handler.setPrototypeOf()
The handler.setPrototypeOf()
method is a trap for the [[SetPrototypeOf]]
object internal method, which is used by operations such as Object.setPrototypeOf()
.
Try it
Syntax
js
new Proxy(target, {
setPrototypeOf(target, prototype) {
}
});
Parameters
The following parameters are passed to the setPrototypeOf()
method.
this
is bound to the handler.
target
-
The target object.
prototype
-
The object's new prototype or
null
.
Return value
The setPrototypeOf()
method returns true
if the
[[Prototype]]
was successfully changed, otherwise false
.
Description
Interceptions
This trap can intercept these operations:
Or any other operation that invokes the [[SetPrototypeOf]]
internal method.
Invariants
If the following invariants are violated, the trap throws a TypeError
when invoked.
-
If
target
is not extensible, theprototype
parameter must be the same value asObject.getPrototypeOf(target)
.
Examples
If you want to disallow setting a new prototype for your object, your handler's
setPrototypeOf()
method can either return false
, or it can
throw an exception.
Approach 1: Returning false
This approach means that any mutating operation that throws an exception on failure to mutate, must create the exception itself.
For example, Object.setPrototypeOf()
will create and throw a
TypeError
itself. If the mutation is performed by an operation that
doesn't ordinarily throw in case of failure, such as
Reflect.setPrototypeOf()
, no exception will be thrown.
js
const handlerReturnsFalse = {
setPrototypeOf(target, newProto) {
return false;
},
};
const newProto = {},
target = {};
const p1 = new Proxy(target, handlerReturnsFalse);
Object.setPrototypeOf(p1, newProto); // throws a TypeError
Reflect.setPrototypeOf(p1, newProto); // returns false
Approach 2: Throwing an Exception
The latter approach will cause any operation that attempts to mutate, to throw. This approach is best if you want even non-throwing operations to throw on failure, or you want to throw a custom exception value.
js
const handlerThrows = {
setPrototypeOf(target, newProto) {
throw new Error("custom error");
},
};
const newProto = {},
target = {};
const p2 = new Proxy(target, handlerThrows);
Object.setPrototypeOf(p2, newProto); // throws new Error("custom error")
Reflect.setPrototypeOf(p2, newProto); // throws new Error("custom error")
Specifications
Specification |
---|
ECMAScript Language Specification # sec-proxy-object-internal-methods-and-internal-slots-setprototypeof-v |
Browser compatibility
BCD tables only load in the browser