How can I call a function every 30 seconds on each device at the same time?

Question

I will try to correctly explain my problem. I want to generate a number every 30 seconds, for example:

function getRandomNumber(min, max) { return Math.floor(Math.random() * (max - min + 1)) + min; } setTimeout(function(){ getRandomNumber(0,14); }, 30000); 

This works well, but not the same on every device, for example, if I open the page with this code, I will get a random number, but if my friend opens it after, for example, there is a delay 5 seconds after opening the page. Is there any way to fix this? I thought something like getting a time server and executing it according to it might work, but I have no idea how to do this.

Answer 1

There is no perfect solution to this problem, two options are possible here:
Simple and complex, which should be more accurate.

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If you use the same time on all devices *, you can use new Date().getTime() % 30000 for the first call, so they are all synchronized, and then always use the 30000 constant in timeout.

 function getRandomNumber(min, max) { return Math.floor(Math.random() * (max - min + 1)) + min; } setTimeout(function() { getRandomNumber(0, 14); }, new Date().getTime() % 30000); // first time use this 

* Time intervals will not matter, but seconds are minutes

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Another way would be to let the server pass its time, and then guess the delay to fix the load time, and use this instad new Date().getTime()

Answer 2

One of the easiest solutions would be to create a new Date object, read the seconds using getSeconds , then round it to a multiple of 30 and schedule the generator in the next step using setTimeout . Once it starts, you can decide to either schedule it again using the same function, or set the interval using setInterval .

If you don't want to rely on a local clock, I think you need to use a service like ntp, but that requires you to know that the delta applies to time, etc.

Answer 3

You can do this with a single line of PHP.

It works here.

 <script> function getRandomNumber(min, max) { return Math.floor(Math.random() * (max - min + 1)) + min; } // The variable 'startTime' is synchronized with the server var startTime=parseInt(<?php echo date("now")?>); setInterval(function() { startTime++; if(startTime%30==0) document.write(parseInt(getRandomNumber(0, 14)).toString()+"<br>"); }, 1000); </script> 

Note that I used setInterval instead of setTimeout, so it will be repeated.