JS: Reading/Writing Property and Prototype Chain

By Xah Lee. Date: . Last updated: .

Reading property goes up the prototype chain

When a property of a object is looked up (e.g. obj.color), JavaScript first look at the object to see if it has that property, if not, it lookup its parent, and repeat, until a property is found or no more parent.

const xdad = { dog: 333 };
const xson = {};
Reflect.setPrototypeOf(xson, xdad);
console.assert(xson.dog === 333);
// from parent

Accessing non-existent property return “undefined”

console.assert({ dog: 2 }.cat === undefined);

Setting property never go up prototype chain

When setting a value to a key, if the object has the property, its value is modified. If the object does not have the property, its created.

// setting a property never go up prototype chain

const xdad = { dog: 3 };

// create object xson, with xdad as parent
const xson = Object.create(xdad);

xson.dog = 4;

// check
console.assert(Object.hasOwn(xson, "dog"));
console.assert(xson.dog === 4);
console.assert(xdad.dog === 3);