Advantages of All-Javascript Webpages

Question

I have noticed that many big(huge) sites like Google and Facebook when looking to the page source 99% of the source is JavaScript.

Does anybody know the advantages to this approach versus regular HTML+JavaScript pages?

Is it just to add some security or does it have benefits in terms of performance or maintainability?

Answer 1

One reason why I have implemented pages in this pattern is because I wanted to have a client-agnostic server that just serves data packaged in an easily-parseable format (such as JSON) so that the same server could be used to drive a traditional webapp as well as other things such as native Android and iPhone applications without needing any special modifications to server code.

A JavaScript-heavy page allows you to work with such a setup by having the JavaScript request the required data from the server and then use it to construct an interface in HTML. Given that most of the major players have similar concerns with wanting a single server architecture to power an application across a large number of platforms, that may be a contributing factor with respect to why they have chosen to implement their webpages primarily in JavaScript.