js 对象属性检索在幕后如何工作？

Question

I have an object

var person = {
   firstName: 'David',
   lastName: 'Ferrera',
   age: 30
};

and, I want to access 2 properties:

• person['lastName']
• person['lastname']

How does this work behind the scene?

Answer 1

The engine looks at the object itself and checks whether it has the property or not. If yes, it returns the corresponding value. If not it gets the object's prototype (which is just and object as well) and repeats the process. It does that until the property is either found or the currently inspected object doesn't have a prototype. In the last case it returns undefined .

This is all defined in the spec in 9.1.8.1 OrdinaryGet ( O, P, Receiver ) .

Since lastName exists in person , the property is found and Ferrera is returned.

Since lastname does not exists in person , the engine continues to look at its prototype ( Object.getPrototypeOf(person) ). Since it doesn't have lastname and it doesn't have a prototype either, undefined is returned.