ts: deep merge json serializable objects

jsonMerge.ts

function isObject(obj: any) {
  return typeof obj === "object" && obj !== null;
}

function isPlainObject(obj: any) {
  return Object.getPrototypeOf(obj).constructor === Object;
}

/**
 * used to deep merge json serializable objects.
 * skips undefined source values and empty array slots.
 */
export function merge(target: any, source: any) {
  if (isObject(target) && isObject(source)) {
    if (Array.isArray(source)) {
      source.forEach((item, i) => (target[i] = merge(target[i], item)));
      return target;
    } else if (isPlainObject(source)) {
      Object.keys(source).forEach(
        key => (target[key] = merge(target[key], source[key]))
      );
      return target;
    }
  }

  return source !== void 0 ? source : target;
}