class LoadMethod{
  static get LOAD_IMG_FINISH(){
    return "load_img_finish";
  }
  static init(sourceList){
    let img=new Image();
    img.num=0;
    img.sourceList=sourceList;
    img.targetList={};
    img.addEventListener("load",LoadMethod.loadHandler);
    img.src=sourceList[0];
 
  }
  static loadHandler(e){
    let images=this.cloneNode(false);
    let name=this.sourceList[this.num];
    name=name.slice(name.lastIndexOf("/")+1,name.lastIndexOf("."));
    this.targetList[name]={src:images.src,elem:images,width:images.width,height:images.height};
    this.num++;
    if(this.num>this.sourceList.length-1){
      this.removeEventListener("load",LoadMethod.loadHandler);
      let evt=new Event(LoadMethod.LOAD_IMG_FINISH);
      evt.targetList=this.targetList;
      document.dispatchEvent(evt);
      return;
    }
    this.src=this.sourceList[this.num];
 
  }
}
class Drag{
  static dragElem(elem,rect,parent){
    Drag.drageHandler=Drag.mouseHandler.bind(elem);
    elem.rect=rect;
    elem.parent=parent;
    elem.addEventListener("mousedown",Drag.drageHandler);
  }
  static removeDrag(elem){
    elem.removeEventListener("mousedown",Drag.drageHandler);
    Drag.drageHandler=null;
  }
  static mouseHandler(e){
    if(e.type==="mousedown"){
      this.addEventListener("mouseup",Drag.drageHandler);
      this.offsetPoint={x:e.offsetX,y:e.offsetY};
      document.addEventListener("mousemove",Drag.drageHandler);
    }else if(e.type==="mousemove"){
      if(!this.rect){
        this.rect=this.parent.getBoundingClientRect();
      }
      let obj={
        left:e.x-this.offsetPoint.x-this.rect.left+"px",
        top:e.y-this.offsetPoint.y-this.rect.top+"px",
        position:"absolute"
      };
      Object.assign(this.style,obj);
      let elemRect=this.getBoundingClientRect();
      if(elemRect.left<this.rect.left){
        this.style.left="0px";
      }
      if(elemRect.right>this.rect.right){
        this.style.left=this.rect.right-elemRect.width-this.rect.left+"px";
      }
      if(elemRect.top<this.rect.top){
        this.style.top="0px";
      }
      if(elemRect.bottom>this.rect.bottom){
        this.style.top=this.rect.bottom-elemRect.height-this.rect.top+"px";
      }
      let evt=new Event(Drag.ELEM_MOVE_EVENT);
      evt.point={x:this.offsetLeft,y:this.offsetTop};
      this.dispatchEvent(evt);
    }else if(e.type==="mouseup"){
      this.removeEventListener("mouseup",Drag.drageHandler);
      document.removeEventListener("mousemove",Drag.drageHandler);
    }
  }
  static get ELEM_MOVE_EVENT(){
    return "elem_move_event";
  }
}

class ZoomClasses{
  constructor(panrent){
    this.bindHandler=this.loadFinishHandler.bind(this);
    document.addEventListener(LoadMethod.LOAD_IMG_FINISH,this.bindHandler);
    this.zoomView=this.createView();
    panrent.appendChild(this.zoomView);
  }
  createView(){
    if(this.zoomView) return this.zoomView;
    let div=document.createElement("div");
    this.minDiv=document.createElement("div");
    this.maxDiv=document.createElement("div");
    this.dragDiv=document.createElement("div");
    Object.assign(div.style,{
      position:"relative",
    });
    this.minDiv.appendChild(this.dragDiv);
    div.appendChild(this.minDiv);
    div.appendChild(this.maxDiv);
    this.dragDiv.addEventListener(Drag.ELEM_MOVE_EVENT,this.moveHandler.bind(this));
    Drag.dragElem(this.dragDiv,null,this.minDiv);
    this.minDiv.style.float=this.maxDiv.style.float="left";
    return div;
  }
  set width(value){
    this._width=value;
    this.render();
  }
  get width(){
    if(!this._width) this._width=0;
    return this._width;
  }
  set height(value){
    this._height=value;
    this.render();
  }
  get height(){
    if(!this._height) this._height=0;
    return this._height;
  }
  set imgSource(value){
    if(!Array.isArray(value))return;
    this._imgSource=value;
    LoadMethod.init(value);
  }
  set left(value){
    this.zoomView.style.left=value+"px";
  }
  set top(value){
    this.zoomView.style.top=value+"px";
  }
  loadFinishHandler(e){
    this.targetList=e.targetList;
    this.width=this.width || e.targetList["min"].width;
    this.height=this.height || e.targetList["min"].height;
 
  }
  moveHandler(e){
    if(!this.targetList || this.targetList.length<2) return;
    let widthScale=this.targetList["min"].width/this.targetList["max"].width;
    let heightScale=this.targetList["min"].height/this.targetList["max"].height;
    Object.assign(this.maxDiv.style,{
      backgroundPositionX:-e.point.x/widthScale+"px",
      backgroundPositionY:-e.point.y/widthScale+"px",
    });
  }
  render(){
    if(!this.targetList || this.targetList.length<2) return;
    Object.assign(this.minDiv.style,{
      width:this.width+"px",
      height:this.height+"px",
 
      border:"1px solid #000000",
 
      backgroundImage:`url(${this.targetList["min"].src})`,
      position:"relative"
    });
    Object.assign(this.maxDiv.style,{
      width:this.width+"px",
      height:this.height+"px",
      backgroundImage:`url(${this.targetList["max"].src})`,
      position:"relative"
    });
    let widthScale=this.targetList["min"].width/this.targetList["max"].width;
    let heightScale=this.targetList["min"].height/this.targetList["max"].height;
    Object.assign(this.dragDiv.style,{
      width:this.width*widthScale+"px",
      height:this.height*heightScale+"px",
      backgroundColor:"rgba(255,0,0,0.2)",
      position:"absolute"
    })
 
  }
}

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>Title</title>
  <script src="js/LoadMethod.js"></script>
  <script src="js/ZoomClasses.js"></script>
</head>
<body>
  <script>
    let zoom=new ZoomClasses(document.body);
    zoom.imgSource=["img/max.jpg","img/min.jpg"];
 
  </script>
</body>
</html>