isPlainObject.js

/**
 * Tests:
 *  - `isPlainObject(null)`
 *  - `isPlainObject([1, 2, 3])`
 *  - `isPlainObject(window)`
 *  - `isPlainObject(new String('hello'))`
 *  - `isPlainObject(Object.create(null))`
 *  - `isPlainObject({ a: 1, b: 2 })`
 *  - `isPlainObject(new Person('Alex'))`
 *
 *  `Person`:
 * ```
 * function Person(name) {
 *   this.name = name;
 * }
 * Person.prototype = {
 *   getName: function () {
 *     return this.name;
 *   }
 * };
 * ```
 *
 * Ref:
 *  - jQuery 3.x isPlainObject
 */
var toString = Object.prototype.toString;
var hasOwn = Object.prototype.hasOwnProperty;
var fnToString = Function.prototype.toString;
var ObjectFunctionString = fnToString.call(Object);

// Object.getPrototypeOf polyfill: core-js or es5-shim
function getProto(obj) {
  return Object.getPrototypeOf(obj);
}

function isPlainObject (obj) {
  if (obj === null || toString.call(obj) !== '[object Object]') {
    return false;
  }
  
  var proto = getProto(obj);
  
  if (!proto) {
    return true;
  }
  
  var Ctor = hasOwn.call(proto, 'constructor') && proto.constructor;
  
  return typeof Ctor === 'function' && fnToString.call(Ctor) === ObjectFunctionString;
}