TypeError: Cannot add property x, object is not extensible (V8-based)
TypeError: Cannot define property x, object is not extensible (V8-based)
TypeError: can't define property "x": Object is not extensible (Firefox)
TypeError: Attempting to define property on object that is not extensible. (Safari)

Usually, an object is extensible and new properties can be added to it. However, in this case Object.preventExtensions() marked an object as no longer extensible, so that it will never have properties beyond the ones it had at the time it was marked as non-extensible.

In strict mode, attempting to add new properties to a non-extensible object throws a TypeError. In sloppy mode, the addition of the "x" property is silently ignored.

"use strict";

const obj = {};
Object.preventExtensions(obj);

obj.x = "foo";
// TypeError: can't define property "x": Object is not extensible

In both, strict mode and sloppy mode, a call to Object.defineProperty() throws when adding a new property to a non-extensible object.

const obj = {};
Object.preventExtensions(obj);

Object.defineProperty(obj, "x", { value: "foo" });
// TypeError: can't define property "x": Object is not extensible

To fix this error, you will either need to remove the call to Object.preventExtensions() entirely, or move it to a position so that the property is added earlier and only later the object is marked as non-extensible. Of course you can also remove the property that was attempted to be added, if you don't need it.

"use strict";

const obj = {};
obj.x = "foo"; // add property first and only then prevent extensions

Object.preventExtensions(obj);

• Object.preventExtensions()