ClojureScript :advanced builds break JS interop after a compiler version bump

Detect renamed or eliminated external names

1. Build with :advanced and enable pretty‑printed output (or source maps):

   :optimizations :advanced
   :pretty-print true
   

2. Open the generated JavaScript file (or the source‑mapped view) and search for the property names you use from external libraries (e.g., $.ajax, L.map). If the name appears as a mangled identifier like a.bc instead of the original, the Closure Compiler renamed or removed it.
3. Watch the compiler output for warnings such as WARNING: No externs file for .... New warnings after the upgrade indicate missing externs that could lead to renaming.

Likely explanation vs confirmed facts

Likely explanation: A newer ClojureScript compiler bundles a newer Google Closure Compiler that enables stricter externs checking. When an external property is not declared in an externs file, the compiler may rename or drop it, breaking JS interop that worked before.

Confirmed facts:

• The compiler emits a warning for each property access that lacks an externs entry.
• ^:export metadata only protects symbols defined inside the ClojureScript code; it does not affect external globals.
• An externs file that lists the required globals and their properties prevents renaming and dead‑code elimination of those names.
• The js> reader macro emits the literal JavaScript text unchanged; the code inside is treated as a black box and is not subject to Closure Compiler renaming.

Protect runtime‑loaded libraries

• For symbols that come from external globals (libraries loaded at runtime), provide an externs file that lists each needed property or function. Example externs fragment:

  var $ = {};
  $.ajax = function() {};
  var L = {};
  L.map = function() {};
  

• If you need to call a ClojureScript function from plain JavaScript, add ^:export to the corresponding var (or use goog.exportSymbol) so the generated name stays stable.
• Both mechanisms can coexist: externs for the external library, ^:export for the ClojureScript entry points.

js> macro behavior under :advanced

The js> form outputs its contents verbatim. Under :advanced it does not prevent renaming of surrounding ClojureScript symbols, but the code inside the block is not renamed because the Closure Compiler treats it as opaque JavaScript. The block can still be eliminated if its result is never used; to prevent that, either use the result or annotate the surrounding form with ^:export.

Missing diagnostic detail

If you are accessing an external property via bracket notation with a string literal (e.g., $('#elem')['animate']), the name is safe from renaming regardless of externs. Let me know whether your calls use dot notation or bracket‑string notation so I can tell you if an externs file is strictly required.