TypeScript Conventions

This file is self-contained for day-to-day use. Deeper reference (needs the full harness checkout): languages/typescript/CONVENTIONS.md (full examples including generics and import grouping) and frameworks/react/CONVENTIONS.md (React/JSX-specific additions).

Naming

• camelCase — functions, variables, method names. PascalCase — classes, interfaces, type aliases, enum names. UPPER_SNAKE_CASE — module-level constants.
• Enum members: PascalCase.
• Generic type params: T/U for simple one-liners; descriptive names (TEntity, TResponse) for complex or nested generics.

Imports & module structure

Group: external packages → internal modules → type-only imports. Separate each group with a blank line. Prefer import type for type-only imports (helps tools that strip types without full parsing).

import fs from 'fs';
import express from 'express';

import { UserRepository } from './repositories/UserRepository';

import type { User } from './types';

Private members: # over _prefix

Prefer native # private fields (ES2022+/TS 3.8+) in new code — real runtime privacy, not a convention that's still readable via ["_name"]. Don't rewrite working _prefix code on sight; it's not deprecated.

class TokenStore {
  #tokens: Map<string, string> = new Map();   // true runtime privacy
  #rotate(): void { /* ... */ }
}

Null vs. Undefined

Pick based on what absence means, then apply it consistently:

• undefined (via ?) — "not provided / not yet set."
• null — an explicit "no value" the code sets deliberately (nullable column, "user cleared this field").
• Do not mix both for the same kind of absence in one module.

Pitfalls to catch in review

// WRONG: async function catches error, logs it, then resolves — hides failure
async function save(data: Data): Promise<void> {
  try {
    await db.write(data);
  } catch (err) {
    logger.error('save failed', err);
    // caller sees a successful promise despite the failure
  }
}

// RIGHT: rethrow after logging so the caller can handle the failure
async function save(data: Data): Promise<void> {
  try {
    await db.write(data);
  } catch (err) {
    logger.error('save failed', err);
    throw err;  // caller knows this failed
  }
}

// Type-cast escape hatch silences the compiler, not the bug
const result = someValue as SomeType;  // risky without a runtime check
// Prefer a type guard:
function isSomeType(v: unknown): v is SomeType { /* ... */ }

// Non-null assertion hides null/undefined bugs
const name = user!.profile!.name;   // crashes at runtime if null
// RIGHT: optional chaining + fallback
const name = user?.profile?.name ?? 'Unknown';

Testing

tests/ or __tests__/ directories, *.test.ts / *.spec.ts files. Use vitest for Vite-based projects, jest for Node/Next. See patterns/testing/TDD.md for the broader methodology — this skill covers TypeScript-specific structure only.

Formatting & tooling

Defer to the project's tsconfig.json and eslint.config.* — don't impose defaults over an existing configured setup. If unconfigured: strict: true in tsconfig, prettier for formatting (2-space indent, single quotes), eslint with @typescript-eslint/recommended.