JavaScript: The Good Parts

Chapter 4. Functions

Why, every fault's condemn'd ere it be done: Mine were the very cipher of a function. . .

—William Shakespeare, Measure for Measure

The best thing about JavaScript is its implementation of functions. It got almost everything right. But, as you should expect with JavaScript, it didn't get everything right.

A function encloses a set of statements. Functions are the fundamental modular unit of JavaScript. They are used for code reuse, information hiding, and composition. Functions are used to specify the behavior of objects. Generally, the craft of programming is the factoring of a set of requirements into a set of functions and data structures.

Function Objects

Functions in JavaScript are objects. Objects are collections of name/value pairs having a hidden link to a prototype object. Objects produced from object literals are linked to Object.prototype. Function objects are linked to Function.prototype (which is itself linked to Object.prototype). Every function is also created with two additional hidden properties: the function's context and the code that implements the function's behavior.

Every function object is also created with a prototype property. Its value is an object with a constructor property whose value is the function. This is distinct from the hidden link to Function.prototype. The meaning of this convoluted construction will be revealed in the next chapter.

Since functions are objects, they can be used like any other value. Functions can be stored in variables, objects, and arrays. Functions can be passed as arguments to functions, and functions can be returned from functions. Also, since functions are objects, functions can have methods.

The thing that is special about functions is that they can be invoked.