A horrifying globalThis polyfill in universal JavaScript · Mathias Bynens
The globalThis proposal introduces a unified mechanism to access the global this in any JavaScript environment. It sounds like a simple thing to polyfill, but it turns out it’s pretty hard to get right. I didn’t even think it was possible until Toon blew my mind with an unexpected, creative solution. This write-up describes the difficulties with writing a proper globalThis polyfill. Such a polyfil
Asynchronous stack traces: why await beats Promise#then() · Mathias Bynens
Compared to using promises directly, not only can async and await make code more readable for developers — they enable some interesting optimizations in JavaScript engines, too! This write-up is about one such optimization involving stack traces for asynchronous code. The fundamental difference between await and vanilla promises is that await X() suspends execution of the current function, while p
In search of the perfect URL validation regex
To clarify, I’m looking for a decent regular expression to validate URLs that were entered as user input with. I have no interest in parsing a list of URLs from a given string of text (even though some of the regexes on this page are capable of doing that). I also don’t want to allow every possible technically valid URL — quite the opposite. See the URL Standard if you’re looking to parse URLs in
ES2015 const is not about immutability · Mathias Bynens
This seems to be a very common misconception that just won’t die. I keep running into it in blog posts, Twitter discussions, and even books. Here’s my attempt at setting things straight. const creates an immutable binding ES2015 const does not indicate that a value is ‘constant’ or immutable. A const value can definitely change. The following is perfectly valid ES2015 code that does not throw an e
Unicode-aware regular expressions in ES2015 · Mathias Bynens
ECMAScript 2015 introduces two new flags for regular expressions: y enables ‘sticky’ matching. u enables various Unicode-related features. This article explains the effects of the u flag. It helps if you’ve read JavaScript has a Unicode problem first. Impact on syntax Setting the u flag on a regular expression enables the use of ES2015 Unicode code point escapes (\u{…}) in the pattern. // Note: `a
Reserved keywords in JavaScript · Mathias Bynens
Looking for a list of all reserved words in JavaScript? You’ve come to the right place. I recently needed such a list myself, but ended up comparing the reserved keywords in all ECMAScript versions as well. The result is listed below, for future reference. ECMAScript 1 In the beginning, there was ECMAScript 1. It listed the following reserved words: do if in for new try var case else enum null thi
JavaScript has a Unicode problem · Mathias Bynens
The way JavaScript handles Unicode is… surprising, to say the least. This write-up explains the pain points associated with Unicode in JavaScript, provides solutions for common problems, and explains how the ECMAScript 6 standard improves the situation. Unicode basics Before we take a closer look at JavaScript, let’s make sure we’re all on the same page when it comes to Unicode. It’s easiest to th
Valid JavaScript variable names in ES5 · Mathias Bynens
Note: This article is based on the ECMAScript 5 specification. For the updated ES2015 version, see Valid JavaScript variable names in ES2015. Did you know var π = Math.PI; is syntactically valid JavaScript? I thought this was pretty cool, so I decided to look into which Unicode glyphs are allowed in JavaScript variable names, or identifiers as the ECMAScript specification calls them. Reserved word
JavaScript character escape sequences · Mathias Bynens
Having recently written about character references in HTML and escape sequences in CSS, I figured it would be interesting to look into JavaScript character escapes as well. Character codes, code points, and code units A code point (also known as “character code”) is a numerical representation of a specific Unicode character. For example, the character code of the copyright symbol © is 169, which c
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