如何为JavaScript循环输出的每个结果添加唯一序号？

• 循环索引i从0开始，要生成从1起始的编号，直接用i + 1替换代码里的???即可。
• 每次点击提交时，先重置结果变量r为空字符串，避免多次提交后内容重复叠加。

修改后的完整代码：

<html>
 
<body style="text-align: center;">
 
    <h2 id="hi">Enter Your Five Favorite Cities</h2>
    <form class="" action="index.html">
        <input type="text" name="favoriteCities[]" value="" /><br>
        <br>
        <input type="text" name="favoriteCities[]" value="" /><br>
        <br>
        <input type="text" name="favoriteCities[]" value="" /><br>
        <br>
        <input type="text" name="favoriteCities[]" value="" /><br>
        <br>
        <input type="text" name="favoriteCities[]" value="" /><br>
    <br>
        <button type="button" name="button" onclick="favoriteCities()">Submit</button>
   
    </form>
 
    <h3 id="hi">Results</h3>
 
    <p id="output"></p>
    
    <script type="text/javascript">
        function favoriteCities() {
            var r = ""; // 每次调用时重置为空
            var input = document.getElementsByName('favoriteCities[]');
 
            for (var i = 0; i < input.length; i++) {
                var a = input[i];
                r = r + "City #" + (i + 1) + " is " + a.value + "<br>";
            }
            document.getElementById("output").innerHTML = r;
        }
    </script>
    
    
</body>
 
</html>

内容的提问来源于stack exchange，提问作者machinegod