Soon We Can Finally Banish JavaScript to the ShadowRealm

It’s gonna be tough to keep it together on this one. Okay. I got this. I am a professional technical writer . Straight face; all-business. Ahem: if you’ve been following the ongoing work at TC39 (the standards body responsible for maintaining and developing the standards that inform JavaScript) you may have encountered some of their recent work on ShadowRealms — snrk . Sorry! Sorry, I’m good! Just, whew ­— what a name, “ShadowRealms.” Okay, hang on, let me start at the beginning. Maybe that will help. It’s exceptionally likely you’ve seen JavaScript described as “single-threaded” at some point — that’s usually pretty high up on the list of JavaScript fundamentals, alongside “case sensitive,” “whitespace insensitive,” and “bad at math.” That is correct , in the strict “computer science” sense, but it still gets my hackles up a little whenever I see it. I mean, accurate in that JavaScript isn’t multi-threaded, for sure. A script is always executed in a very linear way — top to bottom, left to right, one execution context after another, winding up the call stack and then back down again. It’s just that you eventually come to learn about something like Web Workers , which — not to put too fine a point on this — allow you to execute JavaScript code in another thread . That’s where I think “JavaScript is single-threaded” becomes a less helpful framing, because even though JavaScript isn’t a multi-threaded language , a JavaScript application can make use of multiple threads. It’s a better framing — and every bit as technically accurate — to say that a JavaScript realm is single-threaded. A realm refers to the environment where code is executed: a browser tab is a realm, and within that realm is the single thread where JavaScript is executed — the main thread . A Web Worker is a realm with a worker thread . JavaScript running in a cross-origin iframe is running in that iframe realm’s main thread. We can’t, for example, offload the execution of a single function to another t