var cons = function () {
this.a = 1;
this.b = 2;
}
var obj = new cons();
cons.prototype.b = 3;
cons.prototype.c = 4;
1. create a new empty object; call it `newObj`
2. call the constructor using `newObj` as its `this` context, so that `this.a = 1` is effectively `newObj.a = 1`
3. if the constructor has a `prototype` property defined, invoke `newObj.__proto__ = myconstructor.prototype` to establish the prototype chain on the new object
Hopefully that all made sense!
__proto__ on objects was originally a leaky abstraction in a specific browser. It caught on such that other browsers borrowed it, because they unfortunately had to, but the specs and the browsers all consider it deprecated and warn not to use it: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Refe...
(The standards compliant way to get an object's prototype at runtime is Object.getPrototypeOf(obj), and to change an object's prototype instance at runtime is Object.setPrototypeOf(obj, newPrototype).)