Animation: a failing price of 58230.76 is halved again and again: 29115.38, 909.86, 7.11, 0.45, until it reaches 0.01, the smallest input that still fails.

# Property-Based Testing Course: [Testing & Verification](https://www.devtutorials.ai/learn/testing-verification) · Section: Verifying AI Changes Source: https://www.devtutorials.ai/learn/testing-verification/verifying-ai-changes/property-based-testing > Write rules that must hold for every input, let fast-check try hundreds of random inputs, and watch shrinking find the smallest one that breaks the AI's code. Bounds, sums, round trips and no-change properties. ## What a Property Is So far, every test checked one example: "10% off ₹500 is ₹450". Examples are good. But you only test the inputs you thought of. AI code often breaks on the inputs nobody thought of. **Property** = a rule that must be true for **every** input, not just one. For example: - A discount never makes the price negative. - A discount never makes the price bigger. - The shares of a split bill always add up to the total. **Property-based testing** = a tool makes hundreds of random inputs and checks the rule on each one. **Simple lesson:** an example checks one input. A property checks a rule on hundreds. ### A Real-World Example Think about testing a new pressure cooker. - An example test: cook 1 cup of rice once. It works. - A property test: "the lid never opens while there is pressure". You try it with rice, dal, water, a full pot, a nearly empty pot, many times. - One day, with a nearly empty pot, the lid opens. You found a case nobody planned for. Property tests are the "many times, many ways" check. ## What You Will Learn In this lesson, you will learn: 1. How to write a property with **fast-check**. 2. How **shrinking** finds the smallest input that breaks a rule. 3. Four kinds of good properties: bounds, sums, round trips and "no change". 4. How properties and examples work together. 5. How to tell a real property from one that just copies the code. ## Your First Property ### The Parts of a Property Test A property test has three parts: 1. **Generators** say what random inputs to make. For example, `fc.integer({ min: 0, max: 100 })`. 2. **The rule** is a function that returns `true` when the rule holds. 3. **`fc.assert`** runs the rule on many inputs. It fails if any input breaks the rule. Press **▶ Run tests**. Green. - **What happened:** the tool made 100 random prices and percents, and the rule held for all of them. - **Now try:** open `src/discount.ts` and change `100 - percent` to `100 + percent`. Run again. Red. **This lab uses a small copy of fast-check.** - Real projects install `fast-check` from npm. The code you write is the same. - `fc.float2` is a lab helper for money with 2 decimals, like 12.34. In real fast-check, you can make it with `fc.integer(...).map((p) => p / 100)`. - Runs use a fixed **seed**, so you get the same inputs every time. **Simple lesson:** generators, a rule, and `fc.assert`. That is a property test. ## Shrinking ### The Smallest Failing Input When a property fails, the first failing input is often big and messy, like ₹58,230.76. **Shrinking** = the tool tries smaller and simpler inputs, and keeps the smallest one that still fails.

Shrinking turns a messy failure into the simplest one you can debug.

Press **▶ Run tests**. - **The Tests tab:** red. Read the message: "Counterexample: [0.01]", then every shrink step. - **Counterexample** = an input that breaks the rule. - **Why it fails:** the code rounds to whole rupees. ₹0.01 becomes ₹0. - **Fix it:** change the last line to `Math.round(price * (100 - percent)) / 100`. Run again. Green. **Simple lesson:** shrinking hands you the simplest broken case. Start debugging there. ## Kinds of Good Properties Good properties describe the **result**, without repeating the formula. Here are four kinds that work on most code. ### Bounds **Bounds** = the result always stays inside a range. - A discount is between 0 and the price. - A rating average is between 1 and 5. ### Sums and Counts The parts add up to the whole, and nothing is lost. Press **▶ Run tests**. - **The Tests tab:** red. "Counterexample: [0.01, 2]". ₹0.01 split between 2 people loses the paisa. - **Why:** this version of `splitBill` drops the extra paise. - **A person might never try ₹0.01 for 2 people.** The tool tried it for you. ### Round Trips **Round trip** = going there and back gives the same value. For example, format a number as text, then read it back. Press **▶ Run tests**. Green. - **Now try:** change `toFixed(2)` to `toFixed(1)` in `src/money.ts`. Run again. - **The Tests tab:** red, with a small counterexample like ₹0.01. ### No Change Some inputs should change nothing. - 0% off keeps the price. - Sorting a list that is already sorted gives the same list. **Do not copy the code into the property.** - `applyDiscount(p, x) === Math.round(p * (100 - x)) / 100` repeats the formula. - If the formula is wrong, the property is wrong in the same way. - Describe what must be true of the result instead: bounds, sums, round trips, no change. **Simple lesson:** a good property says what the result must look like, not how to compute it. ## Properties Plus Examples Properties and examples work best together. - **Examples** pin exact answers: "10% off ₹500 is ₹450". - **Properties** guard the rules across many inputs, including odd ones. Press **▶ Run tests**, then **🐞 Plant bugs**. - **The Bug check tab:** one example plus one property catch all five discount bugs. - **Why:** the example catches wrong maths. The 0% property catches special cases and rounding. **Where properties help most with AI code:** - Money and rounding. - Lists: sorting, splitting, merging. - Text: format, parse, trim. - Any rule with "never", "always" or "adds up to". **Simple lesson:** examples check what you know. Properties find what you did not think of. ## TL;DR 1. **Property** = a rule that must hold for every input. 2. **Generator** = says what random inputs to make, like `fc.integer({ min: 0, max: 100 })`. 3. **`fc.assert(fc.property(...))`** = runs the rule on many random inputs. 4. **Counterexample** = an input that breaks the rule. 5. **Shrinking** = finding the smallest input that still breaks it. 6. **Good kinds of property:** - bounds - sums and counts - round trips - no change 7. **Do not copy the formula** into the property. ## Common mistakes | Mistake | Wrong idea | Correct idea | What goes wrong | |---|---|---|---| | Copying the formula | "The property is just the code again." | Describe the result: bounds, sums, round trips. | A wrong formula passes its own check. | | Rule too weak | "It returns a number." | Pick a rule a bug would break. | Every bug still returns a number. | | Rule too strict | "All shares are equal." | Check what is really always true. | The test fails on correct code. | | Ignoring the counterexample | "It is a random fluke." | It is a real input that breaks the rule. | The bug ships for that input. | | Only properties | "Properties replace examples." | Use both. | Exact answers stop being checked. | ## Check Your Understanding ## Exercises ## What to Remember - A property is a rule for every input. The tool makes the inputs. - Shrinking gives you the smallest failing case. - Good properties: bounds, sums and counts, round trips, no change. - Never copy the formula into the property. - Use properties and examples together. ## Further Reading - [fast-check documentation](https://fast-check.dev/docs/introduction/): the real tool, with every generator. - [Vitest getting started](https://vitest.dev/guide/): run property tests with your other tests. Next: **Integration and Contract Tests**. A mocked unit test stays green while the real API changes. You will add a contract test that catches it.