How the 'new' keyword works in JavaScript

The new keyword in JavaScript can sometimes slip up new and experienced programmers alike. We typically associate this word with classical-based object-oriented languages (such as Java, etc.), whereas in JavaScript it works a little differently. In fact, due to the language’s prototypical behaviour, any function can be used as a constructor call, which adds more fuel to the confusion. To demystify the process that occurs, this post will take you through the four operations that occur when the new keyword is invoked on a Function. After describing these steps, we will then work through codifying a user-land function that mimics its behaviour.

It creates a plain ol’ object.
It assigns the supplied Function’s prototype object property as the new object’s [[prototype]] link. All functions are created with a prototype object property which initially just has a constructor member that links back to the function itself.
It invokes the function constructor (with supplied arguments), binding this to the newly created object.
It checks to see if a non-primitive value has been returned from the function call. If so, it will disregard the created object and return that value; otherwise, the newly formed object is returned.

User-land Solution

Now that we know the basic steps that occur when the new keyword is called, we can create a user-land function that mimics this behaviour.

let isObj = o => typeof o == 'object' || typeof o == 'function';

function new_(cons, ...args) {
  let obj = Object.create(cons.prototype); // step 1 and 2
  let ret = cons.apply(obj, args); // step 3
  return isObj(ret) ? ret : obj; // step 4
}