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I have two questions related to the JavaScript's setInterval() method. I haven't found any practical cases (but I guess it's not impossible also) related to these question, but for curiosity I wanted to ask these questions.

  1. What happens if the code to be be executed by the setInterval() takes more time than the time interval provided? Does the previous execution stops and the current one starts executing or both will run in parallel.

  2. What if the whole system (OS) is hanged between the time gap when setInterval() is called? Is it possible that the code can execute with some different interval during this condition? I mean does setInterval() guarantees that the code will be executed at the specified interval only? Thanks

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  1. JavaScript uses single threaded execution. Functions such as setTimeout and setInterval lead many to believe that it is possible to multi-thread in JavaScript. In reality, setInterval and setTimeout merely schedule a function or expression to execute at a specified time and those functions are added to the same single-threaded stack. If the browser is in the middle of processing something else when a setTimeout or setInterval is scheduled to fire, the scheduled functions will execute as soon as the browser can get to it.

  2. setInterval does not guarantee that a function will execute at the specified interval only. setInterval will try to execute a function at the specified time, but any number of things could delay the execution or prevent it from executing altogether.

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Ok, I get it. But what the single thread execution means. Does it mean that for all the scripts in a page there is only one thread, and each tab in the browsers has exactly one thread.
@DigvijayYadav: No, it just defines how JavaScript behaves. How many threads the engine actually uses is implementation-dependent. While Chrome runs every tab in its own process, Opera is known to be a single-process application.
@DigvijayYadav Bergi is correct. For your development purposes, just understand that for a single page/tab JavaScript will only execute in a single thread. If you have a JavaScript function that runs for a long time, then it will block other functions from running until it has returned.
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Quoting this article by John Resing:

If a timer is blocked from immediately executing it will be delayed until the next possible point of execution (which will be longer than the desired delay).

Intervals may execute back-to-back with no delay if they take long enough to execute (longer than the specified delay).

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