John Resig - How JavaScript Timers Work - The Diigo Meta page

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  John Resig - How JavaScript Timers Work http://ow.ly/17UsjT
  – Javascript News (del_javascript) http://twitter.com/del_javascript/statuses/17222587485

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  • Since all JavaScript in a browser executes on a single thread asynchronous events (such as mouse clicks and timers) are only run when there's been an opening in the execution.
  • Since JavaScript can only ever execute one piece of code at a time (due to its single-threaded nature) each of these blocks of code are "blocking" the progress of other asynchronous events. This means that when an asynchronous event occurs (like a mouse click, a timer firing, or an XMLHttpRequest completing) it gets queued up to be executed later
  • Intervals don't care about what is currently executing, they will queue indiscriminately, even if it means that the time between callbacks will be sacrificed.

  • There's a lot that we've learned here, let's recap:

     
     
    • JavaScript engines only have a single thread, forcing asynchronous events to queue waiting for execution.
     
    • setTimeout and setInterval are fundamentally different in how they execute asynchronous code.
     
    • 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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• 01 Apr 10
  

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  At a fundamental level it's important to understand how JavaScript timers work. Often times they behave unintuitively because of the single thread which they are in. Let's start by examining the three functions to which we have access that can construct a

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  • var id = setTimeout(function(){ alert('hello'); }, 1000);
     clearInterval(id);
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  •   setTimeout(function(){
         /* Some long block of code... */
         setTimeout(arguments.callee, 10);
       }, 10);
      
       setInterval(function(){
         /* Some long block of code... */
       }, 10);
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