Is a function a constructor or not? Original src https://github.com/Benvie/Node.js-Ultra-REPL

gistfile1.js

function isConstructor(o){
  return typeof o === 'function'                             // per ES spec any callable is typed 'function'
      && o.prototype != null                                 // filter out absent prototypes
      && (typeof o.prototype === 'object'                    // filter out primitive prototypes
       || typeof o.prototype === 'function')
      && Object.getPrototypeOf(o) !== Object.prototype       // filter out non-constructable DOM interfaces
      && Object.getOwnPropertyNames(o.prototype).length >    // ensure the prototype has at least one property
         {}.hasOwnProperty.call(o.prototype, 'constructor'); // ...besides 'constructor'
}