Pure functions are self-contained code units that always return the same output given the same inputs, without relying on external state or modifying anything outside their scope. They make code more predictable, testable, and reusable, while avoiding common pitfalls like modifying external variables, network requests, and using Date or Math.random().
Pure functions improve code quality by having no side effects, always returning the same output for given inputs, and not modifying external state or performing I/O operations, making them easier to reason about, test, and reuse with benefits like caching and memoization.
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