> But the general benefit you get from JS being single-threaded is the fact that any given callback is transactional. No other code will ever come in and mutate

> But the general benefit you get from JS being single-threaded is the fact that any given callback is transactional. No other code will ever come in and mutate state between two regular lines of JavaScript code. Achieving this is pretty much the whole point of locking mechanisms, so under normal circumstances they aren't necessary for JS. The same claim can be made for Visual Basic. This always sounds like a stronger guarantee than it actually is. In practice, the exact same mistakes get made irrespective of what the runtime provides. Ultimately, people have to learn about race conditions in order to write correct code in a nontrivial system. Brushing the 'majority' of such cases under the carpet just makes it harder to educate them.