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ReferenceError: x is not defined?

Today for a bit of code I was writing, I needed to be able to distinguish “x is not defined” ReferenceErrors from any other error within a try...catch block and handle them differently.

Now I know what you’re thinking. Trying to figure out exactly what kind of error you have programmatically is a well-known fool’s errand. If you express a desire to engage in such a risky endeavor, any JS veteran in sight will shake their head in remembrance of their early days, but have the wisdom to refrain from trying to convince you otherwise; they know that failing will teach you what it taught them when they were young and foolish enough to attempt such a thing.

Despite writing JS for 13 years, today I was feeling adventurous. “But what if, just this once, I could get it to work? It’s a pretty standard error message! What if I tested in so many browsers that I would be confident I’ve covered all cases?”

I made a simple page on my server that just prints out the error message written in a way that would maximize older browser coverage. Armed with that, I started visiting every browser in my BrowserStack account. Here are my findings for anyone interested:

  • Chrome (all versions, including mobile): x is not defined
  • Firefox (all versions, including mobile): x is not defined
  • Safari 4-12 : Can't find variable: x
  • Edge (16 – 18): 'x' is not defined
  • Edge 15: 'x' is undefined
  • IE6-11 and Windows Phone IE: 'x' is undefined
  • UC Browser (all versions): x is not defined
  • Samsung browser (all versions): x is not defined
  • Opera Mini and Pre-Chromium Opera: Undefined variable: x

Even if you, dear reader, are wise enough to never try and detect this error, I thought you may find the variety (or lack thereof) above interesting.

I also did a little bit of testing with a different UI language (I picked Greek), but it didn’t seem to localize the error messages. If you’re using a different UI language, please open the page above and if the message is not in English, let me know!

In the end, I decided to go ahead with it, and time will tell if it was foolish to do so. For anyone wishing to also dabble in such dangerous waters, this was my checking code:

if (e instanceof ReferenceError 
    && /is (not |un)defined$|^(Can't find|Undefined) variable/.test(e.message)) {
    // do stuff
}

Found any cases I missed? Or perhaps you found a different ReferenceError that would erroneously match the regex above? Let me know in the comments!

One thing that’s important to note is that even if the code above is bulletproof for today’s browser landscape, the more developers that do things like this, the harder it is for browser makers to improve these error messages. However, until there’s a better way to do this, pointing fingers at developers for wanting to do perfectly reasonable things, is not the solution. This is why HTTP has status codes, so we don’t have to string match on the text. Imagine having to string match “Not Found” to figure out if a request was found or not! Similarly, many other technologies have error codes, so that different types of errors can be distinguished without resulting to flimsy string matching. I’m hoping that one day JS will also have a better way to distinguish errors more precisely than the general error categories of today, and we’ll look back to posts like this with a nostalgic smile, being so glad we don’t have to do crap like this ever again.

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Apps & scripts Articles

Refresh CSS Bookmarklet v2

Almost 11 years ago, Paul Irish posted this brilliant bookmarklet to refresh all stylesheets on the current page. Despite the amount of tools, plugins, servers to live reload that have been released over the years, I’ve always kept coming back to it. It’s incredibly elegant in its simplicity. It works everywhere: locally or remotely, on any domain and protocol. No need to set up anything, no need to alter my process in any way, no need to use a specific local server or tool. It quietly just accepts your preferences and workflow instead of trying to change them. Sure, it doesn’t automatically detect changes and reload, but in most cases, I don’t want it to.

I’ve been using this almost daily for a decade and there’s always been one thing that bothered me: It doesn’t work with iframes. If the stylesheet you’re editing is inside an iframe, tough luck. If you can open the frame in a new tab, that works, but often that’s nontrivial (e.g. the frame is dynamically generated). After dealing with this issue today once more, I thought “this is just a few lines of JS, why not fix it?”.

The first step was to get Paul’s code in a readable format, since the bookmarklet is heavily minified:

(function() {
	var links = document.getElementsByTagName('link');
	for (var i = 0; i < links.length; i++) {
		var link = links[i];
		if (link.rel.toLowerCase().match(/stylesheet/) && link.href) {
			var href = link.href.replace(/(&|%5C?)forceReload=\d+/, '');
			link.href = href + (href.match(/\?/) ? '&' : '?') + 'forceReload=' + (new Date().valueOf())
		}
	}
})()

Once I did that, it became obvious to me that this could be shortened a lot; the last 10 years have been wonderful for JS evolution!

(()=>{
	for (let link of Array.from(document.querySelectorAll("link[rel=stylesheet][href]"))) {
		var href = new URL(link.href, location);
		href.searchParams.set("forceReload", Date.now());
		link.href = href;
	}
})()

Sure, this reduces browser support a bit (most notably it excludes IE11), but since this is a local development tool, that’s not such a big problem.

