Object.setPrototypeOf()

setPrototypeOf(obj, prototype)

The Object.setPrototypeOf() method sets the prototype of an object to another object or null. This method changes the object’s internal [[Prototype]] property, modifying its prototype chain at runtime. While powerful, frequent prototype changes can impact performance.

Syntax

Object.setPrototypeOf(obj, prototype)

Parameters

Parameter	Type	Description
obj	object	The object whose prototype to set
prototype	object | null	The new prototype for the object

Examples

Setting a prototype on a new object

const animal = {
  speak() {
    return `${this.name} makes a sound`;
  }
};

const dog = { name: 'Rex' };
Object.setPrototypeOf(dog, animal);

console.log(dog.speak()); // "Rex makes a sound"

Changing prototype of existing object

const proto1 = { a: 1 };
const obj = { b: 2 };

Object.setPrototypeOf(obj, proto1);
console.log(obj.a); // 1
console.log(obj.b); // 2

Setting prototype to null

const obj = { x: 1 };
Object.setPrototypeOf(obj, null);

console.log(Object.getPrototypeOf(obj)); // null
console.log(obj.toString); // undefined - no inherited methods

Mixin-like behavior

const canFly = {
  fly() {
    return `${this.name} is flying`;
  }
};

const bird = { name: 'Sparrow' };
Object.setPrototypeOf(bird, canFly);

console.log(bird.fly()); // "Sparrow is flying"

Common Patterns

Dynamic object composition

function extendObject(target, mixins) {
  for (const mixin of mixins) {
    Object.setPrototypeOf(target, mixin);
  }
  return target;
}

const canSwim = { swim() { return 'swimming'; } };
const canWalk = { walk() { return 'walking'; } };

const amphibian = extendObject({ name: 'Frog' }, [canSwim, canWalk]);
console.log(amphibian.swim()); // "swimming"
console.log(amphibian.walk()); // "walking"

Performance Note

Changing prototypes is expensive because it affects property lookups. For performance-critical code, use Object.create() to set the prototype at creation time instead.

See Also

• Object.create() — Create object with specified prototype
• Object.keys() — Get array of object keys