Javascript (Jquery), increment/decrement++/-- operator 计算错误

Question

我正在用左右两个按钮制作幻灯片，当单击按钮时，图像 src 发生变化，新图像出现。图像的位置存储在一个数组中，++ 运算符用于将照片移动到下一张，-- 用于上一张有时它不起作用我必须双击按钮工作。为什么会这样？

---

HTML

<div class="row">
        <div class="col-xs-1">
            <a href="" id="left"><img src="images/left.png" width="80px"    height="80px"></a>
        </div>
        <div class="col-xs-6">
            <img src="images/1.jpg" id="image">
        </div>
        <div class="col-xs-1">
            <a href="" id="right"><img src="images/right.png" width="80px"  height="80px"></a>
        </div>
    </div>

查询

var count = 1;

        var imagesLocation = ['images/1.jpg',
                              'images/2.jpg',
                              'images/3.jpg',
                              'images/4.jpg',
                              'images/5.jpg'];

        $('#right').on('click',function(e){
            e.preventDefault();
            if(count < 6){
                $('#image').attr('src',imagesLocation[count]);
                count++;
            }
        });

        $('#left').on('click',function(e){
            e.preventDefault();
            count--;
            $('#image').attr('src',imagesLocation[count]);

        });

Answer 1

数组索引从 0 开始，而不是 1

var count = 0;
//----------^--- update here

var imagesLocation = ['images/1.jpg',
  'images/2.jpg',
  'images/3.jpg',
  'images/4.jpg',
  'images/5.jpg'
];

$('#right').on('click', function(e) {
  e.preventDefault();
  if (count < imagesLocation.length) {
    //--------^----- .length can be use instead, so this will work even if array size changed
    $('#image').attr('src', imagesLocation[++count]);
    //-------------------------------------^--  ++count will increment the count, then returns the incremented count value    
  }
});

$('#left').on('click', function(e) {
  e.preventDefault();
  if (count > 0) {
    //------^---- to avoid negative index    
    $('#image').attr('src', imagesLocation[--count]);
    //-------------------------------------^--  --count will decrements the count, then returns the decremented count value
  }
});

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js"></script>
<div class="row">
  <div class="col-xs-1">
    <a href="" id="left">
      <img src="images/left.png" width="80px" height="80px">
    </a>
  </div>
  <div class="col-xs-6">
    <img src="images/1.jpg" id="image">
  </div>
  <div class="col-xs-1">
    <a href="" id="right">
      <img src="images/right.png" width="80px" height="80px">
    </a>
  </div>
</div>