Utility Types Development Tutorials, Guides & Insights

Intersection types combine multiple types into one. An object of an intersection type must satisfy all the combined types.

interface Person {
  name: string;
}

interface Employee {
  employeeId: number;
}

type EmployeePerson = Person & Employee;

const john: EmployeePerson = {
  name: 'John Doe',
  employeeId: 1234,
};

type IsString<T> = T extends string ? 'string' : 'not string';

type Test1 = IsString<string>; // 'string'
type Test2 = IsString<number>; // 'not string'

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