pure-function-pattern

To write deterministic business logic in TypeScript, enforce pure-function patterns where functions rely only on provided inputs and return consistent outputs without side effects. Inject runtime data as explicit parameters and colocate unit tests to cover all branches.

Handling errors in pure functions requires returning a stable discriminated union that explicitly expresses success or specific error states. This explicit purity contract ensures your TypeScript modules remain side-effect-free and fully testable across all logic branches.

To refactor TypeScript code and remove hidden side effects, extract core logic into pure modules with explicit input-output behavior. Inject runtime data as parameters instead of accessing external state, ensuring functions produce deterministic results for validation and transformation tasks.

Yes, the pure function pattern works for validation and data transformation tasks by enforcing explicit input-output behavior. Functions process injected runtime data deterministically, returning well-defined error unions for failures, making the logic highly testable and reusable.

You should use discriminated unions for error handling in functional TypeScript because they provide an explicit purity contract for success and failure states. This approach avoids side effects from thrown exceptions, ensuring deterministic outputs that are easy to unit test.

You should not use pure functions when tasks inherently require immediate side effects, such as direct database writes or network calls. Pure functions are designed for deterministic validation, transformation, and calculation, where runtime data must be injected as explicit parameters.