What is the purpose of `package-lock.json` (or `yarn.lock`) and why is it important when dealing with dependency issues?

`package-lock.json` (for npm) or `yarn.lock` (for Yarn) files record the exact versions of all dependencies in your project and their transitive dependencies. This ensures that everyone working on the project uses the same versions, preventing unexpected errors caused by different versions being installed. Using these lock files consistently is vital for reproducible builds and avoiding dependency conflicts, which can lead to the 'unable to resolve reference' error.