Question

在此程序中 - 页面应每2秒更新一次但是跑步被 - Uncaught TypeError: Cannot set property 'value' of undefined打断了在这一行 - document.formal.input.value="Count: " + counter;
为什么会这样？有什么问题？

代码：

<html>
    <head>
        <title>Waiting example</title>
            <script>
                var counter=0;
                // call function after evty 2 sec
                id = window.setTimeout("Update();", 2000);

                function Update() {
                    counter++;
                    window.status="Count " + counter;
                    document.formal.input.value="Count: " + counter;
                    // waiting after next value
                    id=window.setTimeout("Update();", 2000);
                }               
            </script>
    </head>
        <body>
            <h1>Waiting Example</h1>
            <hr>
                Value in line status & page are update evry 2 sec.
                Click the button Reset for launch counter from zero, and on stop for stop counter.
            <hr>
                <form name="formal">
                <input type="text" name="input1" size="40"><br>
                <input type="button" value="RESET" onClick="counter=0;"><br>
                <input type="button" value="STOP" onClick="window.clearTimeout(id);"><br>
            <hr>
        </body>
</html>

问题：

如何解决这个问题？
为什么会这样？

Answer 1

使用

document.formal.input1.value="Count: " + counter;

的 LIVE DEMO

Answer 2

请改用setInterval。

var counter=0;
// call function after evty 2 sec
id = window.setInterval(Update, 2000);

function Update() {
     counter++;
     window.status="Count " + counter;
     document.formal.elements["input1"].value="Count: " + counter;
}

示例：http://jsfiddle.net/bLet2/

Answer 3

我解决了你的麻烦。将行更改为此 - document.form1.input1.value
这是代码：

<html>
    <head><title>Timeout Example</title>
        <script>
            var counter = 0;
            // call Update function in 2 seconds after first load
            ID=window.setTimeout("Update();",2000);
            function Update() {
               counter++;
               window.status="The counter is now at " + counter;
               document.form1.input1.value="The counter is now at " + counter;
            // set another timeout for the next count
               ID=window.setTimeout("Update();",2000);
            }
        </script>
    </head>
        <body>
            <h1>Timeout Example</h1>
            <hr><p>
                The text value below and the status line are being updated every two seconds.
                Press the RESET button to restart the count, or the STOP button to stop it.
            </p><hr>
            <form NAME="form1">
                <input TYPE="text" NAME="input1" SIZE="40"><br>
                <input TYPE="button" VALUE="RESET" onClick="counter = 0;"><br>
                <input TYPE="button" VALUE="STOP" onClick="window.clearTimeout(ID);">
            </form>
            <hr>
        </body>
</html>

每2秒更新一次页面 - 未捕获的TypeError

3 个答案: