Why should you care?

Automated tests catch regressions when you refactor.

See it live — copy this example

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// sum.test.js
function sum(a, b) { return a + b; }
test("adds numbers", () => {
  expect(sum(2, 3)).toBe(5);
});

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What happened?

• test runs one case.
• expect(...).toBe checks equality.
• npm test runs the suite.

Practice next

1. Write sum test with expect().toBe.
2. npm install -D jest and add test script.
3. Run npm test on save.
4. Add test for divide by zero throwing Error.
5. Use test.each table for multiple add cases.

Remember

test / expect Run with npm test Start with pure functions

ScriptVerse utils package

Jest covers formatINR and parseAmount with 40 tests run in GitHub Actions on every PR.

Outcome: Jest catches regressions in shared ScriptVerse libraries used by multiple apps.