How to get client id at external javascript file

When I use embedded javascript functions I can get client id of elements with this code:

document.getElementById('<%=buttonXXX.ClientID%>' )

But now I am using external javascript file for caching and faster rendering
and this code does not work any more for getting client id’s of elements, it gives error.

How can I get client id’s of elements at external javascript file using 2.0 , netframework 3.5 , c# , iis 7.5


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Method 1

I can suggest 2 ways.

First way

define your variables before call the javascript, inside the .aspx file that can be compiled.

var ButtonXXXID = <%=buttonXXX.ClientID%>
// and now include your javascript and use the variable ButtonXXXID

Second way

in the external javascript file, write your code as:

function oNameCls(ControlId1) {

    this.ControlId1 = ControlId1;

    this.DoYourWork1 = function() {
        // use the control id.
        // this.ControlId1

And call your actions like.
    // init - create
    var <%=this.ClientID%>MyCls = new oNameCls('<%=Control1.ClientID%>');
    // do your work

calling the action this way you prevent overwrite one action from one control with the same action from other controls on the same page.

Method 2

You could use a class selector. jquery might greatly simplify your life here. So you could apply a special class to the control:

<asp:LinkButton ID="foo" CssClass="foo" runat="server" Text="foo" />

and in your external javascript file once the DOM is ready you could reference the button using a class selector:
$(function() {
    var fooButton = $('.foo');

Method 3

in script file (test.js)

function test(ControlID1) {
  var controlId = document.getElementById(ControlID1);
  controlId.onchange = function () {        

in .aspx file
<script type="text/javascript">
   var callTest = test('<%=txtSelected.ClientID%>');
   window.onload = callTest;

Method 4

I use the following code in my .js file, when I have no other better choice.


Method 5

I know old question but with jquery u can use this approache:

Aspx file

<asp:Button ID="btnCalculate" ClientID="btnCalculate" runat="server" />

external js file

All methods was sourced from or, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0

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