Optional vs undefined

optional.ts

// Do these syntaxes produce the same result?
let x: { foo?: string };
let y: { foo: string | undefined };

x = {}; // ✅ The value can be omitted entirely, even `undefined`
y = {}; // ❌ The value must be explicitly provided, even if it is `undefined`
y = { foo: undefined }; // ✅ `undefined` is explicitly assigned