I was just wondering how you guys manage complexity in large JS Applications? Since JavaScript is so flexible and HN is full of very smart people, I'm curious to know how everyone handles it.
I'm also pretty sure that for someone who is just starting out, this would be tremendously useful for them to know.
By not making them large in the first place.
Take for instance the Facebook home when you're logged in. It looks like one unified app. But what you might not know is that it may be comprised of several different "applets" that are totally independent of one another. They may possibly not use the same framework, nor share code, and could possibly be built by several teams which are proficient in different languages and tools.
iirc, Spotify's app isn't built as one huge app. It's several different "applets" running in iframes. This ensures that each applet is totally isolated (so they don't kill each other even if they use different frameworks and versions) and self-contained (so they're testable individually, reducing the need for integration tests).
Also: garbage data makes every aspect of your business a living hell. Clean data throughly before storing it and migrate data as business rules change. You should be able to make A LOT of assumptions about your data without having to parse it a dozen ways.
Maybe the website http://superherojs.com/ will help with your question.
Frameworks are a big help – partially for the tools they provide, but also for the consistency. "We have a lot of code, but it's all structured in a standard way" makes things easier to understand. It's also a way of having confidence in the architecture: you know many other people have built successful apps with it, rather than something you've invented yourself.
The concepts in Redux are very appealing to me: your application has one state, and that state gets changed via actions. You have a function that, given a state and an action, produces a new state. Your application can render itself from that state. This makes testing straightforward: your business logic tests boil down to "Given this starting state, and this action, my output state should look like this." Similarly, your UI is now easy to test: "Given this state, my component should render like this," covers displaying information, and "When I interacted with this element, was an action emitted?" covers user interaction. Traditionally hairy areas are now more straightforward: "I need to test with a logged-in user" or "I need to test a successful payment" is all about the data in the state.
Another technique you can use is a publish/subscribe architecture: individual parts of your application publish events, and can subscribe to events they care about. Now, your InboxCounter widget doesn't have to know anything about how messages are received or displayed; as long as it listens for "message:added" and "message:removed" events, it can display an accurate count. Similarly, the AJAX polling for new messages doesn't need to be coupled to the message display or the counters or other elements; it just has to check for new messages, and publish events when new ones are received. This should make it easier to add new components and refactor old ones (as long as you're emitting the same events, nothing else should have to be updated), but it can be hard to figure out what events get emitted, and where they come from. Tests help.
On a much smaller level, using promises instead of callbacks can help reduce "pyramid code" - sections of the codebase with extreme indentation. They also make it easier to handle failure cases; writing "Make these 3 co-dependent AJAX requests, and if any fail then display an error message" will be more concise and clearer with promises.
Finally, don't over-complicate things. Frameworks, dependency injection, and the like add a lot of upfront complexity to a project, but reduce it later on. If all your application needs is an occasional "When X happens, show/hide this DOM element" then simple jQuery event listeners will be a lot quicker to build and easier to understand. Don't build a Swiss watch when what you needed was an egg timer.