Mapped types

What a session covers

The tutor works through these with you, in conversation and code. A session ends when you have shown them, not when a video does.

1. Explain how a mapped type loops over the keys of another type
2. Write a mapped type that makes every property readonly or optional
3. Read the definition of a built-in utility type and explain it

In practice

A few lines that show Mapped types doing its job.

type Readonlyish<T> = { readonly [K in keyof T]: T[K] };
type Async<T> = { [K in keyof T]: () => Promise<T[K]> };
type Flags<T> = { [K in keyof T as `has${Capitalize<string & K>}`]: boolean };

interface Settings { theme: string; volume: number }
type SettingsFlags = Flags<Settings>;   // { hasTheme: boolean; hasVolume: boolean }

The common mistake

What to know first

Your roadmap puts these before Mapped types, unless you already know them.

• Partial, Pick and Omit

How it stays learned

Once a session shows you understand Mapped types, it comes back for review. Each review takes a minute and tells the memory model how well you still know it.

1. Day 3First recall, while it is fresh
2. Day 7Second, a week on
3. Day 18Last, when it has settled

Where it leads

Topics that build on Mapped types.

• Conditional types