深入理解Javascript箭头函数中的this

function Countdown(seconds) {
 this._seconds = seconds;
}
Countdown.prototype._step = function() {
 console.log(this._seconds);
 if (this._seconds > 0) {
  this._seconds -= 1;
 } else {
  clearInterval(this._timer);
 }
};
Countdown.prototype.start = function() {
 this._step();
 this._timer = setInterval(function() {
  this._step();
 }, 1000);
};

new Countdown(10).start();

Countdown.prototype.start = function() {
 var self = this;
 this._step();
 this._timer = setInterval(function() {
  self._step();
 }, 1000);
};

Countdown.prototype.start = function() {
  this._step();
  this._timer = setInterval(function() {
    this._step();
  }.bind(this), 1000);
};

Countdown.prototype.start = function() {
  this._step();
  this._timer = setInterval(() => {
    this._step();
  }, 1000);
};