style attribute

1. DOM cannot manipulate CSS files directly, indirectly by manipulating style attributes of elements

   var oDiv = document.getElementsByTagName('div')[0];
   oDiv.style.width = '100px';
   oDiv.style.backgroundColor = 'red';
   // Style attribute is an inline style and cannot derive the contents of a CSS file definition
   // If there is no corresponding inline style, an empty string is returned
   

2. Be careful

   • Attribute using hump notation
   • Assignment uses string
   • Composite values are best disassembled to improve performance, such as border
   • style.float uses style.cssFloat instead of conflict-free reserved words

getComputedStyle method

1. getComputedStyle under window, returns the style attributes that the element takes effect

   window.getComputedStyle(elem, null);
   var width = window.getComputedStyle(div, null)['width']; // Return string
   window.getComputedStyle(elem, 'after'); // Returns the style attributes of the after pseudo element
   

2. The data returned by getComputedStyle is converted to specific units, such as em to px, hexadecimal color to rgb/rgba, etc.

3. getComputedStyle is not supported in IE8 and below and can be replaced with elem.currentStyle

   function getStyles(elem) {
       if (window.getComputedStyle {
           return window.getComputedStyle(elem, null);
       } else return elem.currentStyle;
   }
   

offsetWidth property

1. offsetWidth, offsetHeight can get the physical size of an element

   var oDiv = document.getElementsByTagName('div')[0];
   oDiv.offsetWidth; 
   oDiv.offsetHeight; 
   // offsetWidth = border + padding + width
   

2. offsetWidth, offsetHeight include padding, which is inconvenient for some development scenarios. It is best to use the getComputedStyle method

Pseudo Element Style

1. :: berfore,:: after pseudo-element style cannot be directly changed by method

2. Changing the style of pseudo elements is usually done by modifying the clssName of the associated element

   .box1::after{
       content: "";
       background-color: red;
   }
   .box2::after{
       content: "";
       background-color: black;
   }
   

   var oDiv = document.getElementsByClassName('box1')[0];
   oDiv.className = 'box2';
   // The style of the after pseudo element also changes
   

Style Animation

1. Elemental Motion

   Change an element's display content by changing its style properties, such as drop-down menus, side drawers, pop-up information boxes, and so on

   We often use CSS3 or some encapsulated frames to animate, and animation effects can provide a better interactive experience for the page

2. Style animation with native JS

   • Get the element node to move
   • Style properties that are explicitly to be changed
   • Determine animation time, animation speed, and animation termination conditions
   • Create timer, clear timer under termination condition

3. Example drop-down menu

   html

   <div class="dropdown">
       <a href="javascript:;" class="main">drop-down menu</a>
       <ul class="list">
           <li>1</li>
           <li>2</li>
           <li>3</li>
           <li>4</li>
       </ul>
   </div>
   <script type="text/javascript">
   
       var dropdown = document.getElementsByClassName('dropdown')[0],
           oList = dropdown.getElementsByClassName('list')[0],
           timer = null,
           listHeight = 0,
           speed = 2;
   
       dropdown.onmouseenter = function () {
           clearInterval(timer);
           timer = setInterval(function () {
               if (listHeight >= 200) {
                   clearInterval(timer);
               } else {
                   listHeight = parseInt(getStyles(oList)['height']) + speed;
                   oList.style.height = listHeight + 'px';
               }
           }, 1);
       }
   
       dropdown.onmouseleave = function () {
           clearInterval(timer);
           timer = setInterval(function () {
               if (listHeight <= 0) {
                   clearInterval(timer);
               } else {
                   listHeight = parseInt(getStyles(oList)['height']) - speed;
                   oList.style.height = listHeight + 'px';
               }
           }, 1);
       }
   
       function getStyles(elem) { 
           if (window.getComputedStyle) {
               return window.getComputedStyle(elem, null);
           } else return elem.currentStyle;
       }
   </script>
   

   css

   <style>
           .dropdown {
               width: 100px;
           }
   
           .dropdown .main {
               display: block;
               height: 50px;
               background-color: black;
               color: white;
               text-align: center;
               text-decoration: none;
               line-height: 50px;
           }
   
           .dropdown .list {
               overflow: hidden;
               height: 0;
               margin: 0;
               padding: 0;
               list-style: none;
           }
   
           .list li {
               height: 50px;
               background-color: #999;
               text-align: center;
               line-height: 50px;
           }
   
           .dropdown li:hover {
               background-color: black;
               color: white;
           }
       </style>