CoffeeScript Default Arguments

Question

I'm looking at the following example from CoffeeScript: Accelerated Development

x = true
showAnswer = (x = x) ->
  console.log if x then 'It works!' else 'Nope.'

console.log "showAnswer()", showAnswer()
console.log "showAnswer(true)", showAnswer(true)
console.log "showAnswer(false)", showAnswer(false)

I don't understand why showAnswer(...) undefined shows up for each test.

Nope.
showAnswer() undefined
It works!
showAnswer(true) undefined
Nope.
showAnswer(false) undefined

Please explain the output of each case.

As a side note, you don't need to define x = true prior to your function if you want to give it a default value. (x = true) -> is a valid (and I think preferred) method signature. — Chris Lambe
– Chris Lambe, Commented May 20, 2014 at 15:39

Chris Lambe · Accepted Answer · 2014-05-20 15:35:53Z

4

Don't forget that CoffeeScript, by default, returns the last statement in a function. So what your showAnswer function actually says is:

showAnswer = (x = x) ->
    return console.log if x then 'It works!' else 'Nope.'

or compiled to JavaScript:

showAnswer = function(x) {
  if (x == null) {
    x = x;
  }
  return console.log(x ? 'It works!' : 'Nope.');
};

The other thing to realize is that the console.log method returns undefined. So when you log the result of your showAnswer method, it will print undefined.

If I understand your intention correctly, I would modify your original function to do this:

showAnswer = (x = x) ->
  if x then 'It works!' else 'Nope.'

Alternatively, modify your console.log statements as such:

console.log "showAnswer()"
showAnswer()

console.log "showAnswer(true)"
showAnswer(true)

console.log "showAnswer(false)"
showAnswer(false)

edited May 20, 2014 at 15:35

answered May 20, 2014 at 15:30

Chris Lambe

1118 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

CoffeeScript Default Arguments

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related