You Don't Know JS: this & Object Prototypes

Chapter 5. Prototypes

In Chapters 3 and 4, we mentioned the [[Prototype]] chain several times, but haven’t said what exactly it is. We will now examine prototypes in detail.

Note

All of the attempts to emulate class-copy behavior described previously in Chapter 4, labeled as variations of mixins, completely circumvent the [[Prototype]] chain mechanism we examine here in this chapter.

[[Prototype]]

Objects in JavaScript have an internal property, denoted in the specification as [[Prototype]], which is simply a reference to another object. Almost all objects are given a non-null value for this property, at the time of their creation.

Note: we will see shortly that it is possible for an object to have an empty [[Prototype]] linkage, though this is somewhat less common.

Consider:

var myObject = {
    a: 2
};

myObject.a; // 2

What is the [[Prototype]] reference used for? In Chapter 3, we examined the [[Get]] operation that is invoked when you reference a property on an object, such as myObject.a. For that default [[Get]] operation, the first step is to check if the object itself has a property a on it, and if so, it’s used.

Note

ES6 Proxies are outside of our discussion scope in this book (they will be covered in a later book in the series), but everything we discuss here about normal [[Get]] and [[Put]] behavior does not apply if a Proxy is involved.

But it’s what happens if a isn’t present on myObject that brings our attention now to the [[Prototype]] link of the object.

The default [[Get]] operation ...