Why should you care?

Cheap confidence compared to manual clicking through the app.

See it live — copy this example

Paste into an HTML file or the browser console (F12). Use Run below when the live editor is available.

export function isAdult(age) {
  return age >= 18;
}
// test: expect(isAdult(20)).toBe(true); expect(isAdult(16)).toBe(false);

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Edit the code below and click Run to see the result in Toolliyo’s live editor.

What happened?

• Test pure logic with clear inputs/outputs.
• Mock fetch for modules that call APIs.

Practice next

1. Test isAdult happy path and edge cases.
2. Mock fetch for modules calling APIs.
3. Keep tests fast — no real network.
4. Write test for empty string input returning false.
5. Split one big test into three focused cases.

Remember

Small isolated tests Fast feedback Cover edge cases

ScriptVerse discount engine

Unit tests verify applyCoupon never returns negative totals for edge inputs.

Outcome: Unit tests document expected behavior and block silent logic breaks.