Error when trying to refer to a field by name_问答_开发者

Error when trying to refer to a field by name

开发者 https://www.devze.com 2023-01-02 17:12 出处：网络

I am getting an error (document.my_formm.fieldName.value is null or not an object) from the below code:

相关专题：javascript

I am getting an error (document.my_formm.fieldName.value is null or not an object) from the below code:

<html>
  <head>
    <title>(Type a title for your page here)</ti开发者_开发知识库tle>

    <script language=JavaScript>
    function check_length(my_formm,fieldName)
    {
      alert(fieldName);
      alert(document.my_formm.fieldName.value);
    }
    </script>
  </head>
  <body>
    <form name=my_form method=post>
      <input type="text" onChange=check_length("my_form","my_text"); name=my_text rows=4 cols=30 value="">
      <br>
      <input size=1 value=50 name=text_num> Characters Left
    </form>
  </body>
</html>

Your check_length function is using variables to identify the form and field names, however, by using dot notation, you are referring to a element of document named my_formm. When you are are using variable names, you should use the bracket notation instead:

function check_length(my_formm,fieldName)
{
  alert(fieldName); 
  alert(document[my_formm][fieldName].value); 
}

Also, you should really quote attributes in your input:

<input type="text" onKeyPress="checkCompanyName();" onChange="check_length('my_form', 'my_text');" name="my_text" rows="4" cols="30" value="">

In your javascript you have referred to the form as 'my_formm' i.e. you have an extra 'm' at the end which is not present in the HTML, this could be your problem.

Why does your JavaScript method take in that first parameter if it never uses it?

Just do onChange=check_length(this)

and in your function

function check_length(element)
     {
       // element points to the element in question
       // element.form points to the form if you need it
       alert(element.value); 
     }

In general it would be nice to write WHAT error are you getting...

anyhow checkCompanyName is not defined in the code you wrote.

Also you're passing two strings as variable, they do not have properties...

A better way to do this:

<script type="text/javascript">
function checkLength()
    {
    inp = document.getElementById("myInput");
    len = document.getElementById("len");
    len.value = inp.value.length;
    }
</script>

<input id="myInput" onkeyup="checkLength()" />
<input id="len" />

EDIT AFTER COMMENT:

<script type="text/javascript">
function checkLength(inputname, lenname)
    {
    inp = document.getElementById(inputname);
    len = document.getElementById(lenname);
    len.value = inp.value.length;
    }
</script>

<input id="myInput" onkeyup="checkLength('myInput', 'len')" />
<input id="len" />

document.my_formm looks for a form named my_formm. You need to use the associative array sintax instead, like document[my_formm], which will pass the value in my_formm at runtime, rather than looking for a property in the document object called my_formm (which doesn't exist).