main.js

let startBtn = document.querySelector('#start')
        let stopBtn = document.querySelector('#stop')
        let intervalId;
        let timer = document.querySelector('#timer')
        let counter = 0;

        function change(){
            timer.innerHTML = "Seconds Passed: " + counter;
        }

        startBtn.addEventListener('click', function(){
             intervalId = setInterval(function(){
                console.log('1 second has passed')
                counter += 1;
                change();
            }, 1000)
        })

        stopBtn.addEventListener('click', function(){
            clearInterval(intervalId)
        })