warn_if_object_id2.js

var checkForHexRegExp = new RegExp('^[0-9a-fA-F]{24}$');

function check_serializability(value, id, path) {
  const type = typeof value;

  if (type === 'string' || type === 'boolean' || type === 'number' || type === 'undefined') {
    // primitives are fine
    return;
  }

  if (type === 'object') {
    // nulls are fine...
    if (!value) return;

    // ...so are plain arrays...
    if (Array.isArray(value)) {
      value.forEach((child, i) => {
        check_serializability(child, id, `${path}[${i}]`);
      });
      return;
    }

    // ...and objects
    const tag = Object.prototype.toString.call(value);
    if (tag === '[object Object]') {
      if (
          typeof value.toHexString === 'function' &&
          (value.id instanceof Buffer || typeof value.id === 'string')
      ) {
        return (value.id.length === 12 || (value.id.length === 24 && checkForHexRegExp.test(value.id))) && undefined;
      }
      for (const key in value) {
        check_serializability(value[key], id, `${path}.${key}`);
      }
      return;
    }
  }

  throw new Error(`${path} returned from 'load' in ${id} cannot be serialized as JSON`);
}