You Don't Know JS: this & Object Prototypes

Chapter 3. Objects

In Chapters 1 and 2, we explained how the this binding points to various objects depending on the call-site of the function invocation. But what exactly are objects, and why do we need to point to them? We will explore objects in detail in this chapter.

Syntax

Objects come in two forms: the declarative (literal) form and the constructed form.

The literal syntax for an object looks like this:

var myObj = {
    key: value
    // ...
};

The constructed form looks like this:

var myObj = new Object();
myObj.key = value;

The constructed form and the literal form result in exactly the same sort of object. The only difference really is that you can add one or more key/value pairs to the literal declaration, whereas with constructed-form objects, you must add the properties one by one.

Note

It’s extremely uncommon to use the “constructed form” for creating objects as just shown. You would pretty much always want to use the literal syntax form. The same will be true of most of the built-in objects (explained later).

Type

Objects are the general building block upon which much of JS is built. They are one of the six primary types (called “language types” in the specification) in JS:

• string
• number
• boolean
• null
• undefined
• object

Note that the simple primitives (string, boolean, number, null, and undefined) are not themselves objects. null is sometimes referred to as an object type, but this misconception stems from a bug in the language that causes typeof null to return the string "object" ...