Consider adding `self.location` to the worker global scope.

[John Spurlock]

A common way for Cloudflare-unaware JS libraries to check if they are running in a web worker is: typeof importScripts == 'function'.

Once this passes, they assume other web-standard API like self.location is defined.

Perhaps workerd could use a blob url as the location here instead of leaving it undefined?

[James M Snell]

Not sure there's really sufficient use case justification for this. How would you expect to use it? I'll note that none of the other primary non-browser runtimes (node.js, deno, bun, etc) implement self.location.

[John Spurlock]

I ran into emscripten js boilerplate that does a "am I running in a web worker" check using typeof importScripts = 'function'. That actually passes on CF workers, but later on when it attempts to use self.location (unchecked, assuming it's there since importScripts was), it crashes. It just uses the .href for some default location probing logic.

I have a PR out for that on their side - honestly I was surprised importScripts was defined at all in CF workers. Does anyone use it? Removing importScripts would also fix this particular issue.

[Hood Chatham]

For Emscripten feature detection I've been doing:

    // Force Emscripten to feature detect the way we want
    // They used to have an `environment` setting that did this but it has been
    // removed =(
    globalThis.window = {};       // makes ENVIRONMENT_IS_WEB    = true
    globalThis.importScripts = 1; // makes ENVIRONMENT_IS_WORKER = false
    const p = createEmscriptenModule(emscriptenSettings);
    delete globalThis.window;
    delete globalThis.importScripts;
    const emscriptenModule = await p;

[John Spurlock]

Here's the PR over there in case anyone wants to +1: emscripten-core/emscripten#21010

For workerd tho, I'm still curious why importScripts exists - never seen a single cloudflare worker example where it is utilized

[Hood Chatham]

It's also not implemented:

jsg::Unimplemented importScripts(kj::String s) { return {}; };

Use Unimplemented as the return type of a method to mark the whole method unimplemented.
Attempts to use the API will throw an exception.

So removing it should be pretty harmless...

[James M Snell]

I'd be fine with removing importScripts(...) /cc @kentonv

[James M Snell]

Landed the PR that removes importScripts with a compatibility flag.

[James M Snell]

FWIW, the most reliable means of determining if code is running within the workers environment is to check the value of navigator.userAgent. Given that we've landed the PR removing importScripts under a compatibility flag, I'm going to go ahead and close this. The change likely would not hit production for another week or two.

[John Spurlock]

@jasnell, I'm getting ReferenceError: navigator is not defined when trying to access navigator.userAgent on Cloudflare Workers - am I missing something?

[MrBBot]

@johnspurlock I'm guessing you don't have the global_navigator compatibility flag enabled?

[John Spurlock]

@mrbbot ah right, that new worker had no compatibility flag/date at all

{
  placement: {},
  compatibility_date: "",
  compatibility_flags: [],
  usage_model: "unbound",
  tags: [],
  tail_consumers: [],
  logpush: false,
  bindings: [
    ...
  ]
}

After redeploying with a compat date >= 2022-03-21, navigator.userAgent is defined, thanks!