code-review

name	code-review
description	Reviews TypeScript code using functional programming principles. Use when reviewing PRs or checking code quality.

TypeScript FP Code Review Guide

When you review TypeScript code, check it using a functional programming (FP) mindset. Use these simple guidelines:

1. Avoid Changing State (Immutability)

• Use const and readonly: Do not reassign variables. Mark array and object inputs as readonly (for example, readonly string[] or Readonly<Todo>).
• Do not mutate data: Do not use mutating methods like .push(), .pop(), or directly changing object properties. Instead, return new copies using the spread operator (...) or non-mutating methods like .map(), .filter(), and .concat().

2. Write Pure Functions

• No side effects: A function should only calculate its output based on its inputs. It must not change global variables, write to files, or modify external state directly.
• Predictable behavior: Running the same function with the same inputs must always return the same result.

3. Keep Types Safe and Strict

• No any: Avoid using any. Use unknown or specific type unions instead.
• Avoid type assertions: Try not to use as SomeType. Let TypeScript infer types, or define them explicitly on functions and variables.
• Handle null and undefined: Make sure all optional properties and nullish values are checked safely (using optional chaining ?. or nullish coalescing ??).

4. Prefer Expressions Over Statements

• No imperative loops: Avoid for and while loops. Use array methods like .map(), .filter(), and .reduce() to transform data.
• Avoid reassigning variables: Instead of using let with if/else statements to set a value, use ternary operators (cond ? a : b) or helper functions.

5. Handle Errors Safely

• Avoid throwing exceptions: Instead of throwing exceptions that crash the program, prefer returning a status object (like { success: true, data: T } | { success: false, error: Error }).

---

How to Give Feedback

• Show the code change you suggest.
• Explain the functional programming benefit (e.g., "This avoids side effects" or "This keeps types safe").
• Keep your tone helpful, clear, and simple.