[feritcemkahraman]

With const cx = classNames.bind({ a: 'a_hash' }), cx('constructor') and cx({ constructor: true }) return the native Object function instead of a class string. Other names inherited from Object.prototype can also corrupt combined class lists.

Use a bound mapping only when its value is a string. This keeps unmapped names intact and preserves explicit prototype-named mappings, inherited string mappings, and the existing empty-string fallback. Each mapping is read once.

Both string and object regressions fail before the fix. All 66 Node tests and the TypeScript declaration checks pass with the fix.

Local performance smoke check

Compared the unchanged main bind implementation with this change on Node 24.21.0, using bound styles and the benchmark's string, object, mixed, and nested-array argument shapes. Median of seven alternating runs, 200,000 calls per sample, after warmup, on one logical CPU:

Arguments	Before (ms)	After (ms)
Strings	17.69	17.73
Object	14.68	15.19
Mixed	20.62	20.99
Arrays	64.36	61.21

The browser/JSPerf benchmark and the older Node matrix were not run locally.