• It should return something.
• It should be pure.
• It should do one thing.

In JavaScript a function always returns but you should be explicit about what you want it to return.

function add (x, y) {
  x + y
}

What does this return? How would you fix this function?

A pure function returns the same thing everytime it is called with the same arguments

function add (x, y) {
  return x + y + Math.random()
}

Can you see why this might be frustrating?

function getUsers () {
  return Users.getAll()
}

Can you see why this might be frustrating?

Also I/O is impure. If you are changing the world with your function you can't be so sure that your program will run as intended.

function log (x) {
  console.log(x)
  return x
}

How might this ruin your life?

var lastItem = readLastStdOut(log(printItems(items)))

Holy strawman!

The point is you want to isolate all the gross parts of your program so you know where the bugs will be.

function add (x, y) {
  Users.find(1).update({
    name: 'Sally'
  }) 

  return x + y
}

You should be cringing.

function add (x, y) {
  console.log(x, '+', y, '=', x + y)
  return x + y
}

These are two seperate things. Don't conflate purposes.

But don't be a wimp either. These rules are almost always applicable. If you think you need to break them you are probably wrong (aka: need to write a new function). But break them if you must, it's okay!