c# - Inserting Javascript into ASP.Net page - Stack Overflow

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I have the code

<script type="text/javascript">
    function validateLength(objectSource, args) {
        var a = document.getElementById('<%=txtUserName.ClientID%>').value;
        args.IsValid = (a.length >= 5 && a.length <= 10);
    }
</script>

Which I have in my ASP.NET page.

This works fines. Will validate a textbox to make sure it is between a certain length.

However I am trying to tidy up my page's code. So decided to put this little function into a separate javascript file.

<script type="text/javascript" src="Scripts/MyJS.js">
</script>

Page loads fine and then when the validation es around it fails. I am guessing it cannot access the textbox( txtUserName).

My question is: How can I get this to work?

I get the error message

0x800a138f - Microsoft JScript runtime error: Unable to get value of the property 'value': object is null or undefined

when the javascript fires

I have the code

<script type="text/javascript">
    function validateLength(objectSource, args) {
        var a = document.getElementById('<%=txtUserName.ClientID%>').value;
        args.IsValid = (a.length >= 5 && a.length <= 10);
    }
</script>

Which I have in my ASP.NET page.

This works fines. Will validate a textbox to make sure it is between a certain length.

However I am trying to tidy up my page's code. So decided to put this little function into a separate javascript file.

<script type="text/javascript" src="Scripts/MyJS.js">
</script>

Page loads fine and then when the validation es around it fails. I am guessing it cannot access the textbox( txtUserName).

My question is: How can I get this to work?

I get the error message

0x800a138f - Microsoft JScript runtime error: Unable to get value of the property 'value': object is null or undefined

when the javascript fires

• javascript
• c#
• asp
• asp-4.5

Share Improve this question Follow edited Sep 10, 2015 at 15:01 Sam asked Sep 10, 2015 at 14:56 SamSam 94833 gold badges1313 silver badges3232 bronze badges 10

• 4 <script type="text/javascript" src"Scripts/MyJS.js"> -> <script type="text/javascript" src="Scripts/MyJS.js"> – messerbill Commented Sep 10, 2015 at 14:57
• yeah you forgot an = by src – johnny 5 Commented Sep 10, 2015 at 14:59
• @messerbill that was me forgetting while typing the code here. In my actual code there is an = – Sam Commented Sep 10, 2015 at 15:00
• document.getElementById('<%=txtUserName.ClientID%>') is not found / existent on your page. Have you spelled it correctly? – messerbill Commented Sep 10, 2015 at 15:05
• @messerbill the code works fine when the script is in the page itself. But when loading from a .js file it cannot find it – Sam Commented Sep 10, 2015 at 15:06

 |  Show 5 more ments

2 Answers 2

Sorted by: Reset to default 4

When you include a javascript file, the part of code

<%=txtUserName.ClientID%>

isn't rendered.

You have to change the javascript function in two ways Fixing the name of the control

<script type="text/javascript">
    function validateLength(objectSource, args) {
        var a = document.getElementById('txtUserName').value;
        args.IsValid = (a.length >= 5 && a.length <= 10);
    }
</script>

or adding a parameter

<script type="text/javascript">
    function validateLength(objectSource, args, nameOfControl) {
        var a = document.getElementById(nameOfControl).value;
        args.IsValid = (a.length >= 5 && a.length <= 10);
    }
</script>

then you can call the function from your page and the parameter nameOfControl could be

<%=txtUserName.ClientID%>

for example

validateLength(objectSource, args, <%=txtUserName.ClientID%>)

The <%=txtUserName.ClientID%> runs in context of your page. That will not work in a separate .js file. Separate .js files won't run any asp code. You could do something like:

JS file:

function validateLength(objectSource, args) {
    var a = document.getElementById(window.myValidatorId).value;
    args.IsValid = (a.length >= 5 && a.length <= 10);
}

ASP

<script type="text/javascript">
    window.myValidatorId = '<%=txtUserName.ClientID%>';
</script>

Or perhaps read the id from a hidden input field on the page.

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