The Principles of Object-Oriented JavaScript

Chapter 2. Functions

As discussed in Chapter 1, functions are actually objects in JavaScript. The defining characteristic of a function—what distinguishes it from any other object—is the presence of an internal property named [[Call]]. Internal properties are not accessible via code but rather define the behavior of code as it executes. ECMAScript defines multiple internal properties for objects in JavaScript, and these internal properties are indicated by double-square-bracket notation.

The [[Call]] property is unique to functions and indicates that the object can be executed. Because only functions have this property, the typeof operator is defined by ECMAScript to return "function" for any object with a [[Call]] property. That led to ...