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HTML / CSS / JS

// Mapping over an object type
type Mutable<T> = {
    -readonly [K in keyof T]: T[K]; // remove readonly
};

type Required2<T> = {
    [K in keyof T]-?: T[K]; // remove optional (-)
};

// Key remapping with 'as'
type Getters<T> = {
    [K in keyof T as `get${Capitalize<string & K>}`]: () => T[K];
};

interface UserModel { id: number; name: string; email: string }
type UserGetters = Getters<UserModel>;
// { getId: () => number; getName: () => string; getEmail: () => string }

// Filter keys by value type
type PickByValue<T, V> = {
    [K in keyof T as T[K] extends V ? K : never]: T[K];
};

interface Mixed {
    id: number;
    name: string;
    count: number;
    label: string;
}
type OnlyStrings = PickByValue<Mixed, string>;
// { name: string; label: string }

// Deep partial
type DeepPartial<T> = {
    [K in keyof T]?: T[K] extends object ? DeepPartial<T[K]> : T[K];
};

interface Config {
    server: { host: string; port: number };
    database: { url: string; name: string };
    debug: boolean;
}

const partialConfig: DeepPartial<Config> = {
    server: { port: 8080 }, // only overriding port
};