The code is not open source. I'm pretty sure the normal presentation back-end is PHP, the Live server code is in Java (server side only), the front end is HTML5 and Javascript using Vue, migrating from Angular. According to the last Job posting/link from March, they use Symfony2 for the framework.
is the code for chess.com open source? what language(s) is it written in?
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I would guess it is written in PHP, but what is the toolchain used to develop Chess.com? Seems like a very elaborate program with all the website forum stuff, and animations. must be huge.