JSFiddle - React, Tailwind, and code Playground

HTML

<div id="log">
fff
</div>

JavaScript

function console_log(what){
     document.getElementById("log").innerHTML += "<br>" + what;
}
var timer; 
function execute() { 
    var date = new Date();
    console_log( date.getMinutes() + ' : ' + date.getSeconds() );
}
 
function metronom() {
    execute();  
    timer=setInterval(execute,3000);
}
 
window.onfocus = function () { 
    metronom(); 
} 
 
window.onblur = function () {  
    clearInterval(timer); 
    //delete timer; // пробую власноручно видалити timer, не допомагає
    console_log("sleep...");   
}
 
//window.onload = function() { 
    metronom(); 
//}