JavaScript Handler setPrototypeOf() Method

JavaScript handler.setPrototypeOf() method in JavaScript is a trap for Object.setPrototypeOf() method and it returns a Boolean value.

Syntax:

const p = new Proxy(target, {
      setPrototypeOf: function(target, prototype) {
  }
});

Parameters: This method accepts two parameters as mentioned above and described below: 

• target: This parameter is the target object.
• prototype: This parameter is the object’s new prototype or null.

Return value: This method returns a boolean value. It returns true if the [[Prototype]] was successfully changed.

Below examples illustrate the handler.setPrototypeOf() method in JavaScript:

Example 1: In this example, we will see the use of the handler.setPrototypeOf() method in JavaScript.

javascript

const handler1 = {
    setPrototypeOf(gfg, gfgProto) {
        gfg.geneticallyModified = true;
        return false;
    }
};

const gfgProto = {};
const gfg = {
    geneticallyModified: false
};

const proxy1 = new Proxy(gfg, handler1);
console.log(Reflect.setPrototypeOf(proxy1, gfgProto));
console.log(gfg.geneticallyModified);

let soo = {
    foo: 1
}
let proxy = new Proxy(soo, {
    setPrototypeOf(target, newProto) {
    }
});
console.log('a' in proxy);

Output:

false
true
false

Example 2: In this example, we will see the use of the handler.setPrototypeOf() method in JavaScript. It throws a custom error.

javascript

const handlerThrows = {
    setPrototypeOf(target, newProto) {
        throw new Error('custom error');
    }
};

const newProto = {}, target = {};

const p2 = new Proxy(target, handlerThrows);
console.log(Object.setPrototypeOf(p2, newProto));
console.log(Reflect.setPrototypeOf(p2, newProto));

Output:

Error: custom error

Supported Browsers: The browsers supported by handler.setPrototypeOf() method are listed below: 

• Google Chrome 49 and above
• Edge 12 and above
• Firefox 49 and above
• Opera 36 and above
• Safari 10 and above

We have a complete list of Javascript Proxy/handler methods, to check those go through the Javascript Proxy/handler Reference article.