Lock Privileged Method in OOP Javascript

I just curious about this question in my head about lock/fixed method in oop javascript. This is the example :

function Counter() {
  this.count = 0;
  this.increaseCount = function() {
    this.count++;
  }
  this.displayCount = function() {
    console.log(this.count);
  }
}
var counter1 = new Counter();
counter1.increaseCount = function() {
  this.count += 2;
}
counter1.increaseCount();
counter1.displayCount(); // 2

How to lock/fixed method so it cannot be changed by instances? In example I want to lock increase method always add one.

Edit : I think its different from read-only properties, because it's property. I very curious about method lock/fixed in javascript. Even i use

Object.defineProperty(Counter.prototype, 'increaseCountt', {
  get: function() { this.count++; },
  writtable: false
});

i called it "counter1.increaseCountt" not "counter1.increaseCountt()" because it's not method.

When i used Object.freeze after creating instance like this:

var counter1 = new Counter();
Object.freeze(counter1);

the count result always zero.