xai-org/xai-cookbook/fix-a-bug

Fixing a bug in tiny-checkout

tiny-checkout is the checkout logic of a small web store. It's plain JavaScript in ES modules with no dependencies, so there's nothing to install.

Where things are

• src/money.js turns prices in dollars into whole cents and formats cents for display.
• src/cart.js keeps the cart's lines and adds up the subtotal.
• src/coupons.js looks up coupon codes and works out their discount.
• src/checkout.js puts it together: subtotal, shipping, discount, and total.
• test/<name>.test.js has the tests for src/<name>.js.

Running the tests

• Every test: node --test
• One file: node --test test/money.test.js

The output lists each test with ✔ or ✖, then a summary. A failing test shows the expected and actual values.

Steps

1. Run node --test to see where things stand before you change anything.
2. Find the code behind the symptom. Start from what the report describes, read the files involved, and follow the calls with grep -rn.
3. Add a test that reproduces the report to the test file of the module you'll fix, and run it to watch it fail. A fix without a failing test first isn't done.
4. Fix the cause, not the symptom, with the smallest change that works.
5. Run node --test again, and keep going until every test passes.
6. Finish with two or three sentences on what was wrong and what you changed.

Conventions

• Amounts are whole cents everywhere. Dollars become cents once, in toCents(), when a product goes into the cart.
• Tests use node:test and node:assert/strict, with one test() per behavior, named after the behavior in plain words, like the existing ones.
• Keep the style of the file you're editing: two-space indents, double quotes, semicolons.
• Don't add dependencies or change package.json.