Generics Development Tutorials, Guides & Insights

You can constrain generics to ensure that the types passed to them meet certain requirements. This is useful when you want to make sure that the generic type has certain properties or methods.

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

const point: Point = { x: 10, y: 20 };
const xValue = getProperty(point, "x");

function identity<T>(arg: T): T {
  return arg;
}

const num = identity<number>(42); // T is replaced with number
const str = identity<string>('Hello'); // T is replaced with string

In this example, T is a type variable that allows the identity function to accept and return any type.

type Action = 'create' | 'update' | 'delete';
type Entity = 'user' | 'post';

type LogMessage = `${Action}_${Entity}`;

function logAction(action: LogMessage) {
  console.log(`Logging action: ${action}`);
}

logAction('create_user'); // valid
logAction('update_post'); // valid
// logAction('read_user'); // error

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

const person = { name: 'Alice', age: 30 };

const name = getProperty(person, 'name'); // valid
const age = getProperty(person, 'age');   // valid
// const gender = getProperty(person, 'gender'); // error