Lexical scoping in a for loop enclosing a promise?

Question

I have an ids object, which maps id strings to product objects.

for id of ids
  product = ids[id]
  console.log product # Prints out something different each loop. :)
  Product.create(product).then ->
    console.log product # Only prints out the last id each loop. :(

I'm using a library for database interactions, which exposes promises (indicated by the then function above). I'm trying to print out the product variable inside the then function, but I only seem to be getting the last id in ids, so it looks like it's a scoping issue. How can I scope the product variable properly so that it prints out a different product in the then function each loop?

Answer 1

@false did find the right duplicate describing your issue. Indeed, you've got a scoping issue where product is non-local to the loop body, and you get the last item only from your asynchronous callbacks.

How can I scope the product variable properly so that it prints out a different product in the then callback?

In idiomatic coffeescript, you will use the do notation for the IEFE in the loop:

for id of ids
  do (product = ids[id]) ->
    console.log product
    Product.create(product).then ->
      console.log product

Or, drawing the property value directly from the of-loop:

for id, product of ids
  do (product) ->
    …