Move code from one implementation to another function by function, tests first and code second, and check two implementations of one app for parity, with jscpd --compare as the progress measure and a coverage map binding each function to its tests. Use when porting a library or app to another language or framework (Java to Kotlin, JavaScript to Rust, a Python library to TypeScript, an iOS app to Android), when asked what is left to port, or when comparing the Android and iOS versions of an app.