JavaScript is client-side. So no, you cannot interact with files/processes on the server, via JavaScript. You need a server-side language for that. You need BOTH, server-side, and client-side, to develop robust web applications. I recommend PHP for server-side code.

What is the coding experience like? I can't really say... I've coded in so many languages, one is pretty much like the other. With two exceptions: PostScript, which is a page description language, so not relevant, and ASP.NET, which is Microsoft's Web language - I find that to be a horrible, tedious coding experience.

Think of JavaScript as adding richness to the user interface. It's also your first line of defense, for data validation. It "scripts" the UI. So, for example, imagine a user clicks a link to get more detail on a product. That detail may need to come (almost certainly) from the server, but it's JavaScript that creates the new "popup" window to display that data. In that detail page, the user may select something, which now needs to be added to a table on the parent page... only JavaScript can do that.

You may want the user to enter data in a certain format, but with JavaScript's regular expressions, you can ENFORCE that format.

There might be a preset spending limit for this user, which came from the server, and was stored in a JavaScript variable. JavaScript can keep a running total of what they've "spent" on the page, and popup a warning dialog box if they exceed this limit.

They might have several actions they can take on a certain page, but some actions are only viable when the user has entered certain information. JavaScript can enable/disable buttons and hyperlinks, and toggle the visibility of elements, in response to user actions.