Suppose I have a master page where I have written some javascript to access the value of a particular asp.net text box, which resides in one of its content pages, but not in all the pages. This piece of javascript code can't be moved to that particular content page because of some restrictions.
Now I need a way to determine whether or not that text box exists in the page, which will imply that the content page containing that text box has been loaded.
How I can I do that in javascript?
var productElement = document.getElementById("<%= YourTextBoxID.ClientID%>");
if (productElement != null)
{
// Code here when the Element Exists.
}
Hope that helps!
EDIT (after you clarification):
In your master page:
<% if (FindControl("YourTextBoxID") != null) {
//do smthng, you can add some jscript that should be executed, or something else you need
} %>
var elem = document.getElementById('myElement');
if(elem)
{
// Do stuff when elem exists
}
Anything that is undefined or null (in this case the element that doesn't exist) will equivocate to false. So, if it exists, it becomes true, and the if statement will run.
On client side, to access control, you need to use ClientID property of control in your content page
<script type="text/javascript">
var control = document.getElementById("<%=YourControl.ClientID%>")
</script>
On master page, your code should be,
<script type="text/javascript">
function IsTextBoxExist(){
if(control != null){
...
}
}
</script>
Be sure that IsTextBoxExist calls when the page loading complete. like
<body onload="isTextBoxExist()"
If you can use jQuery, you can use an "Attribute Contains Word" selector. Say your server-side text box id is 'text_box'. The client-side 'name' attribute will contain the server-side id (something like "ctl00$ctl00$text_box). So, to select it, use:
var textBox = $("input[name~=text_box]");
if (textBox.length != 0) {
// text box found - do something here
}
Without jQuery, you could iterate through the form.elements collection and test each name:
for (var i; i++; i < forms[0].elements.length) {
if (forms[0].elements[i].name.indexOf("text_box") != -1) {
// text box found - do something here
break;
}
}