Single page applications (SPAs) are meant to provide a better user experience. But, not only do they cause many usability problems, they're much harder to design and build.
Before getting to the issues, let's clarify what an SPA is.
What's an SPA?
You might associate MVC, AJAX and client-side templating with an SPA. But these are not the defining characteristics of an SPA. We can use all of these things to build a rich, ROCA-style website.
What really defines an SPA as such, is the fact that client-side JS handles the routing instead of the browser. That is, the application hijacks the behaviour that browsers inherently provide natively for free.
Put like that, it's hardly surprising that SPAs cause so many problems:
1. History and fast back
Browsers store history so that pages load quickly when the user clicks back. Daniel Puplus explains in Building Single Page Applications that:
“When a user presses the browser’s back button they expect the change to happen quickly and for the page to be in a similar state to how it was last time they saw it.
“In the traditional web model the browser will typically be able [to] use a cached version of the page and linked resources.
“In a naive implementation of a SPA hitting back will do the same thing as clicking a link, resulting in a server request, additional latency, and possibly visual data changes.”
If we want users to enjoy the same, fast experience, we need to mimic the native browser behaviour in JS.
First, the application will need to store pages in memory, local storage, client-side databases or cookies. Second, the application will need to determine when to retrieve these pages. As part of this it will need to differentiate between:
- a user changing the URL (by clicking a link or typing a URL in the location bar); and
- manually pressing back or forward which isn't simple.
2. Scroll position
Browsers remember the scroll position of pages you've visited. Daniel Puplus explains how SPAs cause trouble:
“Lots of sites get this wrong and it’s really annoying. When the user navigates using the browser’s forward or back button the scroll position should be the same as it was last time they were on the page. This sometimes works correctly on Facebook but sometimes doesn’t. Google+ always seems to lose your scroll position.”
SPAs don't navigate in the traditional sense. Therefore the application must store the scroll position and apply it when the user presses back or forward.
3. Cancelling navigation
Consider what a browser gives us for free:
- When a user clicks the cancel button, the browser will stop any in-flight requests.
- When a user clicks a link, the browser will again, stop any in-flight requests and make a new request.
As SPAs retrieve whole (data for) pages with AJAX, there could be several requests in-flight. The first page request could finish last. And, a user may click (and therefore request) the same link twice. This is problematic because:
- it's inefficient;
- the user's data allowance could be eaten up unnecessarily; and
- it causes visual glitches as a subsequent page request finishes (when the browser would normally have cancelled it).
The application will need to handle duplicate requests. And the interface will need a cancel button, which is obviously undesirable.
4. Unsaved changes
Sometimes an application needs to warn users of losing unsaved changes when they leave a page. The
beforeunload event lets us do this.
But, the application can't use this because users don't request real pages. This means the app will need to reimplement this behaviour.
5. Search engine ranking
For many SPAs, SEO is an afterthought, which is problematic because retro fitting server side rendering is not trivial. The alternative is to create a special website just for bots which isn't ideal either. Either way, it's another problem that needs consideration and action.
6. Loading CSS and JS
If an SPA grows to a significant size, loading the entire application on load will be slow. This leads to conditionally loading CSS and JS with a script loader.
The problem is that script loading is notoriously difficult and contains hacks. This slows down development and reduces the reliability of the application.
Analytics tools track page views by default. But SPA pages aren't really pages. This means the application will have to notify the analytics tool that a pseudo page has been loaded.
8. Automated functional testing
Selenium knows when a page has finished loading. However, SPAs load pages with AJAX. This means that:
- tests are more complicated and must handle timeouts; and
- the tests will be slower to execute.
9. Memory leaks
Pages are "long lived" increasing the chance of memory leak issues. This can degrade UX and cause battery drain on mobile devices.
10. Loading indicators
Browsers show a loading indicator when the page is loading. But, SPAs use AJAX to load pages, so we need to implement a custom one which is more work.
Also, the browser's indicator displays progress. That is, a user can tell how long until the page loads. JS can only state that something is loading, not how long left there is. This leads to users clicking the link again which exacerbates problem 3.
Furthermore, the user's browser provides a predictable and familiar loading indicator, no matter which website they visit. When we use JS to do this, we break the third of Henny Swan’s UX principles, design with familiarity in mind.
11. It's going to fail
Everyone has JS, Right? explains that JS can fail in many ways. Maybe it doesn't load. Maybe the browser doesn't recognise the script and fails. SPAs depend on many different enhancements, and they tend not to conform to Progressive Enhancement. This means when it does fail, the resulting experience will be a blank page.
12. They're probably slower
Given that they require a whole load of extra code to load and fix all these issues, it's probably going to be slower. And don't assume AJAX is always faster.
13. They're inaccessible
I'll defer to Craig's excellent article: One-page-applications are not accessible.
14. They just don't feel right
I'll defer to Stefan's excellent article: Why I hate your Single Page App
Ironically, SPAs are harder to both design and build. And yet, they typically produce slow, disagreeable and exclusive experiences.
Twitter, Lifehacker and Delicious have reverted to traditional architectures because of these issues which you can read in:
We should let the browser manage the browsing experience, and spend our time solving real problems.
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