Singleton Pattern

1// Database connection singleton
2class Database {
3  private static instance: Database | null = null;
4  private connected: boolean = false;
5 
6  // Private constructor prevents direct instantiation
7  private constructor(private connectionString: string) {}
8 
9  // Static method to get the single instance
10  static getInstance(connectionString?: string): Database {
11    if (!Database.instance) {
12      if (!connectionString) {
13        throw new Error("Connection string required for first initialization");
14      }
15      Database.instance = new Database(connectionString);
16    }
17    return Database.instance;
18  }
19 
20  async connect(): Promise<void> {
21    if (this.connected) {
22      console.log("Already connected");
23      return;
24    }
25 
26    console.log(`Connecting to ${this.connectionString}...`);
27    // Simulated connection
28    await new Promise((resolve) => setTimeout(resolve, 100));
29    this.connected = true;
30    console.log("Connected!");
31  }
32 
33  async query(sql: string): Promise<unknown[]> {
34    if (!this.connected) {
35      throw new Error("Not connected to database");
36    }
37    console.log(`Executing: ${sql}`);
38    return []; // Simulated result
39  }
40 
41  async disconnect(): Promise<void> {
42    if (!this.connected) return;
43    console.log("Disconnecting...");
44    this.connected = false;
45  }
46 
47  // For testing - reset the singleton
48  static resetInstance(): void {
49    Database.instance = null;
50  }
51}
52 
53// Configuration singleton using a class
54class Config {
55  private static instance: Config;
56  private settings: Map<string, unknown> = new Map();
57 
58  private constructor() {
59    // Load default settings
60    this.settings.set("apiUrl", "https://api.example.com");
61    this.settings.set("timeout", 5000);
62    this.settings.set("debug", false);
63  }
64 
65  static getInstance(): Config {
66    if (!Config.instance) {
67      Config.instance = new Config();
68    }
69    return Config.instance;
70  }
71 
72  get<T>(key: string): T | undefined {
73    return this.settings.get(key) as T | undefined;
74  }
75 
76  set<T>(key: string, value: T): void {
77    this.settings.set(key, value);
78  }
79}
80 
81// Usage example
82async function main(): Promise<void> {
83  // Database singleton
84  const db1 = Database.getInstance("postgres://localhost:5432/mydb");
85  const db2 = Database.getInstance(); // Returns same instance
86 
87  console.log("Same instance?", db1 === db2); // true
88 
89  await db1.connect();
90  await db1.query("SELECT * FROM users");
91 
92  // Config singleton
93  const config = Config.getInstance();
94  console.log("API URL:", config.get<string>("apiUrl"));
95 
96  config.set("debug", true);
97  console.log("Debug mode:", config.get<boolean>("debug"));
98}
99 
100main().catch(console.error);