How make object property private but accesible for inner methods

Question

Let's say I have an object that contains a string and a function that does something with that string. But I want that string to remain private.

let object = {
   someData: 'test', // this is public
   modifyData: function() {
      return this.someData + ' needs to be private';
    }
}

I want to have the property someData private from outside but accesible for the method modifyData (or further new methods if added at some point).