THE FREE ENGINEERING LAB

Good engineering
starts with evidence.

Make a request. Find the bug. Prove the fix.
Learn the testing habits that make software more dependable.

No payment required Work at your own pace About 40 minutes

API FOUNDATIONS

…/ 03 lessons

Your next useful skill starts here.

LESSON 01 / 03 · 10 MIN

Make the contract testable

Turn an API requirement into explicit, executable checks.

Imagine you are shipping a checkout API for a community learning project. A successful request is only half the story. Your service must also explain why a bad request was rejected, so the application can help the person using it.

Our quote API accepts a whole-number quantity from 1 to 10. Each item costs 2,500 cents. A quantity of 2 therefore produces a total of 5,000 cents. A fractional or missing quantity returns status 400 and the error code INVALID_QUANTITY. The status describes the broad outcome; a stable error code explains the specific problem. Testing both catches failures that a status-only check would miss.

Read the API contract POST /sandbox/quote

Teaching simulator: requests run entirely in your browser. They do not contact a server, create orders, or take payments.

quantity: Required integer, 1–10 inclusive. Missing, fractional, or below 1: 400 INVALID_QUANTITY. Above 10: 400 QUANTITY_LIMIT.
coupon: Optional. Omit or leave empty for no discount. LEARN10 gives 10% off and ignores letter case. Any other value: 400 INVALID_COUPON.
Success: 200 with currency, quantity, unitPrice, subtotal, discount, and total. Money uses USD cents. Unit price is 2500; total = subtotal − discount.
Validation order: Quantity type and minimum, then maximum, then coupon. Additional request fields are ignored.
Builds: Reference follows this contract. Candidate intentionally accepts 0, rejects 10, and checks coupon letters case sensitively.

YOUR CHALLENGE

The fractional-quantity test has the wrong expectation. Use the contract to repair it, then run all three cases against the reference build.

Send quantity 2 in the request explorer and inspect the response.
Try quantity 1.5. Compare the actual status and error code with the contract.
In the JSON suite, change the fractional test to expect status 400 and error code INVALID_QUANTITY. Keep the normal and missing-quantity checks.
Run the suite. A passing score is useful only when every required piece of evidence is present.

Your workbench

LOCAL SIMULATOR

Select a buildMatches the documented API contract.

01 Explore a requestTry an input. Inspect the response.

POST /sandbox/quote

quantity JSON value · empty means missingcoupon optional

RESPONSE

// Send a request to inspect
// its actual response here.

// Nothing leaves your browser.

Write your assertions

JSON data. Match the status and the body fields that matter.

JSON assertion suite for Make the contract testable

1–12 cases · status is required · body fields are partial matches

03 / REVIEW THE EVIDENCE

What did your tests prove?

Run your suite to see the result of every assertion.

The checklist below measures coverage as well as correctness.

Required evidence

Check quantity 2 returns status 200 and total 5000

Check quantity 1.5 returns INVALID_QUANTITY with status 400

Check a missing quantity returns INVALID_QUANTITY with status 400

Run a reference suite with every assertion passing

TAKE THIS WITH YOU

Write the expected behavior from a requirement before looking at the implementation. Copying every observed response into a test can preserve a bug.

Complete the evidence checklist to finish this lesson.

Loading Engineer Africa…

Good engineeringstarts with evidence.

Make the contract testable

Your workbench

Write your assertions

What did your tests prove?

Required evidence

Good engineeringstarts with evidence.

Make the contract testable

Your workbench

Write your assertions

What did your tests prove?

Required evidence

Good engineering
starts with evidence.

Good engineering
starts with evidence.