How to force a variable to be local in coffeescript?

Question

Given the following code:

outer=1
f=->
  local=1
  outer=0
  local+outer

coffeescript creates a var for local but re-ueses outer:

var f, outer;

outer = 1;

f = function() {
  var local;
  local = 1;
  outer = 0;
  return local + outer;
};

Which is what your expect.

However, if you use a local variable in a function it depends on the outer scope if the variable is declared local or not. I know this is a feature, but it was causing some bugs, because I have to check all outer scopes for variables with the same name (which are declared before my function). I wonder if there is a way to prevent this type of bug by declaring variables local?

Answer 1

This kind of error usually comes up when you aren't using appropriately descriptive variable names. That said, there is a way to shadow an outer variable, despite what the accepted answer says:

outer=1
f=->
  do (outer) ->
    local=1
    outer=0
    local+outer

This creates an IIFE, with outer as it's one argument. Function arguments shadow outer variables just like the var keyword, so this will have the behavior you expect. However, like I said, you should really just name your variables more descriptively.

Answer 2

No, that feature is explicitly not available in CoffeeScript (emphasis mine):

This behavior is effectively identical to Ruby's scope for local variables. Because you don't have direct access to the var keyword, it's impossible to shadow an outer variable on purpose, you may only refer to it. So be careful that you're not reusing the name of an external variable accidentally, if you're writing a deeply nested function.