Use firebug to view the details of the for loop

var container = document.getElementById ("container"),

     oDiv = container.getElementsByTagName("div"),  
     width = oDiv[0].offsetWidth;  
  for(var i = oDiv.length-1;i>=0;i--){    
    oDiv[i].style.left = (oDiv.length-1-i)*50+"px";    
  }  
  
  
  
  foroDiv[]left
  firebug
Mar.19,2021

chrome has a feature in its development tool called "Break on." You can see this menu item by right-clicking on the DOM node. The trigger condition of the breakpoint can be set to the deletion of the node, any change in the properties of the node, or a change in one of its child nodes.

Menu