JS: Get Set Property

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

Accessing Non-Existent Property Return “undefined”

console.assert({ a: 2 }.b === 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 = { kk: 3 };

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

xson.kk = 4;

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