ExecuteAsyncScript not working with Firefox

[blutorange]

We have several tests that make use of executeAsyncScript. When we started running the tests on Firefox, we noticed that they failed with the error message

Expected async operation, instead received: {}

After a little bit of digging, it seems to be an issue Firefox and how ExecuteAsyncScript checks for whether an object is a promise.

I could create a simple Maven project that illustrates the cause of the error

selenese-java-exec-async-ff.zip

Unpack the file, cd into the directory, make sure you have Chrome and Firefox installed on your system, then run mvn test. This runs the same 2 tests, once with Firefox and once with Chrome. Note that it fails with Firefox.

For reference, on my system I've got Chrome Version 99.0.4844.74 (Official Build) (64-bit) and Firefox 98.0.2 (64-bit) Tested with Ubuntu and Windows, so doesn't seem to be OS-specific.

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To summarize what happens, as far as I understand it, is as follows:

• The ExecuteAsyncScript command uses Promise.resolve(p) === p to check whether an object is a promise.
• When you have an HTML page with something like window.getPromise = () => Promise, then use the WebDriver to execute JavaScriptExecutor#executeAsyncScript("arguments[0](Promise === window.getPromise())"), you get false. The promise constructor from the HTML page and the Promise constructor from the script we run via WebDriver appear to be different.
  • Similarly, Promise.resolve(p) === p fails when p is a Promise that was obtained from a function in a script block in the HTML page.
  • It seems that FF uses two different realms for the script in the HTML page and for the script executed via WebDriver?

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So it seems like we might need another way to check for whether an object is a Promise, though I'm not sure how that check could look like.

• This single-line NPM module checks whether a value is truthy, an object or a function, and that is has a then property which is a function.
• Angular has a utility method that just checks for truthiness and a then property which is a function.
• A comment on this stackoverflow answer also warns about using Promise.resolve(p) === p as it wouldn't work for different realms (which I suspect is the issue here).
• The spec also seems to allow thenables