type mpObj<T> = { [k in keyof T | string | number | symbol]: any };
export function apply<L, R>(
  target: mpObj<L>,
  patchItem: mpObj<R>
): Partial<L> & Partial<R>;
export function apply<L, R>(target: mpObj<L>, patchItem: mpObj<R>): R;
export function apply<L, R>(target: mpObj<L>, patchItem: mpObj<R>): {};
export function apply<L, R>(target: mpObj<L>, patchItem: null): null;
export function apply<L, R>(target: mpObj<L>, patchItem: string): string;
export function apply<L, R>(target: mpObj<L>, patchItem: number): number;
export function apply<L, R>(target: mpObj<L>, patchItem: undefined): undefined;
export function apply<L, R>(target: mpObj<L>, patchItem: R[]): R[];

export function apply(target: any, patchItem: any): any {
  /**
   * If the patch is anything other than an object,
   * the result will always be to replace
   * the entire target with the entire patch.
   */
  if (typeof patchItem !== "object" || Array.isArray(patchItem) || !patchItem) {
    return JSON.parse(JSON.stringify(patchItem)); //return new instance of variable
  }

  if (
    typeof patchItem === "object" &&
    patchItem.toJSON !== undefined &&
    typeof patchItem.toJSON === "function"
  ) {
    return patchItem.toJSON();
  }
  /** Also, it is not possible to
   * patch part of a target that is not an object,
   * such as to replace just some of the values in an array.
   */
  let targetResult = target;
  if (typeof target !== "object") {
    //Target is empty/not an object, so basically becomes patch, minus any null valued sections (becomes {} + patch)
    targetResult = { ...patchItem };
  }

  Object.keys(patchItem).forEach((k) => {
    if (!targetResult.hasOwnProperty(k)) targetResult[k] = patchItem[k]; //This ensure the key exists and TS can't throw a wobbly over an undefined key
    if (patchItem[k] === null) {
      delete targetResult[k];
    } else {
      targetResult[k] = apply(targetResult[k], patchItem[k]);
    }
  });
  return targetResult;
}