Should I write tests even if I'm the only developer?

Shouldn't the QA engineer write tests for me?

Codebase size

Team size

(git blame is not enough)

(complexity grows exp.)

Technology

(types? encapsulation? lint?)

Code quality

(refactoring)

$$$

(server is down = millions of $ go to the toilet)

Write test code as you write application code (TDD)

80/20 rule: which 20% of tests will cover 80% of the cases?

Think about testing when you write application code (separation of concerns, DI, service layer, pure functions)

Use tooling - test coverage

Think what code is most crucial

Top to bottom

Is being run the most

Is being used the most

Is complex (math, data processing, async)

Can have edge cases (usually unit tests)

Has a crucial business value (Auth, Payments...)

1 unit = function / class

all else is mocked

Component test?

Less mocking

Series of endpoint tests

visit a URL => interact => check the result

POST /api/register => POST /api/login => GET /api/kingdom

/login => fill the login form => check that I'm on /kingdom

Usually via external tool (Selenium, Cypress.io, Puppeteer)

Testing against real backend

qa.app.com

Acceptance tests

E2E tests

Nice, but rarely done:)