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Minification removes class identifiers causing observable runtime change in Class.name #1973
Description
- Rollup Plugin Name: @rollup/plugin-terser
- Rollup Plugin Version: 1.0.0
- Rollup Version: 4.59.0
- Operating System (or Browser): any
- Node Version: 22.19.0
- Link to reproduction: https://stackblitz.com/edit/rollup-repro-dxhgy1x9?file=dist%2Fmain.js
Description
When using @rollup/plugin-terser, named class declarations are transformed into anonymous class expressions during minification. This causes clazz.name and this.constructor.name to return an empty string ("") instead of the original class name. This results in observable runtime differences between development and production builds.
Behavior
Terser removes the class identifier and converts:
class MyClass {}
into:
class {}
This creates an anonymous class expression.
According to the ECMAScript specification:
ClassDeclaration → has a name
Anonymous ClassExpression → name === ""
Therefore:
clazz.name === ""
this.constructor.name === ""
Why This Is Problematic
This is not just cosmetic minification.
Class.name and constructor.name are frequently used for:
- logging
- debugging
- dependency injection
- metadata systems
- serialization
- runtime introspection
This creates:
- different runtime behavior in dev vs prod
- silent semantic changes
- hard-to-debug production issues
Important Note
I understand that Terser does not guarantee preservation of Function.name.
However, this transformation introduces observable semantic differences between builds, not just identifier mangling.
Workaround
Setting:
terser({ keep_classnames: true })
preserves the expected behavior.
Activity
shellscape commentedon Mar 8, 2026
@CharlieHelps using the reproduction link and the issue description, please do a best-effort triage of this issue and recommend a reasonable fix. If you have questions, please ask them first and provide 1-4 suggested answers in
a b c dformat.charliecreates commentedon Mar 8, 2026
Thanks for the ping — I did a best-effort triage from the reproduction and current plugin code.
What I found
@rollup/plugin-terser,renderChunksets defaults ofmodule: truefor ES output andtoplevel: truefor CJS output (packages/terser/src/module.ts).minify(code, options)inpackages/terser/src/worker.ts), with no extra class-name-specific transform.keep_classnames/keep_fnames), so.name/constructor.namechanges are expected unless preserved.Triage conclusion
This looks like expected upstream Terser behavior surfaced by plugin defaults, rather than a plugin-specific semantic rewrite bug.
Recommended fix
packages/terser/README.mdthat default minification can change runtime class/function names.keep_classnames: true(and optionallykeep_fnames: true) as the intended opt-in when runtime name introspection is required.packages/terser/test/test.jscovering default behavior vskeep_classnamesandmodule/topleveloverrides.