postmessage | Backwards compatible window.postMessage

So, NOTHING about the code inside your async function supports promises. You can't just throw random asynchronous (but not promise-based) code inside an async function and expect anything to work. An async function will work just fine with promise-based asynchronous functions that you await. Otherwise, it knows nothing about your asynchronous operations in there. That's why calling encode() returns immediately without waiting for anything to complete.

In addition, return transcode_data is inside an asynchronous callback. Returning inside that callback just goes back into the system code that called the callback and is dutifully ignored. You're not returning anything there from the async function itself. You're returning to that callback.

Since your operation is not promise-based, to solve this, you will have to make it promise-based by wrapping it in a promise and then manually resolved or rejecting that promise when needed with the proper values. You can do that like this:

You can try like this, in Test.js: My english is not really good, so I can't explan it so much.

useEffect always cache state and reuse for its callback func, in this case is "currentNumber". So the "countUp" func always have currentNumber = 0. Only When dependencies has change, it will recreate its callback func and apply new state to it.

Try to create a new class which inherit from WKScriptMessageHandler to handle the DidReceiveScriptMessage method .

Modify your code as below

1. Change the Handler

Most epub viewers that I'm familiar with use a web engine for rendering. So to get your SVG to be visible in an epub, it probably needs to be visible in a web browser.

Trying your second SVG, I don't see anything in Chrome or Firefox. Or in a generic image viewer.

If I add a stroke= value to your path, I see "something" in the browser, but it's a tiny little thing. As if the viewbox and/or the use settings are confusing the browser.

You cannot directly use router. declare the express app first and attach the router to it.

window in your iframe's context point's to the iframe's Window. You are posting a message to the iframe's content from the iframe's content.
What you want is to post a message to the parent context, so do

When working with WorkerThread code like this, I found that the best way to distribute jobs was to have the WorkerThread ask the main thread for a job when the WorkerThread knew that it was done with the prior job. The main thread could then send it a new job in response to that message.

In the main thread, I maintained a queue of jobs and a queue of WorkerThreads waiting for a job. If the job queue was empty, then the WorkerThread queue would likely have some workerThreads in it waiting for a job. Then, any time a job is added to the job queue, the code checks to see if there's a workerThread waiting and, if so, removes it from the queue and sends it the next job.

Anytime a workerThread sends a message indicating it is ready for the next job, then we check the job queue. If there's a job there, it is removed and sent to that worker. If not, the worker is added to the WorkerThread queue.

This whole bit of logic was very clean, did not need atomics or shared memory (because everything was gated through the event loop of the main process) and wasn't very much code.

I arrived at this mechanism after trying several other ways that each had their own problems. In one case, I had concurrency issues, in another I was starving the event loop, in another, I didn't have proper flow control to the WorkerThreads and was overwhelming them and not distributing load equally.

The saveMessage function is declared async

1. Site permissions should be added to host_permissions, not permissions, more info.
2. create + executeScript in ManifestV3 is bugged so either a) go back to ManifestV2 until this is fixed or b) use the workaround below.

Wait for the URL to be set: