What are union types and intersection types? When do you use each?

Union types (A | B) — a value can be one of several types. You need type narrowing to use type-specific operations.

type StringOrNumber = string | number;

function format(val: StringOrNumber): string {
  if (typeof val === 'string') {
    return val.toUpperCase(); // narrowed to string
  }
  return val.toFixed(2); // narrowed to number
}

// Discriminated union — literal type narrows the variant
type Shape =
  | { kind: 'circle'; radius: number }
  | { kind: 'square'; side: number };

function area(s: Shape) {
  switch (s.kind) {
    case 'circle': return Math.PI * s.radius ** 2;
    case 'square': return s.side ** 2;
  }
}

Intersection types (A & B) — a value must satisfy ALL types simultaneously. Useful for composing object shapes.

type Serializable = { serialize(): string };
type Loggable = { log(): void };

type SerializableLogger = Serializable & Loggable;
// Must have both serialize() AND log()

// Merging object types
type Admin = User & { adminLevel: number };

// Conflict: if A and B have same property with incompatible types
type Conflict = { id: string } & { id: number };
// id becomes: string & number = never — unusable