Constructor pattern to allow both new and functional usage

ctor.js

function Ctor(options) {
  if(!(this instanceof Ctor)) { // same as (!Ctor.prototype.isPrototypeOf(this))
    return Ctor.apply(Object.create(Ctor.prototype), arguments)
  }
  
  // ...
  
  return this
}

instance-new.js

var Ctor = require("ctor")

var instance = new Ctor({/*...*/})

instance.js

var ctor = require("ctor")

var instance = ctor({/*...*/})

function Ctor(param1, param2){
  if(!Ctor.prototype.isPrototypeOf(this)) {
    return Ctor.apply(
      Object.create(Ctor.prototype),
      arguments // needed if more than one param
    )
  }
  // code
  return this // imperative when `new` isn't called, as there is no default `returnValue`
}

Updated according to your comment.
Thanks !