[amelius]

I also want to be able to run javascript (and the rest of the browser) deterministically in a test environment, and run tests, each with different timing characteristics.

Of course, I want to run my server in the same way.

So perhaps this is better done inside some "deterministic virtual machine", where the browser and server are run deterministically hand-in-hand.

[jpeterson]

First of all, the browser environment is the asynchronous part here, not JavaScript. JavaScript is a language that happens to be interpreted on top of that environment. You'd run into similar issues with any other language over an asynchronous execution model.

Second, you can actually execute browser and node code deterministically if you like, by providing alternative implementations of setTimeout, setInterval, setImmediate, etc. that serialize execution of the callbacks in the order of your choice.