TypeScript Pro

Senior-level TypeScript expertise for production projects. Focuses on strict type safety, zero-any tolerance, and TypeScript's full type system capabilities.

When Invoked

1. Review tsconfig.json and eslint.config.js for project conventions
2. For build system setup, invoke the just-pro skill (covers just vs make)
3. Apply type-first development and established project patterns

Core Standards

Non-Negotiable:

• Strict mode enabled with all compiler flags
• NO explicit or implicit any - use unknown and narrow
• NO type assertions to circumvent the type system (as any, as unknown as T)
• NO dangling promises - await, return, or void explicitly
• All exported functions have explicit return types
• ESLint strict-type-checked passes with project configuration
• Table-driven tests for multiple cases

Foundational Principles:

• Single Responsibility: One module = one purpose, one function = one job
• No God Objects: Split large classes/objects; if it has 10+ methods or properties, decompose
• Dependency Injection: Pass dependencies via constructor/params, don't instantiate internally
• Small Interfaces: Prefer many small types over few large ones; compose with intersection types
• Composition over Inheritance: Use object composition and mixins, not deep class hierarchies

Project Setup (TypeScript 5.5+)

Version Management with mise

mise manages language runtimes per-project. Ensures all contributors use the same Node version—no "works on my machine" issues.

# Install mise (once)
curl https://mise.run | sh

# In project root
mise use node@22

# Creates .mise.toml — commit it
# Team members just run: mise install

New Project Quick Start

# Initialize
npm init -y
npm install -D typescript typescript-eslint @eslint-community/eslint-plugin-eslint-comments vitest husky

# Add scripts to package.json:
npm pkg set scripts.typecheck="tsc --noEmit"
npm pkg set scripts.lint="eslint src/"
npm pkg set scripts.test="vitest run"
npm pkg set scripts.check="npm run typecheck && npm run lint && npm run test"
npm pkg set scripts.prepare="husky"

# Set up pre-commit hook
npm run prepare
echo "npm run check" > .husky/pre-commit
chmod +x .husky/pre-commit

# Verify
npm run check

Required Config Files: Copy references/gitignore → .gitignore, then create tsconfig.json and eslint.config.js per the templates below.

Developer Onboarding

git clone <repo> && cd <repo>
just setup               # Runs mise trust/install + npm ci
just check               # Verify everything works

Or manually:

mise trust && mise install  # Get pinned Node version
npm ci                      # Get dependencies

Why strict configs? Type errors caught at compile time are 10x cheaper than runtime bugs. Strict linting prevents any from leaking through the codebase.

Build System

Invoke the just-pro skill for build system setup. It covers:

• Simple repos vs monorepos
• Hierarchical justfile modules
• TypeScript-specific templates (references/package-ts.just)

Alternative: Use npm scripts directly if just is unavailable.

Quality Assurance

Auto-Fix First - Always try auto-fix before manual fixes:

npx eslint src/ --fix        # Fixes style, imports, etc.
npx tsc --noEmit             # Type check without emit

Verification:

npm run check                # typecheck + lint + test

Pre-commit Hook (automatic if husky configured):

• Runs npm run check before every commit
• Blocks commits with type errors, lint violations, or failing tests

Linting Configuration

eslint.config.js Template

IMPORTANT: When creating a new project, use this complete template. Do not omit rules.

import tseslint from 'typescript-eslint';
import eslintComments from '@eslint-community/eslint-plugin-eslint-comments';

export default tseslint.config(
  ...tseslint.configs.strictTypeChecked,
  ...tseslint.configs.stylisticTypeChecked,
  {
    files: ['src/**/*.ts', 'src/**/*.tsx'],
    languageOptions: {
      parserOptions: {
        projectService: true,
        tsconfigRootDir: import.meta.dirname,
      },
    },
    plugins: {
      '@eslint-community/eslint-comments': eslintComments,
    },
    rules: {
      // === TYPE SAFETY (non-negotiable) ===
      '@typescript-eslint/no-explicit-any': 'error',
      '@typescript-eslint/no-unsafe-argument': 'error',
      '@typescript-eslint/no-unsafe-assignment': 'error',
      '@typescript-eslint/no-unsafe-call': 'error',
      '@typescript-eslint/no-unsafe-member-access': 'error',
      '@typescript-eslint/no-unsafe-return': 'error',
      '@typescript-eslint/no-unsafe-type-assertion': 'error',
      '@typescript-eslint/no-non-null-assertion': 'error',

