JSFiddle - React, Tailwind, and code Playground

JavaScript

function BaseClass(arg1,arg2,arg3) {
    var privateVar = BaseClass.privateVar;

    this.getPrivateVar = function () {
        return privateVar;
    };
}

BaseClass.privateVar = 7500;

function SubClass(arg1, arg2, arg3, privateVar) {
    var args = Array.prototype.slice.call(arguments, 0, 3); // the first 3 args
    var defaultPrivateVar = BaseClass.privateVar;           // save the old value
    BaseClass.privateVar = privateVar;                      // set a new default
    BaseClass.call(this, args);                             // call super
    BaseClass.privateVar = defaultPrivateVar;               // restore old value
}

SubClass.prototype = Object.create(BaseClass.prototype);    // inherit new way

var obj = new SubClass(1, 2, 3, 2500);

alert(obj.getPrivateVar() === 2500 ? "It works!" : "It doesn't work!");