Free course on freeCodeCamp
freeCodeCamp's Quality Assurance with JavaScript certification teaches testing: how to write unit tests, integration tests, and end-to-end tests for JavaScript applications. You use Chai (assertions), Mocha (test runner), and Selenium (browser automation). The course is completely free on freeCodeCamp.org, and you earn a certificate by completing five projects. After finishing, you can write automated tests and understand TDD (Test-Driven Development). The main limitation: testing culture varies by team. Some employers care deeply about it; others treat it as optional.
A study of three Microsoft teams and one IBM team found TDD projects had 40-90% lower pre-release defect density than comparable non-TDD projects (source: Nagappan, Maximilien, Bhat, and Williams, "Realizing Quality Improvement Through Test-Driven Development," Empirical Software Engineering, 2008).
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