💥 Regression Report

When cloning an instance, if you pass in config containing any interpolation options, some interpolation behaviors may break.

This creates a regression in next-i18next, which clones instances on the server and not on the client. Incorrect translations are generated server-side, leading to hydration errors.

I think it's because when creating an instance, options.interpolation gets populated with defaults (here). But when cloning an instance, effectively this is attached to the clone: new Interpolator({ ...oldConfig, ...newConfig, isClone: true }) (here).oldConfig.interpolation gets replaced by newConfig.interpolation, which does not have those defaults.

This probably could be fixed in next-i18next by not passing config into cloneInstance. However, I think it's very surprising that defaults are sometimes used and sometimes not.

Last working version

Worked up to version: 25.6.3

Stopped working in version: 25.7.0

To Reproduce

https://codesandbox.io/p/sandbox/yfgs43

test1 and test2 produce "My list: foo and bar". test3 produces "My list: foo,bar". If you change the version to 25.6.3, test3 works correctly.

Expected behavior

Explained above.

Your Environment

n/a