JestWise

A simple add() function. Looks harmless. Breaks in 3 ways.

This project demonstrates white-box unit testing using Jest — finding edge cases that black-box testing would never catch.

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The bug

function add(a, b) {
  return a + b;
}

Looks fine. But watch this:

Input	Expected	Actual
add("2", 3)	Error	"23" — string concat
add(undefined, 3)	Error	NaN
add(null, 1)	Error	1 — silently wrong

Black-box testing misses all three. White-box testing catches them.

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The fix

function add(a, b) {
  if (typeof a !== "number" || typeof b !== "number") {
    throw new Error("Both inputs must be numbers");
  }
  return a + b;
}

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Test results

7 test cases. 4 passed before fix. 7 passed after.

Test Case	Before Fix	After Fix
add(2, 3) → 5	✅ Pass	✅ Pass
add(0, 0) → 0	✅ Pass	✅ Pass
add(-5, -10) → -15	✅ Pass	✅ Pass
add(MAX_SAFE_INTEGER, 1)	✅ Pass	✅ Pass
add("2", 3) → Error	❌ Fail	✅ Pass
add(undefined, 3) → Error	❌ Fail	✅ Pass
add(null, 1) → Error	❌ Fail	✅ Pass

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Run it yourself

npm install
npm test

Generate HTML report:

npm test -- --json --outputFile=results.json
npx @mallikgalibshahriar/failsafe-report generate results.json --output report.html

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What I learned

Writing tests before fixing code forces you to understand the bug first. The three failing cases — string concat, NaN, silent null — are exactly the kind of bugs that slip into production unnoticed.

Type validation is not optional. It is the first line of defense.

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Tech

• Runtime: Node.js
• Test framework: Jest
• Report: @mallikgalibshahriar/failsafe-report
• Concepts: White-box testing, unit testing, boundary value analysis, type validation

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Author

Sirajul Islam — SQA Engineer
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