Tags: error

Bring Focus to the First Form Field with an Error :: Aaron Gustafson

A handy little script from Aaron to improve the form validation experience.

How To Build Resilient JavaScript UIs — Smashing Magazine

The opening paragraphs of this article should be a mantra recited by every web developer before they begin their working day:

Things on the web can break — the odds are stacked against us. Lots can go wrong: a network request fails, a third-party library breaks, a JavaScript feature is unsupported (assuming JavaScript is even available), a CDN goes down, a user behaves unexpectedly (they double-click a submit button), the list goes on.

Fortunately, we as engineers can avoid, or at least mitigate the impact of breakages in the web apps we build. This however requires a conscious effort and mindset shift towards thinking about unhappy scenarios just as much as happy ones.

I love, love, love the emphasis on reducing assumptions:

Taking a more defensive approach when writing code helps reduce programmer errors arising from making assumptions. Pessimism over optimism favours resilience.

Hell, yeah!

Accepting the fragility of the web is a necessary step towards building resilient systems. A more reliable user experience is synonymous with happy customers. Being equipped for the worst (proactive) is better than putting out fires (reactive) from a business, customer, and developer standpoint (less bugs!).

I think I’ve found some more strange service worker behaviour in Chrome.

It all started when I was checking out the very nice new redesign of WebPageTest. I figured while I was there, I’d run some of my sites through it. I passed in a URL from The Session. When the test finished, I noticed that the “screenshot” tab said that something was being logged to the console. That’s odd! And the file doing the logging was the service worker script.

I fired up Chrome (which isn’t my usual browser), and started navigating around The Session with dev tools open to see what appeared in the console. Sure enough, there was a failed fetch attempt being logged. The only time my service worker script logs anything is in the catch clause of fetching pages from the network. So Chrome was trying to fetch a web page, failing, and logging this error:

The service worker navigation preload request failed with a network error.

But all my pages were loading just fine. So where was the error coming from?

After a lot of spelunking and debugging, I think I’ve figured out what’s happening…

First of all, I’m making use of navigation preloads in my service worker. That’s all fine.

Secondly, the website is a progressive web app. It has a manifest file that specifies some metadata, including start_url. If someone adds the site to their home screen, this is the URL that will open.

Thirdly, Google recently announced that they’re tightening up the criteria for displaying install prompts for progressive web apps. If there’s no network connection, the site still needs to return a 200 OK response: either a cached copy of the URL or a custom offline page.

So here’s what I think is happening. When I navigate to a page on the site in Chrome, the service worker handles the navigation just fine. It also parses the manifest file I’ve linked to and checks to see if that start URL would load if there were no network connection. And that’s when the error gets logged.

I only noticed this behaviour because I had specified a query string on my start URL in the manifest file. Instead of a start_url value of /, I’ve set a start_url value of /?homescreen. And when the error shows up in the console, the URL being fetched is /?homescreen.

Crucially, I’m not seeing a warning in the console saying “Site cannot be installed: Page does not work offline.” So I think this is all fine. If I were actually offline, there would indeed be an error logged to the console and that start_url request would respond with my custom offline page. It’s just a bit confusing that the error is being logged when I’m online.

I thought I’d share this just in case anyone else is logging errors to the console in the catch clause of fetches and is seeing an error even when everything appears to be working fine. I think there’s nothing to worry about.

Update: Jake confirmed my diagnosis and agreed that the error is a bit confusing. The good news is that it’s changing. In Chrome Canary the error message has already been updated to:

DOMException: The service worker navigation preload request failed due to a network error. This may have been an actual network error, or caused by the browser simulating offline to see if the page works offline: see https://w3c.github.io/manifest/#installability-signals

Much better!

How-to: Create accessible forms - The A11Y Project

Another five pieces of sweet, sweet low-hanging fruit:

• Always label your inputs.
• Highlight input element on focus.
• Break long forms into smaller sections.
• Provide error messages.
• Avoid horizontal layout forms unless necessary.

Inclusive Inputs « Texte | ovl – code & design

This is a great walkthough of making a common form pattern accessible. No complex code here: some HTML is all that’s needed.

The Flawed Reasoning Behind the Replication Crisis — Nautilus

Bayesian analysis vs. statistical significance, clearly explained.

It seems that some code that I wrote in Going Offline is haunted. It’s the trimCache function.

First, there was the issue of a typo. Or maybe it’s more of a brainfart than a typo, but either way, there’s a mistake in the syntax that was published in the book.

Now it turns out that there’s also a problem with my logic.

