Mapped type modifiers (readonly, optional) in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the readonly modifier do in a mapped type?

It makes all properties in the mapped type read-only, meaning their values cannot be changed after initialization.

beginner

How do you make all properties optional in a mapped type?

By adding the ? modifier after the property name in the mapped type, like [P in keyof T]?: T[P].

intermediate

What is the effect of using +readonly and -readonly in mapped types?

+readonly adds the readonly modifier to properties, while -readonly removes it if it was present.

intermediate

Explain the difference between +? and -? in mapped types.

+? makes properties optional, -? makes them required (removes optional).

beginner

Show a simple example of a mapped type that makes all properties readonly and optional.

type ReadonlyOptional<T> = { readonly [P in keyof T]?: T[P] };

What does { readonly [P in keyof T]: T[P] } do?

A Makes all properties writable

B Makes all properties of T optional

C Makes all properties of T readonly

D Removes readonly from all properties

The readonly modifier before the property makes all properties readonly.

How do you remove the optional modifier from all properties in a mapped type?

A [P in keyof T]?: T[P]

B [P in keyof T]+readonly T[P]

C [P in keyof T]+?: T[P]

D [P in keyof T]-?: T[P]

-? removes the optional modifier, making properties required.

What does { -readonly [P in keyof T]: T[P] } do?

A Removes readonly from all properties

B Makes all properties optional

C Adds readonly to all properties

D Makes all properties required

-readonly removes the readonly modifier from properties.

Which syntax makes all properties optional in a mapped type?

A [P in keyof T]+?: T[P]

B [P in keyof T]?: T[P]

C [P in keyof T]-?: T[P]

D [P in keyof T]+readonly T[P]

Adding ? after the property name makes properties optional.

What is the result of { readonly [P in keyof T]?: T[P] }?

A All properties are optional and readonly

B All properties are optional and writable

C All properties are required and readonly

D All properties are required and writable

The readonly modifier makes properties readonly, and ? makes them optional.

Explain how to use mapped type modifiers to make all properties readonly and optional in TypeScript.

Think about adding +readonly and ? in the mapped type.

Describe the difference between + and - modifiers in mapped types for readonly and optional properties.

Consider how you can add or remove modifiers explicitly.