Use of .apply() with 'new' operator. Is this possible?

Question

In JavaScript, I want to create an object instance (via the new operator), but pass an arbitrary number of arguments to the constructor. Is this possible?

Answer 1

Yes we can, javascript is more of prototype inheritance in nature.

function Actor(name, age){
  this.name = name;
  this.age = age;
}

Actor.prototype.name = "unknown";
Actor.prototype.age = "unknown";

Actor.prototype.getName = function() {
    return this.name;
};

Actor.prototype.getAge = function() {
    return this.age;
};

when we create an object with "new" then our created object INHERITS getAge(), But if we used apply(...) or call(...) to call Actor, then we are passing an object for "this" but the object we pass WON'T inherit from Actor.prototype

unless, we directly pass apply or call Actor.prototype but then.... "this" would point to "Actor.prototype" and this.name would write to: Actor.prototype.name. Thus affecting all other objects created with Actor...since we overwrite the prototype rather than the instance

var rajini = new Actor('Rajinikanth', 31);
console.log(rajini);
console.log(rajini.getName());
console.log(rajini.getAge());

var kamal = new Actor('kamal', 18);
console.log(kamal);
console.log(kamal.getName());
console.log(kamal.getAge());

Let's try with apply

var vijay = Actor.apply(null, ["pandaram", 33]);
if (vijay === undefined) {
    console.log("Actor(....) didn't return anything 
           since we didn't call it with new");
}

var ajith = {};
Actor.apply(ajith, ['ajith', 25]);
console.log(ajith); //Object {name: "ajith", age: 25}
try {
    ajith.getName();
} catch (E) {
    console.log("Error since we didn't inherit ajith.prototype");
}
console.log(Actor.prototype.age); //Unknown
console.log(Actor.prototype.name); //Unknown

By passing Actor.prototype to Actor.call() as the first argument, when the Actor() function is ran, it executes this.name=name, Since "this" will point to Actor.prototype, this.name=name; means Actor.prototype.name=name;

var simbhu = Actor.apply(Actor.prototype, ['simbhu', 28]);
if (simbhu === undefined) {
    console.log("Still undefined since the function didn't return anything.");
}
console.log(Actor.prototype.age); //simbhu
console.log(Actor.prototype.name); //28

var copy = Actor.prototype;
var dhanush = Actor.apply(copy, ["dhanush", 11]);
console.log(dhanush);
console.log("But now we've corrupted Parent.prototype in order to inherit");
console.log(Actor.prototype.age); //11
console.log(Actor.prototype.name); //dhanush

Coming back to orginal question how to use new operator with apply, here is my take....

Function.prototype.new = function(){
    var constructor = this;
    function fn() {return constructor.apply(this, args)}
    var args = Array.prototype.slice.call(arguments);
    fn.prototype = this.prototype;
    return new fn
};

var thalaivar = Actor.new.apply(Parent, ["Thalaivar", 30]);
console.log(thalaivar);