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

问题:

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?

回答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

回答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'}.

回答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'

回答4:

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

{ "#{foo}": bar }

回答5:

If you're looking to use Coffeescript's minimal syntax for defining your associative array, I suggest creating a simple two line method to convert the variable name keys into the variable values after you've defined the array.

Here's how I do it (real array is much larger):

@sampleEvents = 
   session_started:
          K_TYPE: 'session_started'
          K_ACTIVITY_ID: 'activity'

   session_ended:
          K_TYPE: 'session_ended'

   question_answered:
          K_TYPE: 'question_answered'
          K_QUESTION: '1 + 3 = '
          K_STUDENT_A: '3'
          K_CORRECT_A: '4' #optional
          K_CORRECTNESS: 1 #optional
          K_SECONDS: 10 #optional
          K_DIFFICULTY: 4 #optional


for k, event of @sampleEvents
    for key, value of event
        delete event[key]
        event[eval(key.toString())] = value

The SampleEvents array is now:

{ session_started: 
   { t: 'session_started',
     aid: 'activity',
     time: 1347777946.554,
     sid: 1 },
  session_ended: 
   { t: 'session_ended', 
     time: 1347777946.554, 
     sid: 1 },
  question_answered: 
   { t: 'question_answered',
     q: '1 + 3 = ',
     sa: '3',
     ca: '4',
     c: 1,
     sec: 10,
     d: 4,
     time: 1347777946.554,
     sid: 1 },

回答6:

Try this:

foo = "asdf"

eval "var x = {#{foo}: 'bar'}"
alert(x.asdf)