The language's REPL and debugger

The project's language comes with a REPL to call the code and a debugger to stop it and read its values. Which ones depend on the language and its setup, so they are found in the project rather than assumed.

Finding it

• The language and its environment: the manifest (pyproject.toml, package.json, go.mod) and where the project's interpreter lives (a virtual environment, a version file).
• A project console, if there is one: python manage.py shell, rails console, iex -S mix.
• The debugger: built in (pdb, node inspect) or a development dependency (Delve).

Using it

• Drive them with scripts, not interactive sessions: python -c '...', node -e '...', or a short file run once.
• Give the debugger its commands up front, from a file or the command line, so a run stops, prints and exits on its own.
• Run inside the project's environment (its virtual environment, its config), so values match what the tests see.
• Remove debug output and breakpoints before the work counts as done.

Examples: python -c and pdb (Python), node -e and node inspect (Node.js), Delve (Go), jshell (Java), any debugger through the Debug Adapter Protocol.