Using server side code in JavaScript

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For some reason I want to implement code like

JavaScript

<script language="javascript" type="text/javascript">
    var value = <% TextBox1.Text %>;
   </script>

but its not working, I know one way to do it is like

JavaScript

var value = $('#<%= TextBox1.ClientID %>').val();

but I just want to know why the first one always return an empty value, any idea?

Posted 5-Jun-13 11:15am

alamz7

Updated 5-Jun-13 11:45am

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Richard C Bishop 5-Jun-13 17:20pm

The "<% %>" symbols signify classic ASP. You would need to use VBScript between those tags, not C# and that is provided you can even do that(I am not sure).

alamz7 5-Jun-13 17:36pm

well, <%= %> these tags are used for asp.net like the 2nd code snippet above works perfect, I'm just wonder why is the 1st code snippet not working ?

Richard C Bishop 5-Jun-13 17:38pm

Yes, that does work in ASP.Net, but notice the "#" in front. The tags without that signify classic ASP. Another characteristic that gives it aways is the fact that the tags automatically highlight in yellow.

alamz7 5-Jun-13 17:40pm

well, the # above is basically a jquery selector, and its not a part of the tags itself,

alamz7 5-Jun-13 17:38pm

Though one thing I dont know is, what's the difference between <%= %>, <%: %> and <%# %> as I often see them?

Sergey Alexandrovich Kryukov 5-Jun-13 17:42pm

How about just reading the documentation? It would give you the answer much faster.
—SA

Richard C Bishop 5-Jun-13 17:44pm

Well <%# %> is a data binding expression, not sure on the others as I have no context.

Sergey Alexandrovich Kryukov 5-Jun-13 17:41pm

If this is ASP.NET, tag it. And you are not "using server-side code in JavaScript", ever. This is not how Web works.
—SA

alamz7 5-Jun-13 17:47pm

well in this case then the 2nd code snippet should also return and empty value, but its working though some server side code has been used here,

Sergey Alexandrovich Kryukov 5-Jun-13 18:11pm

No. It works because server-side first generates the whole page on server side and insert the value of TextBox1.ClientID as a parameter of text generation. This is delivered as a whole HTML (or just JavaScript) in an HTTP request to the client side. For JavaScript, TextBox1.ClientID is just a hard-coded text expanded as id="someIdValue". It's always useful to see what's going on by looking at pure HTML, via the browser, using "View Page Source" or something like that.

Do you see the point? This is how it really works.
—SA

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Sergey Alexandrovich Kryukov · Answer 1 · 2013-06-05T12:25:00

Please see my comment to the question explaining how an ASP.NET application works with JavaScript. There is no "using", calls between server-side and JavaScript — one is on the server side, another — on the client side.

Now, I'll explain why first code fragment is not working and the second does. This is just JavaScript and jQuery. First of all, TextBox.Text gets hard-coded in the instance of JavaScript text as some text value at the moment. It makes no sense for JavaScript, as this is some arbitrary blah-blah, whatever the value is. It could be

JavaScript

var value =

// or some

JavaScript

var value = blah-blah

Yes, just so, even without quotation marks. I hope you can see it now. Not only it is gibberish to JavaScript, it may not even compile. JavaScript exceptions are usually hidden from the user, so you can imagine the result.

In second case, jQuery is used with id selector. Actually, TextBox1.ClientID generates some HTML id used in HTML, like in

HTML

<input type=text id="someId" ... />

In your second sample, with the actual id value like in my example above, the JavaScript will read:

JavaScript

var value = $('#someId').val();

This is a valid way to get a jQuery wrapper over HTML DOM object ($('#someId'); an element is found by its unique id value), and call its method .val(), to get the current value.

See also:
http://api.jquery.com/category/selectors/[^],
http://api.jquery.com/id-selector/[^].

If you need to learn jQuery (highly recommended), please see:
http://en.wikipedia.org/wiki/JQuery[^],
http://jquery.com/[^],
http://learn.jquery.com/[^],
http://learn.jquery.com/using-jquery-core/[^],
http://learn.jquery.com/about-jquery/how-jquery-works/[^] (start from here).

—SA

Using server side code in JavaScript

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