Почему анимация проходит моментально?

let animateBackground = null; 
let speed = 0.05; 
(function() { 
 
  //Create instance 
  this.Modal = function() { 
 
    //Default options 
    var defaults = { 
      popupSelector: null, 
      openButtonSelector: null, 
      animate: false, 
      castumazePopup: false, 
      overlayClass: false, 
      popupClass: false, 
    }; 
 
    //Extend default properties 
    if (arguments[0] && typeof arguments[0] === 'object') { 
      this.options = extendDefaults(defaults, arguments[0]) 
    }; 
 
    //Default UI elements 
    this.openButton = document.querySelector(this.options.openButtonSelector); 
    this.popUp = document.querySelector(this.options.popupSelector); 
    if (this.options.closeButton) { 
      this.closeButton = document.querySelector(this.options.closeButton); 
    } 
 
    //Initialize popup 
    this.init(); 
 
 
 
  }; 
 
 
 
  //------------Publick methods----------------// 
 
  Modal.prototype.init = function() { 
    this.popUp.style.display = 'none'; 
    this.openButton.addEventListener('click', this.open.bind(this)); 
 
    if (typeof this.popupClass === 'string') { 
      this.popUp.classList.add(this.popupClass); 
    } else { 
      var defaultStyles = { 
        padding: '10px', 
        position: 'absolute', 
        left: '50%', 
        top: '50%', 
        transform: 'translate(-50%, -50%)', 
        backgroundColor: 'white', 
        zIndex: '999', 
      } 
 
      for (property in defaultStyles) { 
        this.popUp.style[property] = defaultStyles[property]; 
      } 
    } 
  } 
 
  Modal.prototype.open = function() { 
 
    initializeEvents.call(this); 
 
    this.addOverlay(); 
 
    computedStyles = getComputedStyle(this.popUp); 
    if (computedStyles.display == 'none') { 
      this.popUp.style.display = 'block' 
    } 
  }; 
 
  Modal.prototype.close = function() { 
 
    this.deleteOverlay(); 
 
    computedStyles = getComputedStyle(this.popUp); 
    if (computedStyles.display == 'block') { 
      this.popUp.style.display = 'none' 
    } 
  } 
 
  Modal.prototype.addOverlay = function() { 
    if (this.options.overlay === true) { 
      this.overlay = document.createElement('div'); 
      this.overlay.classList.add('uzi-popup-overlay'); 
      //Style options 
      if (typeof this.options.overlayClass === 'string') { 
        this.overlay.classList.add(this.options.overlayClass); 
      } else { 
        var defaultStyles = { 
          width: '100%', 
          height: '100%', 
          position: 'fixed', 
          top: '0', 
          left: '0', 
          backgroundColor: 'rgba(0, 0, 0, 0.5)', 
          zIndex: '998', 
        } 
 
        for (property in defaultStyles) { 
          this.overlay.style[property] = defaultStyles[property]; 
        } 
      } 
      //Animation 
      if (this.options.overlayAnimation === true) { 
        this.overlay.style.opacity = '0'; 
        this.popUp.parentNode.insertBefore(this.overlay, this.popUp); 
        animateBackground = setInterval(backgroundFade, 10, this.overlay, 'show'); 
      } else { 
        this.popUp.parentNode.insertBefore(this.overlay, this.popUp); 
      } 
 
 
    } 
 
    if (this.overlay) { 
      this.overlay.addEventListener('click', this.close.bind(this)); 
    } 
  } 
 
  Modal.prototype.deleteOverlay = function() { 
    if (this.overlay) { 
      this.overlay.parentNode.removeChild(this.overlay); 
    } 
  } 
 
 
  //------------Private methods----------------// 
 
  //Extend default options function 
  function extendDefaults(source, optionsArr) { 
    var property; 
    for (property in optionsArr) { 
      if (optionsArr.hasOwnProperty(property)) { 
        source[property] = optionsArr[property]; 
      }; 
    }; 
    return source; 
  }; 
 
  //AddEventListeners 
  function initializeEvents() { 
 
    if (this.closeButton) { 
      this.closeButton.addEventListener('click', this.close.bind(this)); 
    }; 
 
 
 
 
  }; 
 
  function buildOut() { 
 
 
  } 
 
 
 
}()); 
 
function backgroundFade(target, type) { 
  let op = Number(target.style.opacity); 
  if (op < 0 || op > 1) { 
    clearInterval(animateBackground); 
  } 
  if (type == 'show') { 
    op += speed; 
  } else if (type == 'hide') { 
    op -= speed; 
  } 
  target.style.opacity = String(op); 
} 
 
 
new Modal({ 
  popupSelector: '.popup', 
  popupClass: false, 
  openButtonSelector: 'button', 
  closeButton: '.close', 
  overlay: true, 
  overlayAnimation: true, 
})

/*Background-animation*/ 
 
.uzi-popup-bganimate { 
  opacity: 0; 
} 
 
body { 
  background-color: red; 
}

<div class="popup"> 
  asdasd asd 
 
  <div class="close">close</div> 
</div> 
<button>button</button>