What does a lockfile actually guarantee you? And where do supposedly reproducible installs still fall over?
A strong answer knows exactly what a lockfile pins, where the gaps are, and can diagnose a 'deploy broke with no code change' from install mechanics
So package.json declares the version ranges, and the lockfile records what those actually resolved to, the exact version of every package, its download URL, and an integrity hash. That way a later install reproduces the same tree instead of re-resolving the ranges against whatever happens to be newest that day. You commit the lockfile, and in CI you run npm ci, which installs strictly from it. It deletes node_modules first, it fails if the lockfile and package.json disagree, and it never writes back to the lockfile. Plain npm install can update it, I think. For the ranges, caret accepts minor and patch updates, and tilde is patches only. Put together you get reproducible builds, basically, the same lockfile produces the same dependency tree and the same behavior on every developer machine and every deploy.