What are union and intersection types?

Question

Accepted Answer

A **union** (`A | B`) means "either A **or** B." An **intersection** (`A & B`) means "both A **and** B at once."

```ts
// Union — value is one of several types
type Status = "loading" | "success" | "error"; // string literal union
let s: Status = "loading"; // only these three allowed

function id(x: string | number) { /* x is string OR number */ }
```

## Working with unions: you can only use what's common

```ts
function format(x: string | number) {
  // x.toFixed(2); // ❌ toFixed doesn't exist on string
  if (typeof x === "number") return x.toFixed(2); // ✅ narrowed to number
  return x.toUpperCase();                          // ✅ here it's a string
}
```

Until you **narrow** a union, you can only access members present on *every* member of the union — the compiler protects you.

## Intersection — combine shapes

```ts
type Name = { name: string };
type Age = { age: number };
type Person = Name & Age; // must have BOTH name and age

const p: Person = { name: "Ann", age: 30 }; // both required
```

Intersections are great for composing/mixing object types (e.g. adding props to an existing type).

## Why it matters

Unions model "one of several possibilities" — the foundation of safe state modeling (e.g. discriminated unions for reducer state), function overloads, and nullable types (`T | null`).

Intersections compose types together.

Together they make TypeScript's type system expressive enough to mirror real data precisely.