Typescript Clean Code

Clean TypeScript: Complete Reference

Enforces all Clean Code principles from Robert C. Martin's Chapter 17, adapted for TypeScript.

Comments (C1-C5)

• C1: No metadata in comments (use Git)
• C2: Delete obsolete comments immediately
• C3: No redundant comments
• C4: Write comments well if you must
• C5: Never commit commented-out code

Environment (E1-E2)

• E1: One command to build (npm install or pnpm install)
• E2: One command to test (npm test or pnpm test)

Functions (F1-F4)

• F1: Maximum 3 arguments (use interfaces for more)
• F2: No output arguments (return values)
• F3: No flag arguments (split functions)
• F4: Delete dead functions

General (G1-G36)

• G1: One language per file
• G2: Implement expected behavior
• G3: Handle boundary conditions
• G4: Don't override safeties
• G5: DRY - no duplication
• G6: Consistent abstraction levels
• G7: Base classes don't know children
• G8: Minimize public interface
• G9: Delete dead code
• G10: Variables near usage
• G11: Be consistent
• G12: Remove clutter
• G13: No artificial coupling
• G14: No feature envy
• G15: No selector arguments
• G16: No obscured intent
• G17: Code where expected
• G18: Prefer instance methods
• G19: Use explanatory variables
• G20: Function names say what they do
• G21: Understand the algorithm
• G22: Make dependencies physical
• G23: Prefer polymorphism to if/else
• G24: Follow conventions (ESLint, Prettier)
• G25: Named constants, not magic numbers
• G26: Be precise
• G27: Structure over convention
• G28: Encapsulate conditionals
• G29: Avoid negative conditionals
• G30: Functions do one thing
• G31: Make temporal coupling explicit
• G32: Don't be arbitrary
• G33: Encapsulate boundary conditions
• G34: One abstraction level per function
• G35: Config at high levels
• G36: Law of Demeter (no train wrecks)

TypeScript-Specific (TS1-TS10)

These adapt the Java-specific rules (J1-J3) to TypeScript conventions:

• TS1: Use explicit types on public interfaces — TypeScript's equivalent of Java's static typing
• TS2: Use Enums or const objects, not magic constants — same principle as J3
• TS3: Prefer named exports over default exports — clearer imports and better refactoring
• TS4: Always add explicit types everywhere — never rely on type inference for variables, parameters, return types, or callbacks
• TS5: Return statement formatting — ONLY return; (no value) inline, return value always with braces
• TS6: Interfaces in separate files — never mix interfaces with functions in the same file
• TS7: No underscore for unused parameters — use descriptive name or omit entirely
• TS8: Use undefined instead of null — consistent absence representation
• TS9: Never use any — use unknown, generics, or proper types
• TS10: No destructuring — access properties directly via dot notation
• TS11: No single-letter variables — use descriptive names in loops, map, filter, reduce

// TS5: Empty return (guard clause) — single line, no braces
// Bad
if (data === undefined) {
    return;
}

// Good - ONLY when return has NO value
if (data === undefined) return;

// TS5: Return with value — ALWAYS use braces (NEVER inline!)
// Bad - returns with values should NEVER be inline
if (data === undefined) return false;
if (items.length === 0) return [];
if (foundIds.has(id)) return false;

// Good - returns with values ALWAYS get braces
if (data === undefined) {
    return false;
}
if (items.length === 0) {
    return [];
}
if (foundIds.has(id)) {
    return false;
}

// TS6: Interfaces in separate files
// Bad - mixing interfaces with functions
// file: userService.ts
interface User {
    id: string;
    name: string;
}
const createUser = (name: string): User => { /* ... */ };

// Good - separate files
// file: types/user.ts
interface User {
    id: string;
    name: string;
}
// file: userService.ts
import { User } from './types/user';
const createUser = (name: string): User => { /* ... */ };

// TS7: No underscore for unused parameters
// Bad
const handleEvent: EventHandler = async (_, context): Promise<void> => {
    await processContext(context);
};

// Good - use descriptive name even if unused
const handleEvent: EventHandler = async (event, context): Promise<void> => {
    await processContext(context);
};

// TS8: Use undefined instead of null
// Bad
const findUser = (id: string): User | null => {
    return users.get(id) ?? null;
};

// Good
const findUser = (id: string): User | undefined => {
    return users.get(id);
};

// TS9: Never use any
// Bad
const processData = (data: any): any => { /* ... */ };

// Good - use unknown and narrow, or generics
const processData = <T>(data: T): T => { /* ... */ };
const parseInput = (data: unknown): ParsedData => {
    if (!isValidInput(data)) {
        throw new Error("Invalid input");
    }
    return data as ParsedData;
};

// TS10: No destructuring — use dot notation
// Bad
const { x, y, z } = vector;
const distance: number = Math.sqrt(x * x + y * y + z * z);

const { name, email, age } = user;
console.log(name, email, age);

// Good - access properties directly
const distance: number = Math.sqrt(
    vector.x * vector.x + vector.y * vector.y + vector.z * vector.z
);

console.log(user.name, user.email, user.age);

// TS11: No single-letter variables — use descriptive names
// Bad
users.map((u) => u.name);
items.filter((i) => i.active);
numbers.reduce((a, b) => a + b, 0);
for (const x of events) { /* ... */ }

// Good
users.map((user: User): string => user.name);
items.filter((item: Item): boolean => item.active);
numbers.reduce((sum: number, value: number): number => sum + value, 0);
for (const event of events) { /* ... */ }

Names (N1-N7)

• N1: Choose descriptive names
• N2: Right abstraction level
• N3: Use standard nomenclature
• N4: Unambiguous names
• N5: Name length matches scope
• N6: No encodings
• N7: Names describe side effects

Tests (T1-T9)

• T1: Test everything that could break
• T2: Use coverage tools
• T3: Don't skip trivial tests
• T4: Ignored test = ambiguity question
• T5: Test boundary conditions
• T6: Exhaustively test near bugs
• T7: Look for patterns in failures
• T8: Check coverage when debugging
• T9: Tests must be fast (< 100ms each)

Quick Reference Table

Anti-Patterns (Don't → Do)

AI Behavior

When reviewing code, identify violations by rule number (e.g., "G5 violation: duplicated logic"). When fixing or editing code, report what was fixed (e.g., "Fixed: extracted magic number to SECONDS_PER_DAY (G25)").