Why is the default running even though the num is set to 1?

let spans = document.querySelector(`#spans`);
let hrs = document.querySelector(`#hrs`);
let mins = document.querySelector(`#mins`);
let secs = document.querySelector(`#secs`);
let start = document.querySelector(`#start`);
let stop = document.querySelector(`#stop`);
spans.style.fontSize = "10em";
let preFix = 0;
let num = 1;
let secspreNum= 0;
let minspreNum = 0;
let hrspreNum = 0;
let myFunc = ()=> {
    setInterval(()=>{
        switch (num){
            case num===1:
                mins.innerHTML = `0` + num;
                num ===0;
                secs.innerHTML =`0`+  num++; 
                
            default:
                console.log(`default test`)
        }
      
     } , 1000);
};


start.addEventListener(`click`,myFunc)       

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <div id="spans">
    <span id="hrs">00</span>:<span id="mins">00</span>:<span id="secs">00</span><br></div>
    <button id="start">Start</button>
    <button id="stop">Stop</button>

    <script src="go2.js"></script>
</body>
</html>

Hello, I'm trying to make a stopwatch atm but I got stuck into a problem. Trying to use the switch statement to set the mins to add up a number if the secs reach 10. But before doing that I've been testing this switch statement and for some reason it runs default statement instead of case . Why is it like that? I set the num variable to 1 and it still runs the default one instead of num===1 . Did I missunderstood something? Thanks !

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