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Editorial Team
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Asked: 2026-05-24T17:53:31+00:00

Consider this JavaScript Code: <script type=text/javascript> function loop(Message){ document.getElementById(‘output’).innerHTML = document.getElementById(‘output’).innerHTML + Message +

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Consider this JavaScript Code:

<script type="text/javascript">

function loop(Message){
document.getElementById('output').innerHTML = 
document.getElementById('output').innerHTML + Message + '</br>';
}

window.setInterval("loop('Message1')", 1000); //Prints "Message1" every 1 Seconds
window.setInterval("loop('Message2')", 3000); //Prints "Message2" every 3 Seconds

</script>

<body>
<div id="output"></div>
</body>

Output:
Message1
Message1
Message2
Message1
Message1
Message1
Message2
...

And now Consider this PHP-Code:

<?php
while(true){               // Print "Message1" every 1 Second
echo 'Message 1 </br>';
sleep(1);
}



while(true){              //This Code will never be executed, 
echo 'Message 2 </br>';    //because the First Loop Blocks the Process!!!!!
sleep(3);
}

?>

Output:
Message1
Message1
Message1
Message1
Message1
Message1
...

Why doesn’t the first JavaScript-Loop stop the whole Process like the first while-Loop in PHP?

I know that Javascript is SingleThreaded, so I suppose, that JavaScript can’t just start a new Thread to process the Second Loop. So I am woundering how JavaScript doesn’t block here?

Could someone explain it to me?

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1 Answer

  1. Editorial Team

    Because setInterval is specifically not sleep.

    setInterval is more like a scheduler. It marks a time at which something should be executed and lets the main thread forget about it completely until that time arrives.

    sleep on the other hand is basically a no-op loop, which lets nothing else execute while it’s looping.

    This is important, since the use cases for PHP and Javascript are entirely different. A PHP script has a start and an end and it executes in a linear fashion. Javascript is used for interactivity, which means it needs to respond to various things (mouse clicks, keyboard input, timers, AJAX callbacks) in a non-linear fashion as they happen. If the main thread was blocked waiting for a timer, nothing would happen and the interface would appear frozen.

    Here’s a good article by jQuery’s John Resig about Javascript timers: http://ejohn.org/blog/how-javascript-timers-work/

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