The Prototype Chain: How Property Lookup Works

The Prototype Chain: How Property Lookup Works

Lookup Process

When you access obj.property:

1. Check obj itself.
2. Check obj.__proto__ (its prototype).
3. Check obj.__proto__.__proto__.
4. Continue until found or reach null.
5. If not found, return undefined.

Example

const animal = { legs: 4 };
const dog = Object.create(animal);
dog.name = "Rex";
dog.name;   // "Rex" (own property)
dog.legs;   // 4 (from animal, the prototype)
dog.tail;   // undefined (not found in the chain)

The End of the Chain

Object.getPrototypeOf({}); // Object.prototype
Object.getPrototypeOf(Object.prototype); // null (end of the chain)

hasOwnProperty

dog.hasOwnProperty("name"); // true (own)
dog.hasOwnProperty("legs"); // false (inherited)

The Takeaway

Property lookup: check the object, then its prototype, then the prototype's prototype, up to null. If not found, return undefined. Use hasOwnProperty to check if a property is own vs inherited. The chain ends at Object.prototype whose prototype is null.