To recap, this is a function that takes two arguments: the name of a cache, and the maximum number of items that cache should hold.

function trimCache(cacheName, maxItems) {

First, we open up the cache:

caches.open(cacheName)
.then( cache => {

Then, we get the items (keys) in that cache:

cache.keys()
.then(keys => {

Now we compare the number of items (keys.length) to the maximum number of items allowed:

if (keys.length > maxItems) {

If there are too many items, delete the first item in the cache—that should be the oldest item:

cache.delete(keys[0])

And then run the function again:

.then(
    trimCache(cacheName, maxItems)
);

A-ha! See the problem?

Neither did I.

It turns out that, even though I’m using then, the function will be invoked immediately, instead of waiting until the first item has been deleted.

Trys helped me understand what was going on by making a useful analogy. You know when you use setTimeout, you can’t put a function—complete with parentheses—as the first argument?

window.setTimeout(doSomething(someValue), 1000);

In that example, doSomething(someValue) will be invoked immediately—not after 1000 milliseconds. Instead, you need to create an anonymous function like this:

window.setTimeout( function() {
    doSomething(someValue)
}, 1000);

Well, it’s the same in my trimCache function. Instead of this:

cache.delete(keys[0])
.then(
    trimCache(cacheName, maxItems)
);

I need to do this:

cache.delete(keys[0])
.then( function() {
    trimCache(cacheName, maxItems)
});

Or, if you prefer the more modern arrow function syntax:

cache.delete(keys[0])
.then( () => {
    trimCache(cacheName, maxItems)
});

Either way, I have to wrap the recursive function call in an anonymous function.

Here’s a gist with the updated trimCache function.

What’s annoying is that this mistake wasn’t throwing an error. Instead, it was causing a performance problem. I’m using this pattern right here on my own site, and whenever my cache of pages or images gets too big, the trimCaches function would get called …and then wouldn’t stop running.

I’m very glad that—witht the help of Trys at last week’s Homebrew Website Club Brighton—I was finally able to get to the bottom of this. If you’re using the trimCache function in your service worker, please update the code accordingly.

Management regrets the error.

Idiosyncrancies of the HTML parser - The HTML Parser Book

This might just be the most nerdily specific book I’ve read and enjoyed. Even if you’re not planning to build a web browser any time soon, it’s kind of fascinating to see how HTML is parsed—and how much of an achievement the HTML spec is, for specifying consistent error-handling, if nothing else.

The last few chapters are still in progress, but you can read the whole thing online or buy an ePub version.

Whenever I create a fetch event inside a service worker, my code roughly follows the same pattern. There’s a then clause which gets executed if the fetch is successful, and a catch clause in case anything goes wrong:

fetch( request)
.then( fetchResponse => {
    // Yay! It worked.
})
.catch( fetchError => {
    // Boo! It failed.
});

In my book—Going Offline—I’m at pains to point out that those arguments being passed into each clause are yours to name. In this example I’ve called them fetchResponse and fetchError but you can call them anything you want.

I always do something with the fetchResponse inside the then clause—either I want to return the response or put it in a cache.

But I rarely do anything with fetchError. Because of that, I’ve sometimes made the mistake of leaving it out completely:

fetch( request)
.then( fetchResponse => {
    // Yay! It worked.
})
.catch( () => {
    // Boo! It failed.
});

Don’t do that. I think there’s some talk of making the error argument optional, but for now, some browsers will get upset if it’s not there.

So always include that argument, whether you call it fetchError or anything else. And seeing as it’s an error, this might be a legitimate case for outputing it to the browser’s console, even in production code.

And yes, you can output to the console from a service worker. Even though a service worker can’t access anything relating to the document object, you can still make use of window.console, known to its friends as console for short.

My muscle memory when it comes to sending something to the console is to use console.log:

fetch( request)
.then( fetchResponse => {
    return fetchResponse;
})
.catch( fetchError => {
    console.log(fetchError);
});

But in this case, the console.error method is more appropriate:

fetch( request)
.then( fetchResponse => {
    return fetchResponse;
})
.catch( fetchError => {
    console.error(fetchError);
});

Now when there’s a connectivity problem, anyone with a console window open will see the error displayed bold and red.

If that seems a bit strident to you, there’s always console.warn which will still make the output stand out, but without being quite so alarmist:

fetch( request)
.then( fetchResponse => {
    return fetchResponse;
})
.catch( fetchError => {
    console.warn(fetchError);
});

That said, in this case, console.error feels like the right choice. After all, it is technically an error.

