21. How do you define and work with union types and intersection types in TypeScript?

Union types allow a variable to be assigned multiple types. They can be defined using the | operator. On the other hand, intersection types combine multiple types into one that contains the properties of all types, using the & operator.

Example of union types:

let value: string | number;
value = 'Hello'; // OK
value = 10; // OK
value = true; // Error: not a string or number

Example of intersection types:

interface Person {
  name: string;
  age: number;
}
interface Employee {
  jobTitle: string;
}

let employee: Person & Employee = { name: 'John', age: 30, jobTitle: 'Developer' };
// 'employee' has properties from both 'Person' and 'Employee'

Union types are useful when a value can be one of several types, and intersection types are useful when you want to combine multiple interfaces or types into one.