Design Patterns in TypeScript

// Type guard functions
function isString(value: unknown): value is string {
  return typeof value === "string";
}

function isUser(value: unknown): value is User {
  return (
    typeof value === "object" &&
    value !== null &&
    "id" in value &&
    "name" in value
  );
}

// Exhaustive switch with never
type Shape = 
  | { kind: "circle"; radius: number }
  | { kind: "rect"; width: number; height: number }
  | { kind: "triangle"; base: number; height: number };

function getArea(shape: Shape): number {
  switch (shape.kind) {
    case "circle": return Math.PI * shape.radius ** 2;
    case "rect": return shape.width * shape.height;
    case "triangle": return 0.5 * shape.base * shape.height;
    default: {
      const _exhaustive: never = shape; // Error if a case is missed!
      return _exhaustive;
    }
  }
}

// Builder pattern
class QueryBuilder<T> {
  private filters: string[] = [];
  private sortField?: keyof T;
  private limitCount?: number;

  where(field: keyof T, value: string): this {
    this.filters.push(`${String(field)} = '${value}'`);
    return this; // Return 'this' for chaining
  }

  orderBy(field: keyof T): this {
    this.sortField = field;
    return this;
  }

  limit(count: number): this {
    this.limitCount = count;
    return this;
  }

  build(): string {
    let query = "SELECT * FROM table";
    if (this.filters.length) query += " WHERE " + this.filters.join(" AND ");
    if (this.sortField) query += ` ORDER BY ${String(this.sortField)}`;
    if (this.limitCount) query += ` LIMIT ${this.limitCount}`;
    return query;
  }
}

// Usage with type safety
interface Product { name: string; price: number; category: string; }
const query = new QueryBuilder<Product>()
  .where("category", "electronics")
  .orderBy("price")
  .limit(10)
  .build();

// Singleton pattern
class Database {
  private static instance: Database;
  private constructor() {} // Private constructor prevents new Database()

  static getInstance(): Database {
    if (!Database.instance) {
      Database.instance = new Database();
    }
    return Database.instance;
  }
}