Now, let’s extend this to support iframes as well:

{
	let $$ = (selector, root = document) => Array.from(root.querySelectorAll(selector));
	
	let refresh = (document) => {
		for (let link of $$("link[rel=stylesheet][href]", document)) {
			let href = new URL(link.href);
			href.searchParams.set("forceReload", Date.now());
			link.href = href;
		}

		for (let iframe of $$("iframe", document)) {
			iframe.contentDocument && refresh(iframe.contentDocument);
		}
	}

	refresh();
}

That’s it! Do keep in mind that this will not work with cross-origin iframes, but then again, you probably don’t expect it to in that case.

Now all we need to do to turn it into a bookmarklet is to prepend it with javascript: and minify the code. Here you go:

Refresh CSS

Hope this is useful to someone else as well 🙂
Any improvements are always welcome!

Credits

  • Paul Irish, for the original bookmarklet
  • Maurício Kishi, for making the iframe traversal recursive (comment)
Categories
Articles

HTML APIs: What they are and how to design a good one

I’m a strong believer in lowering the barrier of what it takes to create rich, interactive experiences and improving the user experience of programming. I wrote an article over at Smashing Magazine aimed at JavaScript library developers that want their libraries to be usable via HTML (i.e. without writing any JavaScript). Sounds interesting? Read it here.

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Articles Thoughts

One year of pastries

Last September, I was approached by Alex Duloz, who invited me to take part in his ambitious new venture, The Pastry Box Project. Its goal was to gather 30 people (“bakers”) every year who are influential in their field and ask them to share twelve thoughts — one per month. For 2012, that field would be the Web. I was honored by the invitation and accepted without a second thought (no pun intended). The project was quite successful and recently we all (almost) agreed for The Pastry Box Project to become a book, whose profits will be donated to charity.

The initial goal of the project was to gather thoughts somehow related to the bakers’ work. Although many stuck to that topic, for many others it quickly drifted away from that, with them often sending thoughts that were general musings about their lives or life in general. For me …well lets just say I was never good at sticking to the topic at hand. 😉

The Pastry Box showed me that I want a personal blog so I made one today. I will still publish personal stuff here, as long as it’s even remotely web-related, so not much will change. However, my interests range to more than the Web, so I will now have another medium to express myself in. 🙂

Since 2012 is now over, I decided to gather all my “pastries” and publish them in two blog posts: I will post the more techy/professional ones below and the more general/personal ones in my personal blog. Since most of them were somewhere in the middle, it wasn’t easy to pick which ones to publish where. I figured the best solution is to allow for some overlap and publish most of them in both blogs.

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Articles Thoughts

In defense of reinventing wheels

One of the first things a software engineer learns is “don’t reinvent the wheel”. If something is already made, use that instead of writing your own. “Stand on the shoulders of giants, they know what they’re doing better than you”. Writing your own tools and libraries, even when one already exists, is labelled “NIH syndrome”  and is considered quite bad.

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Articles

A List Apart article: Every time you call a proprietary feature “CSS3”, a kitten dies

My first article in ALA was published today, read it here:

Every time you call a proprietary feature “CSS3”, a kitten dies

Some comments about it on twitter:

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Articles

Optimizing long lists of yes/no values with JavaScript

My newest article on Smashing Magazine’s coding section is for the geekiest among you. It’s about how you can pack long lists of boolean values into a string in the most space-efficient way. Hope you enjoy it 🙂

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Articles Personal

Help the community: report browser bugs

Thought I’d let you know that my Smashing Magazine article with that title was published today. It discusses why, how, when and where to report browser bugs, as well as how to make a good bug report.

Get comfortable and make a big cup of coffee before you dive in, as it’s quite long (4000 words).

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Articles

Tag editing UIs

I had to build the edit tags interface for an application I’m working on, so I took a good look at how these are implemented across many popular applications nowadays. It seems there are a few patterns that are used over and over, and I’m unsure which one is the most preferable by users, they all have their advantages and disadvantages. In this post I’m going to describe these patterns and list some of the pros and cons I think they have. For simplicity, I will focus on the tag editing interface itself, ignoring any tag suggestions and other extra features.

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Articles

The curious case of border-radius:50%

Admittedly, percentages in border-radius are not one of the most common use cases. Some even consider them an edge case, since most people seem to set border-radius in pixels or –rarely– ems. And since it’s not used very frequently, it’s still quite buggy. A bit of a chicken and egg case actually: Is it buggy because it’s used rarely or is it used rarely because it’s buggy? My vote would go to the first, so the purpose of this post is to let people know about why percentages in border-radius are incredibly useful and to highlight the various browser whims when it comes to rendering them.