Factory Pattern

1// Product interface
2interface Logger {
3  log(message: string): void;
4  error(message: string): void;
5  warn(message: string): void;
6}
7 
8// Concrete implementations
9class ConsoleLogger implements Logger {
10  log(message: string): void {
11    console.log(`[LOG] ${message}`);
12  }
13 
14  error(message: string): void {
15    console.error(`[ERROR] ${message}`);
16  }
17 
18  warn(message: string): void {
19    console.warn(`[WARN] ${message}`);
20  }
21}
22 
23class FileLogger implements Logger {
24  constructor(private filename: string) {}
25 
26  private write(level: string, message: string): void {
27    const entry = `[${new Date().toISOString()}] [${level}] ${message}\n`;
28    // In real implementation, write to file
29    console.log(`Writing to ${this.filename}: ${entry.trim()}`);
30  }
31 
32  log(message: string): void {
33    this.write("LOG", message);
34  }
35 
36  error(message: string): void {
37    this.write("ERROR", message);
38  }
39 
40  warn(message: string): void {
41    this.write("WARN", message);
42  }
43}
44 
45class JsonLogger implements Logger {
46  private emit(level: string, message: string): void {
47    const entry = JSON.stringify({
48      timestamp: new Date().toISOString(),
49      level,
50      message,
51    });
52    console.log(entry);
53  }
54 
55  log(message: string): void {
56    this.emit("info", message);
57  }
58 
59  error(message: string): void {
60    this.emit("error", message);
61  }
62 
63  warn(message: string): void {
64    this.emit("warn", message);
65  }
66}
67 
68// Factory
69type LoggerType = "console" | "file" | "json";
70 
71interface LoggerConfig {
72  type: LoggerType;
73  filename?: string;
74}
75 
76class LoggerFactory {
77  static create(config: LoggerConfig): Logger {
78    switch (config.type) {
79      case "console":
80        return new ConsoleLogger();
81      case "file":
82        if (!config.filename) {
83          throw new Error("Filename required for file logger");
84        }
85        return new FileLogger(config.filename);
86      case "json":
87        return new JsonLogger();
88      default:
89        throw new Error(`Unknown logger type: ${config.type}`);
90    }
91  }
92}
93 
94// Usage
95const consoleLogger = LoggerFactory.create({ type: "console" });
96consoleLogger.log("Application started");
97 
98const fileLogger = LoggerFactory.create({
99  type: "file",
100  filename: "app.log"
101});
102fileLogger.error("Something went wrong");
103 
104const jsonLogger = LoggerFactory.create({ type: "json" });
105jsonLogger.warn("Low memory warning");