我需要了解此代码的工作原理，“ this”的用法

Question

有人可以帮我理解这段代码，this.fn和this._fnSuccess的工作原理，它们在任何地方都没有定义

   class Swear {
    constructor(fn) {
    this.fn = fn;
  }



    _resolve(data) {
        this._fnSuccess(data);
      }



    then(fnSuccess) {
        this._fnSuccess = fnSuccess;
        this.fn(this._resolve.bind(this));
      }
    }

    const swear = new Swear((resolve) => {
      resolve('data');
    })
      .then(data => {
        console.log('swear', data);
      })