Confusion about CoffeeScript variable scope

Question

I am trying to understand how CoffeeScript variables are scoped. According to the documentation:

This behavior is effectively identical to Ruby's scope for local variables.

However, I found out that it works differently.

In CoffeeScript

a = 1
changeValue = -> a = 3
changeValue()
console.log "a: #{a}" #This displays 3

In Ruby

a = 1
def f
  a = 3
end
puts a #This displays 1

Can somebody explain it, please?

Answer 1

Ruby's local variables (starting with [a-z_]) are really local to the block they are declared in. So the Ruby behavior you posted is normal.

In your Coffee example, you have a closure referencing a. It's not a function declaration.

In your Ruby example, you don't have a closure but a function declaration. This is different. The Ruby equivalent to your Coffee is:

a = 1
changeValue = lambda do
   a = 3
end
changeValue()

In closures, local variables present when the block is declared are still accessible when the block is executed. This is (one of the) powers of closures!