meteor/packages/check/isPlainObject.js

// Copy of jQuery.isPlainObject for the server side from jQuery v3.1.1.

const class2type = {};

const toString = class2type.toString;

const hasOwn = Object.prototype.hasOwnProperty;

const fnToString = hasOwn.toString;

const ObjectFunctionString = fnToString.call(Object);

const getProto = Object.getPrototypeOf;

export const isPlainObject = obj => {
  let proto;
  let Ctor;

  // Detect obvious negatives
  // Use toString instead of jQuery.type to catch host objects
  if (!obj || toString.call(obj) !== '[object Object]') {
    return false;
  }

  proto = getProto(obj);

  // Objects with no prototype (e.g., `Object.create( null )`) are plain
  if (!proto) {
    return true;
  }

  // Objects with prototype are plain iff they were constructed by a global Object function
  Ctor = hasOwn.call(proto, 'constructor') && proto.constructor;
  return typeof Ctor === 'function' && 
    fnToString.call(Ctor) === ObjectFunctionString;
};