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TypeScript Academy · Lesson

Modifiers in Mapped Types: readonly and ?

Add or remove modifiers with +/- in mapped types.

Lesson 2 of 413 steps

Modifiers in Mapped Types: readonly and ? is a free TypeScript Academy lesson on CoddyKit. This is lesson 2 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the TypeScript Academy learning path, and your progress syncs across the web and the CoddyKit app. The TypeScript Academy course includes 4 lessons in total.

Welcome

Mapped types can add or remove `readonly` and optional (`?`) modifiers from properties using the `+` and `-` prefixes.

Adding Modifiers with +

The `+` prefix (implicit by default) adds a modifier to each property in the mapped type.
type Readonly<T> = { +readonly [K in keyof T]: T[K] }; // + is implicit
type Optional<T> = { [K in keyof T]+?: T[K] };

Removing readonly with -

Use `-readonly` to strip the readonly modifier from properties.
type Mutable<T> = { -readonly [K in keyof T]: T[K] };
const mutableUser: Mutable<Readonly<User>> = { id: 1, name: 'Alice' };
mutableUser.id = 2; // OK now

Removing ? with -

Use `-?` to remove the optional modifier, making all properties required.
type Required<T> = { [K in keyof T]-?: T[K] };
// Now all optional properties become required

Combining - and -?

You can combine both removals to create a mutable, required version of a type.
type Concrete<T> = { -readonly [K in keyof T]-?: T[K] };

Partial and Required Are Opposites

Partial adds `?` with +?. Required removes it with -?.
// From TypeScript lib:
type Partial<T> = { [K in keyof T]?: T[K] };   // adds ?
type Required<T> = { [K in keyof T]-?: T[K] };  // removes ?

Modifier Preservation by Default

Without explicit + or -, mapped types preserve existing modifiers from the source type.
interface User { readonly id: number; name?: string; }
type Copy<T> = { [K in keyof T]: T[K] };
type C = Copy<User>; // readonly id preserved, name? preserved

Practical Use: Patch Type

Create a Patch type that makes all properties optional and also removes readonly for updates.
type Patch<T> = { -readonly [K in keyof T]?: T[K] };

DeepReadonly

Apply readonly recursively with a conditional type inside the mapped type.
type DeepReadonly<T> = {
  readonly [K in keyof T]: T[K] extends object ? DeepReadonly<T[K]> : T[K]
};

DeepPartial

Similarly, apply ? recursively for a deeply optional type.
type DeepPartial<T> = {
  [K in keyof T]?: T[K] extends object ? DeepPartial<T[K]> : T[K]
};

Modifier Combinations Table

+readonly = add readonly. -readonly = remove readonly. +? = add optional. -? = remove optional. All four can be combined.

Quick Check

What does `{ [K in keyof T]-?: T[K] }` produce?
Answer it in the interactive lesson

Recap

Use +/- prefixes before readonly and ? in mapped types to explicitly add or remove those modifiers. -? implements Required, -readonly implements Mutable. Combine both for complete mutability control.
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Frequently Asked Questions

Is the “Modifiers in Mapped Types: readonly and ?” lesson free?

Yes — the full text of “Modifiers in Mapped Types: readonly and ?” is free to read here on the web. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the TypeScript Academy course, upgrade to CoddyKit PRO. The TypeScript Academy course includes 4 lessons in total.

What will I learn in “Modifiers in Mapped Types: readonly and ?”?

Add or remove modifiers with +/- in mapped types. You practise TypeScript Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start TypeScript Academy?

No prior experience is required. TypeScript Academy on CoddyKit is structured for beginners through advanced learners, so you can start here or from the beginning and move at your own pace. This is lesson 2 of 4.

How long does the “Modifiers in Mapped Types: readonly and ?” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this TypeScript Academy lesson?

Yes. Every TypeScript Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

All lessons in this course

  1. Mapped Types: Transforming Object Types
  2. Modifiers in Mapped Types: readonly and ?
  3. Template Literal Types: String Patterns
  4. Key Remapping with as in Mapped Types
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