webstorm: what does “Element is not exported” warning mean?

For me it worked to mark all “private” properties with a prefixed underscore. Obviously Webstorm/IntelliJ recognized the properties as something that should not be exported.

export class someOne {
  constructor(param) {
    this._test = param;
  }

  useTest(){
    return this._test;
  }
}


console.log(new someOne(123).useTest());