Object-Oriented JavaScript

Functions are Data

This is an important concept that we'll need later on—functions in JavaScript are actually data. This means that the following two ways to define a function are exactly the same:

function f(){return 1;}
var f = function(){return 1;}

The second way of defining a function is known as function literal notation.

When you use the typeof operator on a variable that holds a function value, it returns the string "function".

>>> function f(){return 1;}
>>> typeof f

"function"

So JavaScript functions are data, but a special kind of data with two important features:

• They contain code
• They are executable (can be invoked)

As you saw before, the way to execute a function is by adding parentheses after its name. As the next example demonstrates, ...