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Understanding TypeScript Generics

January 15, 2025
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TypeScript: A powerful tool in modern web development

TypeScript Generics Explained

Generics are one of TypeScript's most powerful features. They allow you to write flexible, reusable code while maintaining type safety. In this comprehensive guide, we'll explore how generics work and their practical applications in real-world scenarios.

basic-generic.ts
1function identity<T>(arg: T): T {
2  return arg;
3}
4

5const stringResult = identity<string>('Hello');
6const numberResult = identity<number>(42);
7const objectResult = identity<{id: number}>({ id: 1 });

Type Inference

TypeScript can often infer the generic type automatically. You can call identity('Hello') without explicitly specifying <string>.

Generic Interfaces

Generics are particularly useful when working with interfaces. Here's how you can create flexible, type-safe interfaces:

generic-interface.ts
1interface Repository<T> {
2  get(id: string): Promise<T>;
3  save(item: T): Promise<void>;
4  update(id: string, item: Partial<T>): Promise<void>;
5  delete(id: string): Promise<void>;
6}
7

8interface User {
9  id: string;
10  name: string;
11  email: string;
12}
13

14// Type-safe user repository
15class UserRepository implements Repository<User> {
16  async get(id: string): Promise<User> {
17    // Implementation here
18  }
19  // Other methods...
20}

Generic Constraints

Sometimes you want to limit the types that can be used with your generic function. This is where constraints come in:

constraints.ts
1interface HasLength {
2  length: number;
3}
4

5function logLength<T extends HasLength>(arg: T): number {
6  console.log(arg.length);
7  return arg.length;
8}
9

10// These work
11logLength('Hello');  // string has .length
12logLength([1, 2, 3]);  // arrays have .length
13

14// This would error
15// logLength(123);  // numbers don't have .length

Best Practice

Use constraints when you need to access specific properties or methods of the generic type parameter.

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Using generics to build scalable applications