TypeScript Advanced Patterns

Deep dive into advanced TypeScript patterns for building type-safe, maintainable applications.

Type-Level Programming

TypeScript's type system is Turing complete. Here are some powerful patterns:

Conditional Types

TypeScript
1type IsString<T> = T extends string ? true : false;
2
3type A = IsString<"hello">; // true
4type B = IsString<42>; // false

Mapped Types

TypeScript
1type Readonly<T> = {
2  readonly [P in keyof T]: T[P];
3};
4
5type Partial<T> = {
6  [P in keyof T]?: T[P];
7};

Mapped types are the foundation of many utility types in TypeScript.

Template Literal Types

Building type-safe route handlers:

TypeScript
1type HTTPMethod = 'GET' | 'POST' | 'PUT' | 'DELETE';
2type Route = '/users' | '/posts' | '/comments';
3
4type Endpoint = `${HTTPMethod} ${Route}`;
5// Results in: 'GET /users' | 'GET /posts' | 'POST /users' | ...
6
7function handleRequest(endpoint: Endpoint) {
8  // Fully type-safe!
9}
10
11handleRequest('GET /users'); // ✓
12handleRequest('GET /invalid'); // ✗ Type error

Branded Types

Preventing type confusion with nominal typing:

TypeScript
1type UserId = string & { readonly brand: unique symbol };
2type PostId = string & { readonly brand: unique symbol };
3
4function getUserId(id: string): UserId {
5  return id as UserId;
6}
7
8function getPost(postId: PostId) {
9  // Implementation
10}
11
12const userId = getUserId("user123");
13const postId = getUserId("post456") as unknown as PostId;
14
15getPost(userId); // ✗ Type error - cannot pass UserId where PostId expected
16getPost(postId); // ✓

Branded types caught 12 type confusion bugs in our codebase before production.

Builder Pattern with Fluent API

Type-safe builder with method chaining:

TypeScript
1class QueryBuilder<T extends object> {
2  private filters: Array<(item: T) => boolean> = [];
3  
4  where<K extends keyof T>(
5    key: K,
6    value: T[K]
7  ): QueryBuilder<T> {
8    this.filters.push(item => item[key] === value);
9    return this;
10  }
11  
12  execute(data: T[]): T[] {
13    return data.filter(item => 
14      this.filters.every(filter => filter(item))
15    );
16  }
17}
18
19// Usage
20const users = new QueryBuilder<User>()
21  .where('age', 25)
22  .where('status', 'active')
23  .execute(userList);

Recursive Types

Handling deeply nested structures:

TypeScript
1type DeepPartial<T> = {
2  [P in keyof T]?: T[P] extends object
3    ? DeepPartial<T[P]>
4    : T[P];
5};
6
7interface Config {
8  database: {
9    host: string;
10    port: number;
11    credentials: {
12      username: string;
13      password: string;
14    };
15  };
16}
17
18// All properties optional at any depth
19const partialConfig: DeepPartial<Config> = {
20  database: {
21    credentials: {
22      username: "admin"
23      // password is optional
24    }
25  }
26};

Type Guards with Type Predicates

TypeScript
1interface Dog {
2  type: 'dog';
3  bark(): void;
4}
5
6interface Cat {
7  type: 'cat';
8  meow(): void;
9}
10
11type Animal = Dog | Cat;
12
13function isDog(animal: Animal): animal is Dog {
14  return animal.type === 'dog';
15}
16
17function handleAnimal(animal: Animal) {
18  if (isDog(animal)) {
19    animal.bark(); // TypeScript knows it's a Dog
20  } else {
21    animal.meow(); // TypeScript knows it's a Cat
22  }
23}

Complex type-level programming can slow down the TypeScript compiler. Balance type safety with build performance.

Key Takeaways

  1. Type Safety: Advanced patterns catch bugs at compile time
  2. Developer Experience: Better autocomplete and IntelliSense
  3. Refactoring: Changes propagate through type system
  4. Documentation: Types serve as inline documentation

Placeholder content - replace with your actual TypeScript experiments.