Javascript document.getElementById似乎不起作用

Question

Javascript document.getElementById似乎不起作用

我有以下代码,并且chrome中的javascript控制台说"无法读取null的innerHTML属性.为什么document.getElementById('display')空出来？

<!DOCTYPE html>
<html>
    <head>
        <title>Chat 3</title>
        <script type="text/javascript">
            function showmsg(str){
                var display = document.getElementById('display');
                display.innerHTML += "<p>" + str + "</p>";
            }

            if("WebSockets" in window){
                pass;
            }else{
                showmsg("Your browser doesn't support WebSockets. Try Google Chrome.");
            }
        </script>

        <style type="text/css">
        #username { 
            padding: 0px;
            margin: 0px;
            height: 26px;
            width: 400px;
        }

        /* ADDED container div that wraps onlineusers and display */
        #container {
          margin: 10px 0;
        }

        /* use float: left to put them side-by-side */
        #display { 
            padding: 0px;
            margin: 0px;
            border-style: solid;
            border-width: 1px;
            overflow: auto;
            height: 400px;
            width: 400px;
            float: left;
        }

        #onlineusers { 
            padding: 0px;
            margin: 0px;
            height: 400px;
            width: 200px;
            border-style: solid;
            border-width: 1px;
            float: left;
        }

        /* Added container2 to wrap inputline and sendbutton */
        #container2 {
            margin: 10px 0;
        }

        #inputline { 
            padding: 0px;
            margin: 0px;
            height: 26px;
            width: 350px;
            float: left;
        }

        #sendbutton {
            padding: 0px;
            margin: 0px;
            height: 30px;
            width: 50px;
            float: left;
        }

        /* this is a well used "hack". */
        .clearfix {
          clear: both;
        }
        </style>
    </head>
    <body onload="document.getElementById("username").focus()">
            <input type="text" id="username" />
            <div id="container">
                <div id="display"></div>
                <div id="onlineusers"></div>
                <div class="clearfix"></div>
            </div>

            <div id="container2">
                <input type="text" id="inputline" length="55" />
                <input type="button" id="sendbutton" value="Send" />
            </div>
    </body>
</html>

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Answer 1

Ger*_*hof 7

将脚本部分向下移动到页面底部.否则,它将在页面加载之前执行.

顺便说一句:将脚本放在页面底部甚至是"最佳实践",并由Google,Yahoo&Co推荐

Answer 2

Ama*_*osh 6

你在showmsg整个页面(因此displaydiv)加载之前调用.因此错误.打电话给它onload,它会工作.

将此功能添加到 <head>

function handleLoad()
{
  document.getElementById("username").focus()
  if("WebSockets" in window){
   pass;
  }else{
    showmsg("Your browser doesn't support WebSockets. Try Google Chrome.");
  }
}

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并从中调用它 onload

<body onload="handleLoad()">

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