private_transpile.js

class Test {
  constructor(){/*...*/}
  set(data){
    this.#data = data
  }
  get(){
    return this.#data
  }
}

// transpile to...
function testConstructor(){}
class Test {
  constructor(...a){
    let _data
    const instance = {
      set(data){
        _data = data
      },
      get(){
        return _data
      }
    }
    instance.__proto__ = Test.prototype
    testConstructor.apply(instance, a)
    
    return instance
  }
}


var test = new Test

console.log(1, test instanceof Test)
console.log(2, test.data)
console.log(3, test.get())
test.set('data')
console.log(4, test.get())