JavaScript setInterval and setTimeout

问题:

The next code display the date every 1 sec and then stops.

(function() {
    var i = setInterval(function() {
        console.log(new Date());
    }, 1000);
    console.log("Hi");

    setTimeout(function() {
        clearInterval(i);
    }, 3000);
    console.log("Hola");
})();

Output:

Hi
Hola
Wed Oct 24 2012 13:35:27 GMT+0200 (CEST)
Wed Oct 24 2012 13:35:28 GMT+0200 (CEST)
Wed Oct 24 2012 13:35:29 GMT+0200 (CEST)

But I don't know why Hi and Hola are displayed first. Also, why setTimeout is executed? It is not supposed that setInterval is executed every 1 sec and nothing else can be executed?. (Does the code above runs on the order in which it is written?) Thanks.

回答1:

setTimeout as well as setInterval only register functions (callbacks) but then go straight to the next command.

Therefor Hi and Hola are printed before the first callbacks are called.

First callback will be that of setInterval after 1 sec, then again after 2 sec.. After 3 seconds setTimeout kicks in and clears setInterval.

回答2:

The comments show the execution order

(function() {                           // 0 anonymous function is called inline

    var i = setInterval(function() {    // 1 setInterval **schedules** the function 
                                        //     parameter to be executed every second
        console.log(new Date()); 
    }, 1000);
    console.log("Hi");                  // 2 Display 'Hi' on console

    setTimeout(function() {             // 3 **Schedule** the function parameter to 
                                        //     execute after three seconds
        clearInterval(i);     
    }, 3000);
    console.log("Hola");                // 4 Display 'Hola' on console
})();

                                        // 5 Display date from setInterval function

                                        // 6 Display date from setInterval function

                                        // 7 Display date from setInterval function

                                        // 8 Cancel setInterval executed from 
                                        //     setTimeout function

Hope this clears up your confusion.

回答3:

Everything out setTimeout() and setInterval() is executed anyway. It's not the sleep() PHP's method... sadly!

However, this link could be useful: http://www.phpied.com/sleep-in-javascript/

回答4:

http://ejohn.org/blog/how-javascript-timers-work/

Those two events are asynchronous. They are queued/executed at the next available moment, not at the exact time "setTimeout" or "setInterval" is called.