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The only way I have heard threads are expensive, in the context of handling many io requests, is stack usage. You can tell the os to give less memory (statically determined stack size) to the thread when it's spawned, so this is not a fundamental issue to threads.
Go ahead and spin up a web worker and transfer a bunch of data to it and tell us how long you had to wait.
Time to transfer data to one thread is related to io speed. Why would this have anything to do with concurrency model?
Well I just told you another one, one actually relevant to the conversation at hand, since it's the only one you can use with JavaScript in the context of a web browser.
You cant say async is the fundamentally better model because threading is purposely crippled in the browser.
The conversation at hand is not "how do io in browser". Its "async is not inherently better than threads"
No, because async is fundamentally a paradigm for how to express asynchronous programming, i.e. situations where you need to wait for something else to happen, threading is not an alternative to that, callbacks are.
Threads are callbacks.