      // === PROMISES ===
      '@typescript-eslint/no-floating-promises': ['error', { ignoreVoid: true, ignoreIIFE: true }],
      '@typescript-eslint/no-misused-promises': 'error',
      '@typescript-eslint/require-await': 'error',
      '@typescript-eslint/promise-function-async': 'error',

      // === COMPLEXITY LIMITS (enforced) ===
      'complexity': ['error', { max: 10 }],
      'max-depth': ['error', 4],
      'max-lines-per-function': ['error', { max: 60, skipBlankLines: true, skipComments: true }],
      'max-lines': ['error', { max: 400, skipBlankLines: true, skipComments: true }],
      'max-params': ['error', 4],

      // === BLOCK DISABLING CRITICAL RULES ===
      '@eslint-community/eslint-comments/no-restricted-disable': ['error',
        '@typescript-eslint/no-explicit-any',
        '@typescript-eslint/no-unsafe-assignment',
        '@typescript-eslint/no-unsafe-argument',
        '@typescript-eslint/no-floating-promises',
        'complexity', 'max-lines-per-function', 'max-lines',
      ],
      '@eslint-community/eslint-comments/require-description': ['error', { ignore: ['eslint-enable'] }],

      // === COMMENTS ===
      '@typescript-eslint/ban-ts-comment': ['error', {
        'ts-expect-error': 'allow-with-description',
        'ts-ignore': true,
        'ts-nocheck': true,
        minimumDescriptionLength: 10,
      }],

      // === CONSISTENCY ===
      '@typescript-eslint/explicit-module-boundary-types': 'error',
      '@typescript-eslint/consistent-type-imports': ['error', { prefer: 'type-imports' }],
      '@typescript-eslint/no-unused-vars': ['error', { argsIgnorePattern: '^_', varsIgnorePattern: '^_' }],
    },
  },
  // Relax for tests
  {
    files: ['**/*.test.ts', '**/*.spec.ts'],
    rules: {
      '@typescript-eslint/no-explicit-any': 'off',
      '@typescript-eslint/no-unsafe-assignment': 'off',
      'max-lines-per-function': 'off',
      'max-lines': 'off',
      'complexity': 'off',
      '@eslint-community/eslint-comments/no-restricted-disable': 'off',
    },
  },
  { ignores: ['dist/', 'node_modules/', 'coverage/', '*.js', '*.cjs', '*.mjs'] },
);

Enforced Limits Summary

Critical rules cannot be disabled via eslint-disable comments - the config blocks it.

Quick Reference

Type Safety Patterns

Error Handling

Type System Techniques

• Generic constraints and variance
• Conditional types with infer
• Mapped types with modifiers
• Template literal types
• Index access types (T[K])

Project Organization

project/
├── src/
│   ├── index.ts          # Entry point / exports
│   ├── types/            # Shared type definitions
│   └── lib/              # Implementation
├── tsconfig.json
├── eslint.config.js
├── package.json
└── justfile

Rules: One module = one purpose. Use barrel exports sparingly. Avoid circular dependencies.

Anti-Patterns

• as any or as unknown as T type assertions
• @ts-ignore instead of @ts-expect-error with reason
• Disabling strict checks to fix errors
• Using eslint-disable to bypass type safety or complexity rules (blocked by config)
• Implicit any in function parameters
• Dangling promises without await/void
• Over-complicated generic signatures
• Non-null assertions (the x! operator) instead of proper narrowing
• Truthy/falsy checks on non-booleans
• Functions over 60 lines or files over 400 lines (refactor instead)
• God classes/objects with 10+ methods or properties
• Deep inheritance hierarchies (prefer composition)
• Barrel files that re-export everything (causes circular deps)

Framework-Specific

React 19+: Explicit props typing (avoid FC), use satisfies for configs.

Next.js: Type server components, use Metadata types, type API routes.

Express/Fastify: Type request handlers, use generic route parameters.

See references/integration.md for detailed framework patterns.

AI Agent Guidelines

Before writing code:

1. Read tsconfig.json for compiler options and strict settings
2. Check eslint.config.js for project-specific lint rules
3. Identify existing type patterns in the codebase to follow

When writing code:

1. Start with type definitions before implementation
2. Use unknown and narrow with type guards - never any
3. Handle all promise returns explicitly
4. Add explicit return types to exported functions

Before committing:

1. Run npm run check (standard for single packages)
2. For monorepos at repo root: just check or turbo run check
3. Fallback: npx eslint src/ --fix && npx tsc --noEmit && npm test