ClojureScript Advanced Compilation Renames JavaScript Properties

Answer to the Core Questions

Which interop property names are renamed in the current advanced build output?

Only those properties that the compiler cannot prove are referenced from outside the compiled code are renamed. In practice, any property that is not declared in an externs file (or in the compiler’s built‑in externs) will be mangled. The exact set of names depends on the Closure Compiler version that ClojureScript bundles, so the same source can produce different renamed names across releases.

How does the bundled Closure Compiler version affect externs preservation?

Each ClojureScript release ships with a specific Closure Compiler snapshot. The snapshot determines the built‑in externs that the compiler knows about and the heuristics it uses to decide whether a property is “external.” A newer snapshot may include additional built‑in externs or change the way it interprets your own externs, leading to a different set of preserved property names.

Are missing externs files silently ignored by the build toolchain?

Yes. If you list an extern file in your build configuration but the file is missing, the compiler simply treats the referenced properties as internal and renames them. No warning is emitted, so the build can silently break external JavaScript interop.

How to Identify Renamed Properties in Your Build

1. Run an advanced build (e.g. lein cljsbuild once prod or clojure -M:cljsbuild once prod).
2. Locate the output file – usually target/js/compiled/app.js.
3. Search for property accesses that you expect to stay unchanged:

   grep -o "\.\w\+" target/js/compiled/app.js | sort | uniq
   

4. Compare the list to your original source (or to goog.global references) to spot renamed names.

Controlling Property Renaming with Externs

• Create an extern file, e.g. src/externs/my-externs.js:

  /** @externs */
  var MyObj = {};
  MyObj.foo = {};
  MyObj.bar = {};
  

• Tell the compiler to use it:

  {:compiler-options {:externs ["src/externs/my-externs.js"]}}
  

• Re‑run the advanced build and confirm the names remain unchanged in the output.

Why the Compiler Version Matters

The bundled Closure Compiler version is printed in the build log (look for a line like Closure Compiler version 20241111). If you upgrade ClojureScript, the snapshot may change, and you might need to regenerate or update your externs accordingly.

What to Verify Next

To give you the most accurate recommendation, I need to know which property names you rely on from external JavaScript. If you can share a short list (e.g. myObj.foo, window.myLib.bar), I can show you exactly how to declare them in an extern file that will survive any future compiler upgrade.