Typescript Mapped Types with recursion.ts

// https://stackoverflow.com/a/72366029/5018572

type Nested = {
  foo: string;
  bar: { baz: number; };
  squick: { squeck: string; squawk: boolean; };
  garble: { gobble: string; };
};

type Example = {
  node1: {
    node2: {
      bla: number;
      node3?: {
        node4: {
          title: string;
        };
      };
    };
  };
}

type Flatten<T extends object> = {
  [K in keyof T]-?: (
    x: NonNullable<T[K]> extends infer V
      ? V extends object
        ? V extends readonly any[]
          ? Pick<T, K>
          : Flatten<V> extends infer FV
          ? {
              [P in keyof FV as `${Extract<K, string | number>}.${Extract<P, string | number>}`]: FV[P]
            }
          : never
        : Pick<T, K>
      : never
  ) => void
} extends Record<keyof T, (y: infer O) => void>
  ? O extends unknown
    ? { [K in keyof O]: O[K] }
    : never
  : never

type Unnested = Flatten<Nested>;
type UnnestedExample = Flatten<Example>;

Flatten is a sweet example of using recursion in mapped type. It does not comes default with it.