TypeError: can't set prototype of this object
The JavaScript exception "can't set prototype of this object" occurs when attempting to set the prototype of an object, but the object's prototype is frozen, either by being a built-in immutable prototype object, or by being non-extensible.
Message
TypeError: Immutable prototype object 'Object.prototype' cannot have their prototype set (V8-based) TypeError: #<Object> is not extensible (V8-based) TypeError: can't set prototype of this object (Firefox) TypeError: Cannot set prototype of immutable prototype object (Safari) TypeError: Attempted to assign to readonly property. (Safari)
Error type
What went wrong?
You are using one of the prototype-mutating methods—most notably, Object.setPrototypeOf()
—on an object whose prototype is immutable. Some built-in objects have immutable prototypes, such as Object.prototype
and window
, for security reasons. User objects can also prevent prototype changes by using Object.preventExtensions()
, Object.seal()
, or Object.freeze()
.
Examples
Changing the prototype of a built-in object
A selected few built-in objects have immutable prototypes. For example, you cannot change the prototype of Object.prototype
:
Object.setPrototypeOf(Object.prototype, {});
This prevents you from being able to arbitrarily change the behavior of all objects in the system. The prototype of Object.prototype
is always null
. However, other built-in prototype objects, such as Function.prototype
and Array.prototype
, are not protected by default in this regard.
Changing the prototype of a non-extensible object
If you make an object non-extensible, you cannot change its prototype either:
const obj = {};
Object.preventExtensions(obj);
Object.setPrototypeOf(obj, {});
// TypeError: can't set prototype of this object