a runtime feature detection way to sniff native functions from fakes

is-native-function.js

var isNativeFunction = (function (Function) {
  var
    toString = Function.toString,
    re = new RegExp(
      '^' +
      toString
        .call(Function)
        .replace(/([\/\\[\]{}()^$.*+?_-])/g, '\\$1')
        .replace('Function', '[a-zA-Z$_][\\S]*') +
      '$'
    )
  ;
  return function (fn) {
    return re.test(fn);
  };
}(Function));