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Chapter 02 路 10 minutes

Set up and run it

Install, run the tests, try the CLI, and see the type checker in your editor.

Goal

Streak installed, its tests green, and your first type error on screen.

Steps

  1. Install Node.js 22 from nodejs.org, and a code editor (VS Code understands TypeScript out of the box).

  2. Get the code and install. npm ci installs the exact versions in package-lock.json.

    git clone https://github.com/jars-demo/typescript-demo.git
cd typescript-demo
npm ci

  3. Run the tests.

    npx vitest run streak

  4. Build and use the CLI. tsc turns streak/src/*.ts into JavaScript in streak/dist/, and Node runs that.

    npm run streak:build
npm run streak -- add Read --emoji 馃摎
npm run streak -- list

    Your habits are saved in habits.json. Set STREAK_FILE to keep them somewhere else.

Meet the type checker

Open streak/src/habits.ts, find describe, and change habit.name to habit.nmae. A red squiggle appears at once. Hover it:

Property 'nmae' does not exist on type 'Habit'.

Now run the checker yourself:

npx tsc -p streak --noEmit

Same error, in the terminal. That is the check CI runs on every push. Undo the typo.

The settings that matter

streak/tsconfig.json turns on the strict checks. The most important ones:

"strict": true,
"noUncheckedIndexedAccess": true,
"exactOptionalPropertyTypes": true

strict is the one to never turn off. The other two catch the bugs strict mode still lets through; you will meet both soon.

Check it works

npx vitest run streak prints all tests passed, and npm run streak -- list shows your habit.

Exercise

Run npm run streak -- check read twice. Read the second message. Where does it come from? Use your editor's "find in files" to locate it.