TypeScript Implementation Skill

Quick Start

Basic Types

// Primitive types
let name: string = "Angular";
let version: number = 18;
let active: boolean = true;

// Union types
let id: string | number;

// Type aliases
type User = {
  name: string;
  age: number;
};

Interfaces and Generics

interface Component {
  render(): string;
}

// Generic interface
interface Repository<T> {
  getAll(): T[];
  getById(id: number): T | undefined;
}

class UserRepository implements Repository<User> {
  getAll(): User[] { /* ... */ }
  getById(id: number): User | undefined { /* ... */ }
}

Decorators (Essential for Angular)

// Class decorator
function Component(config: any) {
  return function(target: any) {
    target.prototype.selector = config.selector;
  };
}

// Method decorator
function Log(target: any, propertyKey: string, descriptor: PropertyDescriptor) {
  const originalMethod = descriptor.value;
  descriptor.value = function(...args: any[]) {
    console.log(`Calling ${propertyKey} with:`, args);
    return originalMethod.apply(this, args);
  };
  return descriptor;
}

// Parameter decorator
function Required(target: any, propertyKey: string, parameterIndex: number) {
  // Validation logic
}

Essential Concepts

Advanced Types

Utility Types:

• Partial<T> - Make all properties optional
• Required<T> - Make all properties required
• Readonly<T> - Make all properties readonly
• Record<K, T> - Object with specific key types
• Pick<T, K> - Select specific properties
• Omit<T, K> - Exclude specific properties

Conditional Types:

type IsString<T> = T extends string ? true : false;
type A = IsString<"hello">; // true
type B = IsString<number>; // false

Mapped Types:

type Getters<T> = {
  [K in keyof T as `get${Capitalize<string & K>}`]: () => T[K];
};

Generic Constraints

// Extend constraint
function processUser<T extends User>(user: T) {
  console.log(user.name); // OK, T has 'name'
}

// Keyof constraint
function getProperty<T, K extends keyof T>(obj: T, key: K): T[K] {
  return obj[key];
}

Advanced Features

Type Guards

// Type predicate
function isUser(value: unknown): value is User {
  return typeof value === 'object' && value !== null && 'name' in value;
}

// Discriminated unions
type Shape =
  | { kind: 'circle'; radius: number }
  | { kind: 'square'; side: number };

function getArea(shape: Shape) {
  switch (shape.kind) {
    case 'circle': return Math.PI * shape.radius ** 2;
    case 'square': return shape.side ** 2;
  }
}

Module System

// Export
export interface User { name: string; }
export const API_URL = 'https://api.example.com';

// Import
import { User, API_URL } from './types';
import * as Types from './types'; // Namespace import

Async Programming

// Promises
async function fetchUser(): Promise<User> {
  const response = await fetch('/api/users/1');
  return response.json();
}

// Error handling
async function safeRequest() {
  try {
    const result = await fetchUser();
  } catch (error) {
    console.error('Request failed:', error);
  }
}

Best Practices

1. Avoid any: Use unknown and type guards instead
2. Use strict mode: Enable strict in tsconfig.json
3. Leverage utility types: Reduce code duplication
4. Document complex types: Use JSDoc for clarity
5. Test type definitions: Use type-level tests

Common Patterns

Result Type Pattern

type Result<T, E> =
  | { ok: true; value: T }
  | { ok: false; error: E };

function createUser(data: any): Result<User, string> {
  try {
    // validation and creation
    return { ok: true, value: user };
  } catch (e) {
    return { ok: false, error: e.message };
  }
}

Builder Pattern

class QueryBuilder<T> {
  private query: any = {};

  where(field: keyof T, value: any): this {
    this.query[field] = value;
    return this;
  }

  build() {
    return this.query;
  }
}

Real-World Angular Examples

Service Type Safety

@Injectable()
export class UserService {
  constructor(private http: HttpClient) {}

  getUser(id: number): Observable<User> {
    return this.http.get<User>(`/api/users/${id}`);
  }
}

Component Props

interface ComponentProps {
  title: string;
  items: Item[];
  onSelect: (item: Item) => void;
}

@Component({
  selector: 'app-list',
  template: `...`
})
export class ListComponent implements ComponentProps {
  @Input() title!: string;
  @Input() items: Item[] = [];
  @Output() itemSelected = new EventEmitter<Item>();

  onSelect(item: Item) {
    this.itemSelected.emit(item);
  }
}

Performance Tips

• Use const assertions for literal types
• Leverage structural typing for flexibility
• Use discriminated unions for safe pattern matching
• Avoid circular type dependencies
• Use omit to reduce property access

Resources

• TypeScript Handbook
• Advanced Types
• TypeScript Playground