Make a property that is read-only to the outside world, but my methods can still set

Question

In JavaScript (ES5+), I\'m trying to achieve the following scenario:

1. An object (of which there will be many separate instances) each with a read-only property

Answer 1

I've been doing this lately:

// File-scope tag to keep the setters private.
class PrivateTag {}
const prv = new PrivateTag();

// Convenience helper to set the size field of a Foo instance.
function setSize(foo, size)
{
  Object.getOwnPropertyDiscriptor(foo, 'size').set(size, prv);
}

export default class Foo
{
  constructor()
  {
    let m_size = 0;
    Object.defineProperty(
      this, 'size',
      {
        enumerable: true,
        get: () => { return m_size; },
        set: (newSize, tag = undefined) =>
        {
          // Ignore non-private calls to the setter.
          if (tag instanceof PrivateTag)
          {
            m_size = newSize;
          }
        }
      });
  }

  someFunc()
  {
    // Do some work that changes the size to 1234...
    setSize(this, 1234);
  }      
}

I think that covers all of the OP's points. I haven't done any performance profiling. For my use cases, correctness is more important.

Thoughts?