浅谈JavaScript_DOM学习篇_图片切换小案例

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今天开始学习DOM操作,下面写一个小案例来巩固下知识点.

DOM: document object model (文档对象模型)

根据id获取页面元素 :　如:　var xx = document.getElementById("id");

根据标签获取元素:　如:　var xx = document.getElementsByTagName("div");

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>Title</title>
  <style>
    * {
      margin: 0;
      padding: 0;
    }

    #outer {
      width: 500px;
      /*设置上边距50px 水平居中*/
      margin: 50px auto;
      /*设置边框*/
      padding: 10px;
      background-color: greenyellow;
      /*设置文本居中*/
      text-align: center;
    }

    img {
      width: 500px;
    }
  </style>


  <script>
　　　　　//btn 为按钮id  clickEventFunction 为点击后执行的操作函数
    function addClick(btn, clickEventFunction) {
      var myButton = document.getElementById(btn);
      myButton.onclick = clickEventFunction;
    };

    window.onload = function () {
      (function () {
        var pics = ["imgs/1.png",
          "imgs/2.png",
          "imgs/3.png"];
        var index = 0;
        showPicNum(index);
        var img = document.getElementsByTagName("img")[0];

        // var btn1 = document.getElementById("btn1");
        var btn2 = document.getElementById("btn2");
        addClick("btn1", function () {
          index--;
          if (index <= -1) {
            index = pics.length - 1;
          }
          console.log(index + " ----- ");
          img.src = pics[index];
          showPicNum(index);
        });
        addClick("btn2", function () {
          index++;
          if (index >= pics.length) {
            index = 0;
          }
          console.log(index + " ++++++++ ");
          img.src = pics[index];
          showPicNum(index);
        });

        //
        // btn1.onclick = function () {
        //   index --;
        //   if(index <= -1){
        //     index = pics.length - 1;
        //   }
        //   console.log(index + " ----- ");
        //   img.src = pics[index];
        //   showPicNum(index);
        // };
        // btn2.onclick = function () {
        //   index ++;
        //   if(index >= pics.length){
        //     index = 0;
        //   }
        //   console.log(index + " ++++++++ ");
        //   img.src = pics[index];
        //   showPicNum(index);
        // };
        console.log(index);

        /**
         * 展示当前图片为第几张
         * @param index  当前图片索引
         */
        function showPicNum(index) {
          var descrs = document.getElementById("discs");
          descrs.innerText = "一共" + pics.length + "张图片,当前第" + ++index + "张";
        }
      }())
    };
  </script>
</head>
<body>
<div id="outer">
  <p id="discs"></p>
  <img src="imgs/1.png"/><br>
  <button id="btn1" type="button" value="上一张">上一张</button>
  <button id="btn2" type="button" value="下一张">下一张</button>
</div>
</body>
</html>

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