JavaScript modules are scoped, so nothing can be accessed outside unless explicitly exported.

FYI, you don't even need an IIFE. ES modules are block-scoped by default.

  // Instead of,
  let foo
  (function() {
    const bar = 1
    foo = bar
  })()
  // `bar` is out of scope here

  // You can just,
  let foo
  {
    const bar = 1
    foo = bar
  }
  // `bar` is out of scope here too