Functional Programming Patterns for Business Logic Optimization

This article introduces several core concepts in functional programming and practical solutions for optimizing business code using functional programming techniques. It addresses the common problem of complex business logic with multiple branching paths and long execution flows that often lead to what developers call "legacy code."

The article begins by discussing the challenges of handling complex business logic with nested if-else statements and switch cases, especially as requirements evolve over time. It then introduces the concept of function composition (compose) as a solution, explaining how it breaks down logic into smaller, reusable functions that can be combined to process data through a pipeline.

The article provides code examples showing how to implement basic compose functionality, including handling both synchronous and asynchronous functions. It demonstrates how to extend compose to support async/await patterns, allowing for the combination of functions that may involve API calls or other asynchronous operations.

A key practical solution presented is the koa-compose pattern, which adapts the middleware pattern from the Koa framework. This approach allows for conditional execution of middleware functions, where each middleware can decide whether to proceed to the next step or terminate the process early. The article explains how to implement this pattern with context objects that track execution status and provide data sharing between middleware.

The article also covers middleware design patterns, including simple middleware for checking user login status, parameterized middleware that accept configuration options, and status tracking mechanisms to determine whether all middleware executed successfully or if the process terminated early. It introduces context status enums (pending, fulfilled, rejected) to track execution flow and provides examples of utility middleware for checking completion status.

Throughout the article, the author emphasizes how these functional programming patterns improve code readability, maintainability, and extensibility compared to traditional imperative approaches with nested conditionals.