How to imitate `let` behavior in loops?

[vzakharov]

Here’s a screenshot demonstrating some sample CS code, its translation to JS, and the log showing the incorrect (for me) behavior, along with the correct one:

The code is:

keys = ['a', 'b', 'c']
func = {}

for key in keys
  func[key] = => key
 
console.log func.a()
console.log func.b()

I want a behavior where it logs the key that the function was created with, not the last key assigned (because coffescript seems to be assigning to global variables).

Is there a way to circumvent this?

[vzakharov]

After reading this, I kind of get why CS doesn’t have let, but my question is more about solving a practical issue rather than syntax.

If, for example, I loop through all the properties of an object and want to create something for each property (say, a watcher), I need to be able to refer to that thing that this something was created for. And currently I don’t really see how this is achievable with CS.

[vzakharov]

Found out that I can do it with [].forEach:

Weird, but I’ll try to get used to it...

[vzakharov]

Still keeping it open to get an expert answer on whether my solution is the “ideologically correct” one.

[edemaine]

The standard CoffeeScript solution is:

for key in keys
  do (key) =>
    funcs[key] = => key

I think it'd be great to add the shorthand for let key in keys for this common case, though.

[GeoffreyBooth]

Yes, do is the idiomatic way to create a new scope (like you’re wishing you could have the for loop do, which it does for let/const variables). And since [].forEach takes a callback function, within that callback is a new function scope just like do creates, so that’s also a fine solution and perhaps simpler to understand.

Closing as there has been lots of discussion of let/const elsewhere and I don’t see it getting revisited anytime soon. If we design a way to “opt in” to block-scoped variables, a proposal to do so should get its own thread and also not cause any breaking changes.