web_sys::window() in Web Worker not returning None but throwing an error

[j-devel]

Describe the Bug

Calling web_sys::window() in a Web Worker throws an unexpected runtime error.

In this case, according to its signature (pub fn window() -> Option<Window>), I expect it to return None instead.

References:

• without-a-bundler-no-modules-worker (a custom example)
• index.html and lib.rs (relevant code)

Steps to Reproduce

1. Build the custom example:

$ git clone -b issue-window https://github.com/j-devel/wasm-bindgen
$ cd wasm-bindgen/examples/without-a-bundler-no-modules-worker/
$ wasm-pack build --target no-modules
$ python -m SimpleHTTPServer
Serving HTTP on 0.0.0.0 port 8000 ...

2. Open http://localhost:8000/ in browser and see the Developer Console
3. See error
   

Expected Behavior

web_sys::window() returns None. No runtime errors reported in the Developer Console .

Actual Behavior

Get an Uncaught ReferenceError as in the above screenshot.

Additional Context

n/a

[Pauan]

This is a duplicate of #1046

We've temporarily fixed this in gloo, but we should think about a better more long-term solution for this.

[j-devel]

Woops, I just learnd this is actually a well-known issue; and thanks for the pointers to the existing pragmatic approaches that are basically relying on unchecked_into().

Skimming through the previous discussions in the links, I do share the same view that a more fundamental fix would be for us to do something about instanceof.

The root problem has turned out to be not only about web_sys::window(). I am working on some global-agnostic code where I just want to perform some simple checks like

js_sys::global().has_type::<web_sys::Window>()
js_sys::global().is_instance_of::<web_sys::Window>()
js_sys::global().has_type::<web_sys::WorkerGlobalScope>()
js_sys::global().is_instance_of::<web_sys::WorkerGlobalScope>()

and they all are failing with similar errors thrown when in a 'wrong' context. With the sought-after fundamental fix in mind, I'd like to open a separate issue when I come up with something worth sharing.

And for now, I'm closing this particular issue on resolving global to Window.