Builder Pattern

1// Product class
2interface HttpRequest {
3  method: "GET" | "POST" | "PUT" | "DELETE";
4  url: string;
5  headers: Record<string, string>;
6  body?: unknown;
7  timeout: number;
8}
9 
10// Builder class
11class RequestBuilder {
12  private request: Partial<HttpRequest> = {
13    method: "GET",
14    headers: {},
15    timeout: 5000,
16  };
17 
18  setMethod(method: HttpRequest["method"]): this {
19    this.request.method = method;
20    return this;
21  }
22 
23  setUrl(url: string): this {
24    this.request.url = url;
25    return this;
26  }
27 
28  addHeader(key: string, value: string): this {
29    this.request.headers = {
30      ...this.request.headers,
31      [key]: value,
32    };
33    return this;
34  }
35 
36  setBody(body: unknown): this {
37    this.request.body = body;
38    return this;
39  }
40 
41  setTimeout(ms: number): this {
42    this.request.timeout = ms;
43    return this;
44  }
45 
46  // Type-safe build that requires url
47  build(): HttpRequest {
48    if (!this.request.url) {
49      throw new Error("URL is required");
50    }
51 
52    return this.request as HttpRequest;
53  }
54}
55 
56// Usage with fluent API
57const getRequest = new RequestBuilder()
58  .setUrl("https://api.example.com/users")
59  .addHeader("Accept", "application/json")
60  .build();
61 
62const postRequest = new RequestBuilder()
63  .setMethod("POST")
64  .setUrl("https://api.example.com/users")
65  .addHeader("Content-Type", "application/json")
66  .addHeader("Authorization", "Bearer token123")
67  .setBody({ name: "Alice", email: "alice@example.com" })
68  .setTimeout(10000)
69  .build();
70 
71console.log("GET request:", getRequest);
72console.log("POST request:", postRequest);
73 
74// Generic builder for any type
75class Builder<T extends object> {
76  private obj: Partial<T> = {};
77 
78  set<K extends keyof T>(key: K, value: T[K]): this {
79    this.obj[key] = value;
80    return this;
81  }
82 
83  build(): T {
84    return this.obj as T;
85  }
86}
87 
88interface User {
89  id: number;
90  name: string;
91  email: string;
92}
93 
94const user = new Builder<User>()
95  .set("id", 1)
96  .set("name", "Bob")
97  .set("email", "bob@example.com")
98  .build();
99 
100console.log("User:", user);