时间轴效果的Javascript不起作用

Question

I am trying to create a timeline that when you hover on a certain part, a display pops up displaying information about that specific part of the history. This seems like a simple task but I cannot get it to work. I am new to javascript and I cannot figure out what I am doing wrong. Any advice or help would be appreciated.

<div class="line"></div>
<div class="circle"> </div>
<div class="popup display"></div>

html { 
width: 100%;
height: 400px;
}

.line {
  width: 90%;
  height: 5px;
  background-color: black;
  position: absolute;
  top: 12%;
  left: 5%;
}

.circle {
  width: 20px;
  height: 20px;
  border-radius: 50%;
  background-color: blue;
  position: absolute;
  left: 50%;
  top: 10%;
}

.popup {
  position: absolute; 
  width: 70%;
  height: 400px;
  background-color: black;
  top: 70%;
  left: 50%;
  transform: translate(-50%,-50%);
}

.display {
  opacity: 0;
}


document.querySelector(".circle").addEventListener("mouseover", function(){
  document.querySelector(".popup").classList.remove("display");
});

Answer 1

See this fiddle here: http://jsfiddle.net/chr7p94q/

I added the mouseout so it goes away when you move the mouse off of the element.

document.querySelector(".circle").addEventListener("mouseout", function(){
  document.querySelector(".popup").classList.add("display");
});

Seems like the following for .display would behave the same way rather than using opacity :

.display {
  display: none;
}