How to use Javascript as OOPS for beginner

From what I understand of best practices in the JS world, one should not use the new keyword very often.

For this example, I would suggest this as an alternate approach:

Create a new script file arithmetic.js

var Arithmetic = function(){
  var obj = {
    add: function(a,b) { return a + b; },
    multiply: function(a,b) { return a * b; }
  };
  return obj;
}();

This is known as a self-executing function, and it will create a closure.
I won't go into how that works here, as there are several good articles around the web on the topic.

This should then let you do this;

var resultAdd = Arithmetic.add(a,b);
var resultMul = Arithmetic.multiply(a,b);

There will still be cases where new is useful to you, but I think that it might surprise you how seldom it actually is.