What are generics and how do constraints work?

Summary

Generics are type parameters (<T>) that let functions, classes and types work over many types while preserving type relationships. Constraints (<T extends Something>) restrict what T can be so you can safely use its members.

Longer Answer

Example: function first<T>(arr: T[]): T | undefined returns the same element type it receives. keyof constraints relate parameters: function get<T, K extends keyof T>(obj: T, key: K): T[K]. Defaults (<T = string>) and inference from arguments make generic APIs ergonomic. Avoid unnecessary generics: if T is used only once, a plain type is clearer.

Code Example

function getProp<T, K extends keyof T>(obj: T, key: K): T[K] {
  return obj[key];
}

const user = { id: 1, name: "Ana" };
const n = getProp(user, "name");   // string
// getProp(user, "age");           // Error: '"age"' not in keyof user

function longest<T extends { length: number }>(a: T, b: T): T {
  return a.length >= b.length ? a : b;
}

Key Points

• Type parameters preserve relationships
• extends constraints
• K extends keyof T → T[K]
• Inference from arguments
• Don't over-genericize