Ask Ben: Converting Javascript Variables Into ColdFusion Variables
Posted June 22, 2009 at 9:51 AM
How can I convert a JavaScript variable into a coldfusion variable?
I have been asked this question enough times that I figured it was be time to turn it into an actual Ask Ben blog post for future reference. Often times, when front-end developers start getting into server-side scripting (hopefully using ColdFusion), there is some confusion as to where the various parts of the page rendering work flow are executing. And, because of this confusion, developers get stuck trying to access one part of the work flow from another.
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To clear up the confusion, I think you just need to step back and understand the request-response processing work flow:
- The server gets a request to deliver some content (at the given URL).
- The server constructs the response (in our case, an HTML page containing Javascript).
- The server returns the HTML data to the client (Web Browser).
- The data streams from the Server to the Client.
- The browser renders the response (HTML page) for the user at which time any embedded Javascript code executes.
Once we hit step #4, the ColdFusion server is no longer involved. In step #5, when the client renders the HTML response, it doesn't even have to know where the HTML data came from. As such, when the Javascript executes in step #5, it has no knowledge of nor any access to the ColdFusion portion of the page response work flow - these two points of execution happen at different times on different machines.
Javascript can, however, communicate with the ColdFusion server by making AJAX-based page requests. These HTTP requests are viewed by the ColdFusion server as completely new page requests by the client and do not have access to temporary ColdFusion variables created during previous requests (unless those variables were explicitly cached on the server).
So, the moral of the story is that you can't really turn a Javascript variable into a ColdFusion variable because the two are completely unrelated. Once you understand the page request / response work flow, this becomes obvious. If you want Javascript to send values back to the ColdFusion server for further processing, you can only do so via AJAX requests. I hope that this clears up some confusion.
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Reader Comments
You can always write to a hidden form field if you are processing a form, etc.
Great Post
When you are calling your JavaScript you can always have the included JavaScript file end in a .cfm which would help in some cases... This will allow you to load different content based on variables that you pass... you can always pass variables through JavaScript like so...
<script type="text/javascript"
src="javascript.cfm?var1=bob&var2=frank">
</script>
This essential will pass the variables to coldfusion in which you can convert the variables into coldfusion variables using the url variable... I have used this method a few times but I don't recommend it for really robust applications.
What about <cfwddx action="cfml2js" ... />?
Sure, we can't use ColdFusion vars on the client side, but we can serialize them and return to the browser along with the rest of the JavaScript code.
json format is very helpfull to pass variables between javascript and coldfusion in ajax calls. There are 2 coldfusion functions since cf8 DeserializeJSON() and SerializeJSON().
And from you javascript code you can serialize your JS variables to a json string by using the json serializer( http://www.JSON.org/json2.js ).
@Jody,
Definitely true - a great technique.
@Alexey,
Yeah - using ColdFusion variables to help render the Javascript that ultimately gets executed by the browser is definitely some we can do, and is the easier of the two directions (server to client vs. client to server).
@Patrick,
JSON rocks! 'nuff said.
