handler.getPrototypeOf()
Baseline
Widely available
This feature is well established and works across many devices and browser versions. It’s been available across browsers since January 2020.
The handler.getPrototypeOf() method is a trap for the [[GetPrototypeOf]] object internal method, which is used by operations such as Object.getPrototypeOf().
Try it
const monster = {
eyeCount: 4,
};
const monsterPrototype = {
eyeCount: 2,
};
const handler = {
getPrototypeOf(target) {
return monsterPrototype;
},
};
const proxy = new Proxy(monster, handler);
console.log(Object.getPrototypeOf(proxy) === monsterPrototype);
// Expected output: true
console.log(Object.getPrototypeOf(proxy).eyeCount);
// Expected output: 2
Syntax
new Proxy(target, {
getPrototypeOf(target) {
}
})
Parameters
The following parameter is passed to the getPrototypeOf() method. this is bound to the handler.
target-
The target object.
Return value
The getPrototypeOf() method must return an object or null, representing the prototype of the target object.
Description
Interceptions
This trap can intercept these operations:
Object.getPrototypeOf()Reflect.getPrototypeOf()__proto__Object.prototype.isPrototypeOf()instanceof
Or any other operation that invokes the [[GetPrototypeOf]] internal method.
Invariants
The proxy's [[GetPrototypeOf]] internal method throws a TypeError if the handler definition violates one of the following invariants:
- The result must be either an
Objectornull. - If the target object is not extensible (that is,
Reflect.isExtensible()returnsfalseontarget), the result must be the same as the result ofReflect.getPrototypeOf(target).
Examples
Basic usage
const obj = {};
const proto = {};
const handler = {
getPrototypeOf(target) {
console.log(target === obj); // true
console.log(this === handler); // true
return proto;
},
};
const p = new Proxy(obj, handler);
console.log(Object.getPrototypeOf(p) === proto); // true
Five ways to trigger the getPrototypeOf trap
const obj = {};
const p = new Proxy(obj, {
getPrototypeOf(target) {
return Array.prototype;
},
});
console.log(
Object.getPrototypeOf(p) === Array.prototype, // true
Reflect.getPrototypeOf(p) === Array.prototype, // true
p.__proto__ === Array.prototype, // true
Array.prototype.isPrototypeOf(p), // true
p instanceof Array, // true
);
Two kinds of exceptions
const obj = {};
const p = new Proxy(obj, {
getPrototypeOf(target) {
return "foo";
},
});
Object.getPrototypeOf(p); // TypeError: "foo" is not an object or null
const obj2 = Object.preventExtensions({});
const p2 = new Proxy(obj2, {
getPrototypeOf(target) {
return {};
},
});
Object.getPrototypeOf(p2); // TypeError: expected same prototype value
Specifications
| Specification |
|---|
| ECMAScript® 2026 Language Specification # sec-proxy-object-internal-methods-and-internal-slots-getprototypeof |
Browser compatibility
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