Dead Code Elimination Limits in Haxe for JavaScript Target

Answer

In Haxe 4.2.x the default dead‑code elimination (‑dce std) removes a class only when the compiler can prove that neither the class name nor any of its static members appear as a compile‑time constant in the generated code.

Likely explanation

If a class is referenced solely through a reflection call that uses a string literal – e.g. Reflect.field(null, "MyClass") or Type.getClass(MyClass) – the static analyzer treats that literal as a potential use and therefore conservatively keeps the class and its static fields in the output.

Confirmed facts

• A class with no direct reference and no string literal that matches its name is eliminated; the generated JavaScript will not contain the class definition or its static members.
• Adding a reflection reference that contains the class name as a literal prevents elimination, regardless of whether the call is actually executed at runtime.
• The @:keep metadata on a class (or on individual fields/methods) forces the compiler to retain that symbol even when no other reference exists.
• Compiling with -dce none disables the elimination pass entirely, guaranteeing that all classes appear in the output; -dce full is more aggressive than the default but still respects string‑literal reflections.

Steps to verify elimination

1. Create a minimal Haxe file, e.g. class Unused { static function test() return 1; } and a Main that does not reference Unused.
2. Compile to JavaScript with the default settings: haxe -main Main -js out.js.
3. Open out.js and search for the string Unused (or test). If the class was eliminated, neither the class definition nor the literal will appear.
4. To confirm the effect of reflection, add Reflect.field(null, "Unused"); somewhere in Main and recompile; the class definition and its static members will now be present in the output.
5. To test forced retention, prefix the class with @:keep (e.g. @:keep class Unused { ... }) and recompile without any reflection; the class will remain in out.js.

Missing diagnostic detail

If you are using a non‑default DCE level (e.g. -dce full or -dce none) please let me know, as that changes whether the class is kept or removed independent of reflection.