Fix a bug and prove it with a test

The steps

One step at a time. When a step’s check matches, tick it and the next one opens.

1. 01

   Step 1 of 4: See the bug

   Price 50 minutes of development and compare the result with the maths.

   TerminalCopy

   Run: $ node src/cli.js dev:0.833

   Output: Development, 0.833 h at EUR 95.00 EUR 79.13

   – What happens: 0.833 × 95 is 79.135, which should round up to EUR 79.14.

   In your own project, pick a bug you can show with one command.

   Check it worked

   The Development line ends in EUR 79.13, one cent short.

   It worked — next step Not yet
2. 02

   Step 2 of 4: Pin it with a failing test

   Save a test that states the right answer, then watch it fail.

   test/rounding.test.js · JavaScriptCopy

   import assert from 'node:assert/strict';
   import { test } from 'node:test';
   
   import { priceLine } from '../src/quote.js';
   
   // A fixed rate card, so this test keeps passing when data/rates.json changes.
   const RATES = {
     currency: 'EUR',
     vatPercent: 20,
     roles: { dev: { label: 'Development', hourly: 95 } },
   };
   
   test('rounds half a cent up when pricing part hours', () => {
     // 50 minutes is 0.833 h. 0.833 h at EUR 95.00 is EUR 79.135,
     // which rounds up to EUR 79.14, like every other half cent.
     assert.equal(priceLine({ role: 'dev', hours: 0.833 }, RATES).amountCents, 7914);
   });
   

   TerminalCopy

   Run: $ npm test

   – What happens: The summary shows fail 1. The failure reads actual: 7913 and expected: 7914.

   Check it worked

   Exactly one test fails: rounds half a cent up when pricing part hours. Every other test still passes.

   It worked — next step Not yet Back
3. 03

   Step 3 of 4: Hand Claude the failing test

   Ask Claude to find the cause and fix the code, not the test.

   Paste into Claude CodeCopy

   npm test fails in test/rounding.test.js. Find the cause in src/ and fix it. Don't change the test. When you're done, run npm test and show me the pass and fail counts.

   In order:

   1. Shell

      Claude runs the test

   2. Claude

      Reads src/quote.js

   3. Tool

      Edits the code

   4. Shell

      Runs npm test again

      Repeats until it passes

   Check it worked

   Claude changes src/quote.js, leaves test/rounding.test.js alone, and shows fail 0.

   If it didn’t work

   Claude changed the test instead

   Undo that edit, then say: “The test is right. Fix the rounding in src/quote.js.”

   It worked — next step Not yet Back
4. 04

   Step 4 of 4: Prove the fix yourself

   Run the tests and both quotes yourself, and trust the output over the summary.

   TerminalCopy

   Run: $ npm test

   Run: $ node src/cli.js dev:0.833

   Output: Development, 0.833 h at EUR 95.00 EUR 79.14

   Run: $ node src/cli.js design:10 dev:24 --discount 10

   – What happens: The total is still EUR 3,326.40.

   Check it worked

   The tests show fail 0, the Development line reads EUR 79.14, and the sample total hasn’t moved.

   It worked — finish Not yet Back