Max Ba

<div id="gallery1">
  <input type="button" class="btn1" value="-">
  <input type="button" class="btn2" value="+">
  <div class="result">0</div>
</div>

<div id="gallery2">
  <input type="button" class="btn1" value="-">
  <input type="button" class="btn2" value="+">
  <div class="result">0</div>
</div>

function Slider(el){
    
this.main = document.getElementById(el.id);
this.speed = 0;
this.btn1 = this.main.getElementsByClassName('btn1')[0];
this.btn2 = this.main.getElementsByClassName('btn2')[0];
this.result = this.main.getElementsByClassName('result')[0];

this.updateResult = function(){
  this.result.innerHTML = this.speed;
};

this.method1 = function(){
  if(this.speed > 0){
    this.speed--;
  }
  this.updateResult();
};

this.method2 = function(){
  this.speed++;
  this.updateResult();
};

this.btn1.addEventListener('click', this.method1.bind(this));
this.btn2.addEventListener('click', this.method2.bind(this));

}

new Slider({
  id: "gallery1"
});

new Slider({
  id: "gallery2"
});