Mapped types in TypeScript

Building a new type by walking another's keys.

interface Settings {
  theme: string;
  volume: number;
}

type Nullable<T> = { [K in keyof T]: T[K] | null };
type Getters<T> = {
  [K in keyof T as `get${Capitalize<string & K>}`]: () => T[K];
};

type MaybeSettings = Nullable<Settings>;
type SettingGetters = Getters<Settings>;

const blank: MaybeSettings = { theme: null, volume: null };

How it works

1. [K in keyof T] iterates every property.
2. The value type can be transformed as it maps.
3. as with a template literal renames the keys.

Keywords and builtins used here

• MaybeSettings
• as
• const
• interface
• null
• number
• string
• type

The run, in numbers

Lines

14

Characters to type

335

Tokens

101

Three-star pace

110 tpm

At the three-star pace of 110 tokens a minute, this run takes about 55 seconds.

Type this snippet

Step 1 of 3 in Mapped types, step 4 of 12 in Type-level programming.

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