Unit Test

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Add unit tests for price calculation: 100 gets 20 off, 99 does not, and external payment must not participate.

A unit test tests one independently verifiable function, component, or small module.Pass a fixed price to a tax function and assert the amount, or check that a disabled button does not submit. It isolates outside services so a failure points to a small responsibility. A full login with browser, API, and real database is not a unit test.

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Test Case

Unit Test

How a price rule is checked without connecting payment

price.test.jsprice calculation only

1 import { calculateTotal } from "../price";
2
3 test("adds 8% tax", () => {
4   const result = calculateTotal(100, 0.08);
5   expect(result).toBe(108);
6 });

$ npm test price.test.jswaitingPayment, login, database: not started

External services never enter this workspace, so a failure points more directly to the price function.

Why unit tests keep scope small

Check one small responsibility: Boundary inputs such as 100 gets 20 off and 99 gets none directly test the price rule. On failure, look at the calculation rather than guessing about network or database.

Keep outside dependencies controllable: Payment, network, or current time can be isolated or replaced when they make results slow or unstable. Unit tests are fast and precise, but do not prove systems work together.

Quick checkChoose the best answer

After changing a discount rule, which is best as a unit test?

Pass 99 and 100 to the discount function and assert no discount versus 20 offComplete a real payment in a browser and wait for a bank callbackAsk a colleague whether the checkout page feels right

You can say this to an AI Agent

Add unit tests for the discount calculation: 99 gets no discount and 100 gets 20 off. Do not call real payment or network services; after the change, list both inputs and actual outputs.

Learn next

1Test Double→2Integration Test→