exactOptionalPropertyTypes Distinguishes Missing vs Undefined

With standard optional properties, setting a field to 'undefined' and omitting it entirely are treated identically. This masks bugs where code explicitly sets a field to undefined when it should omit it.

Enable 'exactOptionalPropertyTypes: true' to make optional fields only accept their declared type, not undefined.

// tsconfig.json
{ "compilerOptions": { "exactOptionalPropertyTypes": true } }

interface User {
  name: string;
  nickname?: string; // can be omitted, but NOT set to undefined
}

const u1: User = { name: 'Alice' };              // OK — omitted
const u2: User = { name: 'Alice', nickname: 'Al' }; // OK
const u3: User = { name: 'Alice', nickname: undefined }; // Error!
// Type 'undefined' is not assignable to type 'string'

// To explicitly allow undefined, add it to the type:
interface User2 {
  name: string;
  nickname?: string | undefined;
}

Optional property '?' means the key can be absent from the object. Setting a key to 'undefined' is semantically different — the key exists with an undefined value. These have different behavior in JSON.stringify, 'in' checks, and Object.keys.