NodeOf.d.ts

type JsonObject = { [property: string]: Json };

type JsonArray = Json[];

type Json = null | boolean | number | string | JsonObject | JsonArray;

type NodeOf<T extends Json> = T extends JsonObject
  ? T
  : T extends JsonArray
  ? T[number]
  : never;

type X = {
  property1: string;
  property2: null;
  property3: string;
  property4: boolean;
  property5: boolean;
  property6: number;
  property7: {
    name: string;
  };
};

type XNode = NodeOf<X[keyof X]>;
//=> { name: string; }
  
type Y = {
  property1: number;
  property2: null;
  property3: string;
  property4: boolean;
  property5: boolean;
  property6: number[];
  property7: {
    value: number;
  }[];
};

type YNode = NodeOf<Y[keyof Y]>;
//=> { value: number }