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How to avoid duplicating a formula in both Javascript and PHP?

I'm currently writing a web application in HTML5/Javascript and PHP.

Some employees of the company will need to use it to enter their work schedule. The application will calculate in real time (using Javascript) some complicated legal infos and display the result at the bottom of the page. When he's done, the user clicks the "save" button and everything is sent to the database.

The problem is that I need both the user to see the output in real time and the managers to get the same output from what was saved in the database. I also need to support the case where the user has Javascript disabled. In other words, I need to do the same calculus in both Javascript and PHP.

To make things more complicated, the formula is very complex and will change often (every month or so), so I'd like to avoid maintaining two different versions. Testing one will already be hard enough.

Also I'd like to avoid AJAX to ask the server for the output, because:

  • the user will often build its schedule according to the result of the real-time calculation, so even a 1-2 seconds lag could really be annoying for him
  • if possible I have to support HTML5's offline features, so the user can load the app on his mobile phone, fill its schedule while offline, and then upload it when online

For the moment the only solution I found is to write the formula in a language-agnostic way, then use some way to turn it into PHP code and JavaScript code, but that's not simple.

Solution

  • It sounds like you have essentially two approaches:

    1. You can write a language-independent formula (in a database, or config file, since it changes so frequently) and two code generators (Javascript, PHP). This is actually less scary than it sounds, as long as you pick your input format sanely and you actually mean a formula, not some arbitrary computation.

    2. You can use server-side JavaScript, and write the formula once in JavaScript. Wikipedia has a comparrison of server-side JavaScript implementations. There are even JavaScript interpreters written in PHP, including at least phpjs and J4P5.