TypeScript Type Safety

Patterns for avoiding common TypeScript errors, proper type definitions, and early type checking workflows. Prevents 50-60% of compilation errors.

Overview

Problem: TypeScript errors discovered late, requiring 3-5 fix cycles

Solution: Early type checking, proper type definitions, and systematic error prevention

Impact: Reduce fix cycles from 3-5 to 1-2, save 30-60 seconds per task

Early Type Checking Workflow

Immediate Validation

Run type-check immediately after code changes:

bash

# After making ANY TypeScript changes
npx tsc --noEmit

# Or use npm script
npm run type-check

Workflow:

•Make code edit
•Run npx tsc --noEmit immediately
•Fix any errors
•Continue with next edit or testing

Timing:

•After file edits: Run check immediately
•Before browser testing: Run check first
•Before running tests: Run check first
•Before marking complete: Run check to verify

Common Error Patterns

Pattern 1: Export/Import Validation

Problem: Missing exports for imported functions

Solution: Validate exports before imports

typescript

// ❌ WRONG: Import without checking export
import { someFunction } from './utils';
// Error: Module has no exported member 'someFunction'

// ✅ CORRECT: Check export exists
// In utils.ts
export function someFunction() { }

// Then import
import { someFunction } from './utils';

Validation pattern:

bash

# Before importing, check export exists
grep -r "export.*someFunction" src/

Pattern 2: Type Conflicts

Problem: Type conflicts (class vs type names)

Solution: Use distinct names or proper type definitions

typescript

// ❌ WRONG: Class and type with same name
class Settings { }
type Settings = { }; // Error: Duplicate identifier

// ✅ CORRECT: Use distinct names
class Settings { }
type SettingsData = { };

// Or use interface
interface SettingsData { }

Pattern 3: Property Initialization

Problem: Property initialization in constructors

Solution: Initialize in constructor or mark as optional

typescript

// ❌ WRONG: Uninitialized property
class GameScene {
  private player: Player; // Error: Property 'player' has no initializer
}

// ✅ CORRECT: Initialize in constructor
class GameScene {
  private player: Player;
  
  constructor() {
    this.player = new Player();
  }
}

// Or mark as optional
class GameScene {
  private player?: Player;
}

Pattern 4: Global Type Extensions

Problem: Window extensions not typed

Solution: Declare global type extensions

typescript

// ✅ CORRECT: Declare Window extension
declare global {
  interface Window {
    __TEST__?: {
      ready: boolean;
      sceneKey: string | null;
      gameState: () => any;
      commands: {
        [key: string]: (...args: any[]) => void;
      };
    };
  }
}

Type Definition Patterns

Global Type Extensions

Window interface extensions:

typescript

declare global {
  interface Window {
    __TEST__?: {
      ready: boolean;
      sceneKey: string | null;
      seed: number | null;
      gameState: () => GameState;
      commands: TestCommands;
    };
    game?: Phaser.Game;
  }
}

Union Types

Flexible object types:

typescript

// Union type for flexible objects
type SpriteOrRectangle = Phaser.GameObjects.Sprite | Phaser.GameObjects.Rectangle;

function handleObject(obj: SpriteOrRectangle) {
  if (obj instanceof Phaser.GameObjects.Sprite) {
    // Sprite-specific code
  } else {
    // Rectangle-specific code
  }
}

Discriminated Unions

Better type safety with discriminated unions:

typescript

type GameEvent = 
  | { type: 'coin_collected'; value: number }
  | { type: 'timer_expired' }
  | { type: 'level_complete'; level: number };

function handleEvent(event: GameEvent) {
  switch (event.type) {
    case 'coin_collected':
      // event.value is available
      break;
    case 'timer_expired':
      // event.value is not available
      break;
    case 'level_complete':
      // event.level is available
      break;
  }
}

Property Initialization Patterns

Constructor Initialization

Initialize all properties in constructor:

typescript

class GameScene extends Phaser.Scene {
  private player: Player;
  private score: number;
  private timer: number;

  constructor() {
    super({ key: 'GameScene' });
    // Initialize all properties
    this.player = new Player();
    this.score = 0;
    this.timer = 60;
  }
}

Optional Properties

Mark properties as optional if not initialized:

typescript

class GameScene extends Phaser.Scene {
  private player?: Player;
  private enemies?: Phaser.Physics.Arcade.Group;

  create() {
    // Initialize when needed
    this.player = new Player();
    this.enemies = this.physics.add.group();
  }
}

Definite Assignment Assertion

Use ! when property is definitely assigned later:

typescript

class GameScene extends Phaser.Scene {
  private player!: Player; // Definite assignment assertion

  create() {
    this.player = new Player(); // Assigned in create()
  }
}

Export/Import Validation

Check Exports Before Imports

Validate exports exist:

bash

# Check if export exists
grep -r "export.*functionName" src/

# Check all exports in file
grep -r "^export" src/utils.ts

Consistent Export Patterns

Use consistent export patterns:

typescript

// ✅ CORRECT: Named export
export function utilityFunction() { }

// ✅ CORRECT: Default export
export default class MyClass { }

// ✅ CORRECT: Export list
export { function1, function2, class1 };

// ❌ WRONG: Mixing patterns inconsistently
export function func1() { }
export default function func2() { } // Confusing

Common Error Fixes

Missing Exports

Error: Module has no exported member 'X'

Fix:

•Check if export exists in source file
•Add export if missing
•Verify export name matches import

typescript

// In source file
export function myFunction() { }

// In importing file
import { myFunction } from './source';

Type Conflicts

Error: Duplicate identifier 'X'

Fix:

•Rename one of the conflicting types
•Use namespace to separate
•Use distinct names

typescript

// Fix: Use distinct names
class Settings { }
type SettingsData = { };

Property Initialization

Error: Property 'X' has no initializer

Fix:

•Initialize in constructor
•Mark as optional
•Use definite assignment assertion

typescript

// Fix 1: Initialize in constructor
constructor() {
  this.property = value;
}

// Fix 2: Mark as optional
private property?: Type;

// Fix 3: Definite assignment
private property!: Type;

Workflow Integration

Immediate Type Checking

After code changes:

bash

# 1. Make code edit
edit_file("src/scenes/GameScene.ts", ...)

# 2. Run type-check immediately
npx tsc --noEmit

# 3. Fix errors if any
# 4. Continue with next edit

Before Testing

Always check types before testing:

bash

# Before browser testing
npx tsc --noEmit && agent-browser open http://localhost:3000

# Before running tests
npx tsc --noEmit && npm test

Before Completion

Verify types before marking complete:

bash

# Final type check
npx tsc --noEmit

# If passes, proceed with completion

Best Practices

•Run type-check immediately after code changes
•Fix errors systematically (one type at a time)
•Validate exports before importing
•Use proper type definitions (global extensions, union types)
•Initialize properties in constructor or mark optional
•Use discriminated unions for better type safety
•Check types before testing or marking complete

Integration with Other Skills

•typescript-incremental-check: Uses early type checking workflow
•pre-implementation-check: Validates types before implementation
•task-verification-workflow: Includes type checking in verification

Related Skills

•typescript-incremental-check - Fast type checking patterns
•pre-implementation-check - Pre-task verification
•task-verification-workflow - Task completion verification

Remember

•Check types immediately after code changes
•Fix errors systematically before proceeding
•Validate exports before importing
•Use proper type definitions for global extensions
•Initialize properties correctly
•Prevent 50-60% of compilation errors