https://jsbench.me/dgkh1z1val/1

is-plain-object.js

var _toString = Object.prototype.toString;
var _getPrototypeOf = Object.getPrototypeOf;

function isPlainObject(value) {
  switch(value) {
    default:
      if (_toString.call(value) === '[object Object]') {
        break;
      }
    case null:
    case undefined:
      return false;
  }
  let valueProto = _getPrototypeOf(value);
  let proto = valueProto;
  let nextProto = valueProto;
  while (true) {
    if (nextProto === null) {
      return valueProto === proto;
    }
    proto = nextProto;
    nextProto = _getPrototypeOf(proto);
  }
}