JavaScript and single-threadedness

Question

I always hear that JavaScript is single-threaded; that when JavaScript is executed, it's all run in the same global mosh pit, all in a single thread.

While that may be true, that single execution thread may spawn new threads, asynchronousy reqeiving data back to the main thread, correct? For example, when an XMLHttpRequest is sent, doesn't the browser create a new thread that performs the HTTP transaction, then invoke callbacks back in the main thread when the XMLHttpRequest returns?

What about timers--setTimeout and setInterval? How do those work?

Is this single-threadedness the result of the language? What has stopped JavaScript from having multi-threaded execution before the new Web Workers draft?

Answer 1

XMLHttpRequest, notably, does not block the current thread. However, its specifics within the runtime are not outlined in any specification. It may run in a separate thread or within the current thread, making use of non-blocking I/O.

setTimeout and setInterval set timers that, when run down to zero, add an item for execution, either a line of code of a function/callback, to the execution stack, starting the JavaScript engine if code execution has stopped. In other words, they tell the JavaScript engine to do something after it has finished doing whatever it's doing currently. To see this in action, set multiple setTimeout(s) within one method and call it.