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Show message before script block

I have a massive script to run synchronously, and I want to show the message "Processing, wait ..." before it starts, so the user does not wonder why the page froze for a few seconds. I am trying to do something like:

messageBox.html("Processing, please wait...");

// run hefty script

messageBox.html("Finished!");

But page blocks before the message are displayed, even if the instruction comes messageBox.html()first. Why is this?

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3 answers

Sometimes it makes sense to run a "hefty script" in timeout.

messageBox.html("Processing, please wait...");

setTimeout(function () {
    heftyScript();

    messageBox.html("Finished!");
}, 1);

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- : http://www.html5rocks.com/en/tutorials/workers/basics/

UPDATE:

- , . .

:

function heftyScript() {

  var arr = [...];
  var chunk_start = 0;

  function do_chunk() {

    for( var i = 0; i < 100; ++i ) { // 100 items per chunk
       if( chunk_start >= arr.length)
         return;
       process( arr[chunk_start++] ); // process one element 
    }
    window.setTimeout(do_chunk,10);
  }
  do_chunk();
}
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It depends on where the time is spent. If it loads and parses the JavaScript file, it messageBox.html()must be pure html, or you do it in a script block before referencing an external file. If the time spent launches this long function, then it setTimeout(function () { heftyScript(); messageBox.html('finished'); }, 1);works wonderfully.

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