Problem

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class Car {
  _milesDriven = 0
  drive(distance) {
    this._milesDriven += distance
  }
  getMilesDriven() {
    return this._milesDriven
  }
}
Solution

Historically - in JavaScript, because we lacked the ability to have private class fields, we’d mark them with an underscore and hope for the best. Now, thanks to recently added Private Class features, we can now mark class fields as private by using the # prefix.

class Car {
  #milesDriven = 0
  drive(distance) {
    this.#milesDriven += distance
  }
  getMilesDriven() {
    return this.#milesDriven
  }
}

const tesla = new Car()
tesla.drive(10)
tesla.getMilesDriven() // 10
tesla.milesDriven // undefined