

JavaScript, Prevent Change Property