Runtime Concept
Queue
A JavaScript runtime uses a message queue, which is a list of messages to be processed. Each message has an associated function that gets called to handle the message.
function is called with the message as an input parameter.
Event Loop
Messages
while (queue.waitForMessage()) {
queue.processNextMessage();
}This offers some nice properties when reasoning about your program, including the fact that whenever a function runs, it cannot be preempted and will run entirely before any other code runs (and can modify data the function manipulates). This differs from C++, for instance, where if a function runs in a thread, it may be stopped at any point by the runtime system to run some other code in another thread.
A good practice to follow is to make message processing short and if possible cut down one message into several messages.
Adding Messages
If there is no other message in the queue, and the stack is empty, the message is processed right after the delay. However, if there are messages, the setTimeout message will have to wait for other messages to be processed. For this reason, the second argument indicates a minimum time — not a guaranteed time.