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Suppose I have server. A client loading an HTML file containing a javascript library will have the script executed by the browser. The problem here is that if the client's computer is slow, the processing will take a long time.

So I want to move the processing to the server side. But, instead of having to rewrite the entire javascript library into another language, I simply want to run the javascript on the server.

Googling "server side javascript" directs me to Node.JS, which in my imagination have the capability to do so. But, I cannot find a tutorial which does just that. Does this mean that there really is no easy way to do so? For example, because the javascript script may contain DOM specific things such as document.getElementById(), which does not make much sense on the server side.

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There is no trivial way to simply shift processing of JS from the client to the server.

You need to break the code down into code that must run on the browser (such as, assuming you don't want the browser to load an entirely new page, DOM manipulation) and code that can run on the server.

Then you need to provide a way to pass data between the server and the browser, this is normally done via HTTP (using Ajax).

When you take input from the client you need to send it to the server in an HTTP request (instead of just passing it as an argument to a function). The server needs to read the HTTP request, process it, and make an HTTP response.

The Ajax callback then needs to parse the response and run any client side logic (such as DOM updates) in response.

Note that network communication times will impact performance.

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Thanks! Ajax isn't exactly what I'm looking for, but your comment on breaking down the code really helped. I was being lazy and trying to avoid that, but I guess there is no other way.
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You can't "merge" the client and server in this way. All you could do is process the data on the server and just display it in the client without any further processing. Maybe you should refresh you knowledge about HTTP and how websites are send to the clients. Without any additional tricks, like websockets, comet or ajax polling, you can't access the client after you send the initial website to it. Even than you can just send data to the client.

When you want to stick to Javascript, Node.js is a good option. But even than you would need to send the data you want processed to the server, process it there and send back the processed data in JSON or "display ready" HTML.

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Thanks! I was initially trying to avoid breaking down the code into what must run on the browser and what is not, but -- as @Quentin mentioned -- I guess there is no other way. You are right in reminding me how HTTP works, Ajax isn't the solution I'm looking for. Once I have the portion of the script that is not browser specific, I believe it is straightforward to use Node.js. Then I can send the processed data as the initial website and the client can just run the browser specific scripts. Is that what you are saying?

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