Change updateCounterView to stand alone function
function Metronome(tempo, beatsPerMeasure){
  this.tempo = Number(tempo);
  this.beatsPerMeasure = Number(beatsPerMeasure);
  this.interval = null;
}

Metronome.prototype.start = function(){
  var millisecondsToWait = this.tempoToMilliseconds(this.tempo);
  this.interval = window.setInterval(updateCounterView, millisecondsToWait, this);
}

Metronome.prototype.tempoToMilliseconds = function(tempo){
  return (1000 * 60)/tempo;
}

updateCounterView = function(metronome){
  var counter = document.getElementById("metronome-counter");
  var pastBeat = Number(counter.innerHTML);
  if (pastBeat < metronome.beatsPerMeasure){
    counter.innerHTML = pastBeat + 1;
  } else {
    counter.innerHTML = 1;
  }
}   

Metronome.prototype.stop = function(){
  window.clearInterval(this.interval);
  counter.innerHTML = "";
}
function Metronome(tempo, beatsPerMeasure){
  this.tempo = Number(tempo);
  this.beatsPerMeasure = Number(beatsPerMeasure);
  this.interval = null;
}

Metronome.prototype.start = function(){
  var millisecondsToWait = this.tempoToMilliseconds(this.tempo);
  this.interval = window.setInterval(this.updateCounterView, millisecondsToWait, this.beatsPerMeasure);
}

Metronome.prototype.tempoToMilliseconds = function(tempo){
  return (1000 * 60)/tempo;
}

Metronome.prototype.updateCounterView = function(beatsPerMeasure){
  var counter = document.getElementById("metronome-counter");
  var pastBeat = Number(counter.innerHTML);
  if (pastBeat < beatsPerMeasure){
    counter.innerHTML = pastBeat + 1;
  } else {
    counter.innerHTML = 1;
  }
  

  Metronome.prototype.stop = function(){
    window.clearInterval(this.interval);
    counter.innerHTML = "";
  }
}