Objects and readonly

In plain English

An object type literal describes an object by listing its properties and their types. An optional property is one that may be missing and is written with a question mark. The readonly modifier says a property may be set once but never changed afterwards. An index signature lets an object hold any number of properties that all share one value type. The built-in Record type is a shorthand for that same idea when the keys are strings.

Step by step

1. 1List each property and its type inside curly braces.
2. 2Add a question mark to properties that can be missing.
3. 3Put readonly before a property that must not change.
4. 4Use a Record type for a lookup where keys are strings.
5. 5Assign an object and let the compiler check it matches the shape.

type Lesson = {
  id: string
  title: string
  readonly slug: string
  published?: boolean
}

const lesson: Lesson = {
  id: '1',
  title: 'Objects and readonly',
  slug: 'objects-and-readonly',
}

const scores: Record<string, number> = {
  quiz: 9,
  challenge: 8,
}

const counts: { [key: string]: number } = {
  lessons: 5,
  minutes: 55,
}

Common mistakes

• Adding every possible property up front makes a type harder to use, so only model real data.
• Forgetting the question mark on an optional property forces every object to include it.
• Assigning a new value to a readonly property causes a compiler error because the type forbids it.
• Using any as a shortcut removes the safety the object type was supposed to give you.