同时执行2个功能。 可能吗？

Question

Let's assume I have the following code:

var funcIsRunning = 0;
function a(){
   funcIsRunning = 1;
   // some code ... (maybe lots of code);
   funcIsRunning = 0;
}

Is there any chance that some asynchronous function will catch funcIsRunning flag in '1' position? (eg the callback of XHR or a function that is called via setTimeout, etc.) Or, which is the same, can two functions be executed at the same time (not one after another)?

Just to make it clear... In fact, I don't have to run 2 funcs simultaneously... I'm just wondering how it works.

PS I would be very grateful if you could provide documentation/grounding of your answer Thanks a lot!

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Made a test here that illustrates given answers: http://jsfiddle.net/rz7tvyof/

Answer 1

No, JavaScript is single threaded and runs only one function at a time. Although, if you have a callback function waiting for, say, a network request that happens to return at funcIsRunning = 1 , that callback function will be put on the queue that is handled by the event loop

JavaScript event loop explained

Answer 2

Normally no thanks to event loop. But you can use Web Workers to run code in parallel (with some limitations such as access to DOM etc.).

Worker documentation