| 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.
