in CoffeeScript, how can I use a variable as a key in a hash?

Question

eg:

So:

foo = "asdf"
{foo: "bar"}
eval foo

# how do I get {"asdf": "bar"} ?

# this will throw parse error:
{(eval foo): "bar"}

This is a simple syntax question: how do I get CoffeeScript to construct a hash dynamically, rather than doing it by hand?

Solution 1

For anyone that finds this question in the future, as of CoffeeScript 1.9.1 interpolated object literal keys are supported!

The syntax looks like this:

myObject =
  a: 1
  "#{ 1 + 2 }": 3

See https://github.com/jashkenas/coffeescript/commit/76c076db555c9ac7c325c3b285cd74644a9bf0d2

Solution 2

Why are you using eval at all? You can do it exactly the same way you'd do it in JavaScript:

foo    = 'asdf'
h      = { }
h[foo] = 'bar'

That translates to this JavaScript:

var foo, h;
foo = 'asdf';
h = {};
h[foo] = 'bar';

And the result is that h looks like {'asdf': 'bar'}.

Solution 3

CoffeeScript, like JavaScript, does not let you use expressions/variables as keys in object literals. This was support briefly, but was removed in version 0.9.6. You need to set the property after creating the object.

foo = 'asdf'

x = {}
x[foo] = 'bar'
alert x.asdf # Displays 'bar'

Solution 4

Somewhat ugly but a one-liner nonetheless (sorry for being late):

{ "#{foo}": bar }

View more solutions

Comments

[Trevor Burnham]

To be clear, {(eval(foo)): "bar"} is invalid JavaScript; the object literal syntax only allows literal strings as keys. To support dynamic keys, CoffeeScript would have to convert that code to something like __obj = {}; __obj[eval(foo)] = "bar";.

[Brian Genisio]

+1. I have actually found myself extending my prototypes to include a generic function like getByKey that does this for me when I need to do it often.

[Andrey Mikhaylov - lolmaus]

Why do you need eval in the last line?

[bcherny]

this is pretty clever, but a bad idea from a security/perf/best practice point of view

[Danyel]

Depending on your coffeescript version, this might not be possible.

[Andrii Gladkyi]

That's the answer. Thanks!

[bcherny]

@KonstantinSchubert Yes, it's a special case of string interpolation. See coffeescript.org/#strings

[Konstantin Schubert]

@bcherny Ah, thanks! I wasn't aware that string interpolation requires double quotes.

[phil294]

I dont think you even need the quotes. [ 1 + 2 ]: 3 will also work.