You write a first test with Node's built-in test runner and its assertion module, and read what a failing assertion prints. You choose test files with a glob instead of a folder name, and compare Node's runner with Vitest. Then you install Vitest, write tests and expectations, and run one test file. Finally you write type tests that check a function's types without running it, watch one catch a wrong type, and see a type test outside the project configuration pass without being checked.

You install Oxlint, a linter for TypeScript and JavaScript, and see it report an unused variable that the compiler lets through. You decide when a warning should fail the run, by exit code. You turn on type-aware rules that find a floating promise, see what that promise does when the program runs, and see why typescript-eslint doesn't install next to TypeScript 7 yet. Then you format code with Prettier, and check formatting without changing a file.

You put the compiler, the linter, the tests and the formatter behind npm scripts, chain them so each one runs only if the one before it passed, and watch a planted type error stop the line with exit code 1. You keep node_modules and dist out of Git, and see a GitHub Actions workflow run the same checks on every push. Then you build a reusable project template whose one verify command checks, lints, tests, checks formatting and builds, proven by its contract check.