The original title is "From Scratch to a Million Users — Part 2: What Breaks After Launch"

Short summary

Scaling production apps requires the right testing strategy: write fast unit tests for pure logic, more integration tests for features that connect to real data, and use E2E tests only for critical paths like checkout. Test what users see (not internal state) using React Testing Library to enable safe refactoring.

• •Test pyramid: many unit tests, fewer integration tests, few E2E tests for critical paths
• •Test user behavior, not implementation details—React Testing Library shows how
• •Avoid slow, flaky E2E suites; focus on money paths and feature-level integration

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