How Test-Driven Development's Red-Green-Refactor Cycle Builds Robust Code

Test-Driven Development (TDD) is a coding practice where developers write failing tests before writing any implementation code, following a Red-Green-Refactor cycle. The approach shifts the core question from 'how do I build this?' to 'how would I know this works?', forcing developers to define inputs, outputs, and edge cases upfront. Writing code without tests first often leads to missed requirements that only surface later as bugs, as illustrated by a Fibonacci function that failed on negative inputs after being shipped. TDD addresses this by treating each test as a concrete specification, with the passing test acting as a signal that a given piece of behavior is complete. The result is a reliable test suite that acts as a safety net, reducing the risk of unintended side effects when making future changes.