Paul Yabsley wrote to let me know about an error in Going Offline. It’s rather embarrassing because it’s code that I’m using in the service worker for adactio.com but for some reason I messed it up in the book.

It’s the trimCache function in Chapter 7: Tidying Up. That’s the reusable piece of code that recursively reduces the number of items in a specified cache (cacheName) to a specified amount (maxItems). On page 95 and 96 I describe the process of creating the function which, in the book, ends up like this:

 function trimCache(cacheName, maxItems) {
   cacheName.open( cache => {
     cache.keys()
     .then( items => {
       if (items.length > maxItems) {
         cache.delete(items[0])
         .then(
           trimCache(cacheName, maxItems)
         ); // end delete then
       } // end if
     }); // end keys then
   }); // end open
 } // end function

See the problem? It’s right there at the start when I try to open the cache like this:

cacheName.open( cache => {

That won’t work. The open method only works on the caches object—I should be passing the name of the cache into the caches.open method. So the code should look like this:

caches.open( cacheName )
.then( cache => {

Everything else remains the same. The corrected trimCache function is here:

function trimCache(cacheName, maxItems) {
  caches.open(cacheName)
  .then( cache => {
    cache.keys()
    .then(items => {
      if (items.length > maxItems) {
        cache.delete(items[0])
        .then(
          trimCache(cacheName, maxItems)
        ); // end delete then
      } // end if
    }); // end keys then
  }); // end open then
} // end function

Sorry about that! I must’ve had some kind of brainfart when I was writing (and describing) that one line of code.

You may want to deface your copy of Going Offline by taking a pen to that code example. Normally I consider the practice of writing in books to be barbarism, but in this case …go for it.

Update: There was another error in the code for trimCache! Here’s the fix.

Frequently Asked Questions [CSS Working Group Wiki]

Rebuttals to the most oft-asked requests for browsers to change the way they handle CSS.

Happier HTML5 Form Validation - daverupert.com

Dave uses just a smidgen of JavaScript to whip HTML5’s native form validation into shape.

Instead of being prescriptive about error messaging, we use what the browser natively gives us.

Usability Testing of Inline Form Validation: 40% Don’t Have It, 20% Get It Wrong - Articles - Baymard Institute

I saw Christian speak on this topic at Smashing Conference in Barcelona. Here, he takes a long hard look at some of the little things that sites get wrong when doing validating forms on the fly. It’s all good sensible stuff, although it sounds a bit medical when he takes about “Premature Inline Validation.”

What’s wrong with big data? | New Humanist

The view that more information uncritically produces better decisions is visibly at odds with our contemporary situation.

A superb piece of research and writing by James, skewering the technological determinism that underlies the current faith in “big data.” At best, this misplaced trust is inaccurate; at worst, it is deadly.

To the algorithmic imagination, the practice of journalism and the practice of terrorism appear to be functionally identical.

Styling Broken Images

This is really, really clever. You can’t use generated content (:before and :after) on replaced content. The img element is replaced content …but only when the image actually loads. So if the image fails to load, you can apply specific fallback styles (using :before and :after).

Simple inline error message pattern

This is my go-to method for adding validation messages to forms—I think I first heard about it from Derek—so it’s nice to see it corroborated by Steve:

Add the error message as a child of the label element associated with an input.

You’re So Smart You Turned JavaScript into XHTML

Web developers overwhelmingly rejected the draconian error-handing of XML …and yet today, web developers are embracing that very same error-handling model by rendering everything with JavaScript.

I don’t think it’s the way of the web to have your site fail and show a blank screen because some third-party dependency doesn’t load, JavaScript is turned off or because your developer left a trailing comma in a JavaScript object and didn’t test in Internet Explorer.

The Killing Machines by Mark Bowden in The Atlantic

How to think about drones—an in-depth and fairly balanced article by Mark Bowden on drone strikes and the politics behind them.

In the long run, careful adherence to the law matters more than eliminating another bad actor. Greater prudence and transparency are not just morally and legally essential, they are in our long-term interest, because the strikes themselves feed the anti-drone narrative, and inspire the kind of random, small-scale terror attacks that are bin Laden’s despicable legacy.

mattdiamond/fuckitjs

This is possibly the most horrifying piece of JavaScript ever written. The license is good too.

Bruce Schneier: Are photographers really a threat? | Technology | The Guardian

An excellent article that explodes the ludicrous myth that terrorists like to go around taking pictures of potential targets so therefore photographers are dangerous.