Linters and static analysis

Beginner 8 min read

Tools that read your code without running it and catch mistakes, risky patterns and style drift.

Why it matters

A linter finds an unused variable, a missing await or a likely bug in seconds, before a reviewer or a user does. A formatter ends arguments about style, so reviews focus on what the code does.

Where it fits

What you will learn

Tell a linter, a formatter and a type checker apart.
Run a linter locally and in CI.
Fix or deliberately silence a warning.

Before you start

Automated tests✓ Learned

Formatter — rewrites layout (spaces, line breaks) so all code looks the same. Never changes behaviour.
Linter — reports suspicious code: unused names, unreachable code, risky patterns.
Type checker — proves types line up (TypeScript, mypy, PHPStan) without running the code.

Example

npx eslint .            # JavaScript / TypeScript
ruff check .            # Python: lint
ruff format .           # Python: format

Tip: Add the linter to an existing project gradually: start with the rules that find bugs, fix those, then tighten. A thousand style warnings on day one get ignored.

Run the same commands in CI as locally, with the versions pinned, so "passes on my machine" means "passes in the pipeline".