Mapped types with key remapping in TypeScript

Building a type from another one, renaming the keys as you go.

interface Settings {
  theme: string;
  editorMode: boolean;
  timeLimit: number;
}

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

type OnlyBooleans<T> = {
  [K in keyof T as T[K] extends boolean ? K : never]: T[K];
};

type Nullable<T> = { [K in keyof T]: T[K] | null };

type SettingsGetters = Getters<Settings>;
type Flags = OnlyBooleans<Settings>;

const getters: SettingsGetters = {
  getTheme: () => "dracula",
  getEditorMode: () => true,
  getTimeLimit: () => 60,
};
const flags: Flags = { editorMode: false };
const nullable: Nullable<Settings> = { theme: null, editorMode: true,
  timeLimit: 30 };
console.log(getters.getTheme(), flags.editorMode, nullable.theme);

How it works

1. as in a mapped type rewrites each key.
2. Returning never from the as clause drops the key.
3. Template literals build the new names.

Keywords and builtins used here

• Flags
• Nullable
• SettingsGetters
• as
• boolean
• const
• extends
• false
• interface
• never
• null
• number
• string
• true
• type

The run, in numbers

Lines

28

Characters to type

698

Tokens

203

Three-star pace

110 tpm

At the three-star pace of 110 tokens a minute, this run takes about 111 seconds.

Type this snippet

Step 2 of 3 in Mapped types, step 5 of 12 in Type-level programming.

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