Chapter 3. First-class functions

If you asked a dozen JavaScript programmers what they thought JavaScript got wrong, you’d probably get a dozen different answers. If you asked those same dozen people what JavaScript got right, they’d probably all answer, “First-class functions.” What does it mean that JavaScript has first-class functions, why does it matter, and how does CoffeeScript make functions even better? All will be answered in good time. First, though, what exactly is a function?

A function defines a transformation of input values called parameters to an output value called the return value. You define a function in CoffeeScript with literal notation using an arrow:

That isn’t a very useful function, though. It has no parameters and no return value. The function parameters go to the left of the arrow, and the function body goes to the right of the arrow:

When you invoke a function, you provide arguments, and the function transforms them to produce an output value:

3.1. Computation

3.2. Events

3.3. I/O

3.4. Higher-order functions

3.5. Scope

3.6. Closures

3.7. Putting it together

3.8. Summary