JavaScript Runtime Semantics

Prototype Chain & Inheritance

JavaScript object inheritance is prototype-based. When a property is not found on an object, lookup continues through the object's prototype chain until the property is found or the chain ends at null.

The prototype chain is delegation
Class methods live on the prototype
this is separate from where a method is found
instanceof checks prototype relationships
Prototype mutation affects all delegating objects

Delegation example

const animal = {
  speak() {
    return `${this.name} makes noise`
  },
}

const dog = Object.create(animal)
dog.name = "Milo"

dog.speak() // "Milo makes noise"

The method is found on `animal`; `this` is still `dog` because of the call expression.