函数内的javascript优先级

我试图先打开覆盖层，然后再打开警报框。即使我在覆盖框之后编写了警报框，它也是第一个出现的。所以我需要知道我的代码中存在什么问题

<!DOCTYPE html>
<html>
  <style>
        body {
            margin: 0;
            font-family: 'Lato', sans-serif;
        }            
        .overlay {
            height: 100%;
            width: 0;
            position: fixed;
            z-index: 1;
            top: 0;
            left: 0;
            background-color: rgb(0,0,0);
            background-color: rgba(0,0,0, 0.9);
            overflow-x: hidden;
            transition: 0.5s;
        }            
        .overlay-content {
            position: relative;
            top: 25%;
            width: 100%;
            text-align: center;
            margin-top: 30px;
        }            
        .overlay a {
            padding: 8px;
            text-decoration: none;
            font-size: 36px;
            color: #818181;
            display: block;
            transition: 0.3s;
        }            
        .overlay a:hover, .overlay a:focus {
            color: #f1f1f1;
        }            
        .overlay .closebtn {
            position: absolute;
            top: 20px;
            right: 45px;
            font-size: 60px;
        }            
        @media screen and (max-height: 450px) {
          .overlay a {font-size: 20px}
          .overlay .closebtn {
            font-size: 40px;
            top: 15px;
            right: 35px;
          }
        }
        </style>
        <body>

        <div id="myNav" class="overlay">
          <a href="javascript:void(0)" class="closebtn" onclick="closeNav()">&times;</a>
          <div class="overlay-content">            
          </div>
        </div>

        <h2>Fullscreen Overlay Nav Example</h2>
        <p>Click on the element below to open the fullscreen overlay navigation menu.</p>
        <p>In this example, the navigation menu will slide in, from left to right:</p>
        <span style="font-size:30px;cursor:pointer" onclick="openNav()">&#9776; open</span>

        <script>
        function openNav() {    
          document.getElementById("myNav").style.width = "100%";
          alert("hello");
        }

        function closeNav() {
            document.getElementById("myNav").style.width = "0%";
        }            
        </script>                 
        </body>
        </html>

最佳答案

你的 JavaScript 代码不会在你的

处等待

document.getElementById("myNav").style.width = "100%";

发生 css 转换时的行。它将立即开始执行接下来的任何 JavaScript 行。

如果您希望在转换后发生操作，您需要为 transitionend 添加事件监听器(或者 animationend 如果使用关键帧动画)。

因此，如果您希望在转换后发生警报，您需要将代码更改为类似的内容

var nav = document.getElementById("myNav");
nav.addEventListener("transitionend",function(){
     //this callback is called when transition ends
     alert("hello");
});
nav.style.width = "100%";

演示

var nav = document.getElementById("myNav");
var btn = document.querySelector("button");

function openNav() {

  nav.addEventListener("transitionend", function() {
    //this callback is called when transition ends
    alert("hello");
  });
  nav.style.width = "100%";
}

btn.addEventListener("click", openNav);

#myNav {
  display: block;
  width: 0%;
  height: 100px;
  background: grey;
  transition: width 1s;
}

<div id="myNav">
</div>

<button>Open</button>

关于函数内的javascript优先级，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/39813941/

函数内的javascript优先级

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