Question

Here’s the original code:

res.write JSON.stringify {"#{result.statusCode}": "OK"}

and here’s the error that both the CoffeeScript linter in SublimeText 2 and the “Try CoffeeScript” interpreter on the CoffeeScript site give me:

PARSE ERROR ON LINE 1: UNEXPECTED '('

Obviously there’s no open parens in the code, so I don’t understand the error. Is it a bug in the CoffeeScript parser?

The smallest line of code that does this seems to be something like this:

{"#{a}": ""}

I’m assuming that string interpolation in an object’s key is valid, but I don’t know for sure.

EDIT:
After some investigation it seems that it’s not valid to do the string interpolation in the key because the resulting JavaScript would be invalid.

This:

{"#{a}": "stuff}

would translate to something like:

{ "" + a: "stuff"}

which isn’t valid.

But can someone explain why the error message it gives me is so wrong?

Answer 1

I’m assuming that string interpolation in an object’s key is valid, but I don’t know for sure.

Unfortunately it’s not.

You’ll have to do something like

(json = {})[result.statusCode] = 'OK'
res.write JSON.stringify json

or if you want a one-liner

res.write (-> ((json = {})[result.statusCode] = 'OK') and JSON.stringify json)()

As for the misleading error, CoffeeScript is trying to translate your {"#{a}": ''} into {("" + a): ""} which is not valid JavaScript. CoffeeScript is throwing the error at that left paren.