Object Types and Optional Properties

You can describe an object's shape right where you use it: star: { name: string; mag?: number }. A ? makes a property optional: it may be missing, so its type includes undefined, and TypeScript makes you check before using it. Object literals are checked exactly: a missing required property and a misspelled extra one are both errors.

How it reads

• mag?: number means the property can be left out
• Checking === undefined narrows mag to a plain number afterwards
• When the shape is used in many places, give it a name with an interface

Cloud tip: value ?? fallback supplies a default only when the value is null or undefined.