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Pascual Vila

Frontend Instructor // Code Syllabus

Common Pitfalls & Errors

The Error //

Not committing the lockfile to version control ("it's just generated output")

# Wrong: .gitignore
package-lock.json

# Correct: always commit the lockfile
# (remove it from .gitignore)

The Solution //

The lockfile is the actual source of truth for exact resolved dependency versions — package.json alone only specifies acceptable ranges. Without a committed lockfile, every fresh install can resolve slightly different transitive versions over time, causing "works on my machine" bugs that are extremely hard to trace back to a dependency mismatch.

The Error //

Running npm install in a CI/production pipeline instead of npm ci

# Wrong: can silently mutate the lockfile mid-pipeline
npm install

# Correct: fails loudly on any mismatch
npm ci

The Solution //

npm install will happily modify the lockfile to resolve a mismatch between it and package.json, silently masking a real problem (like someone editing package.json by hand without reinstalling). npm ci refuses to do this — it fails the build immediately if the lockfile and package.json disagree, surfacing the issue before it reaches production.