document.addEventListener("DOMContentLoaded", event => { 
    let btns = [
        document.querySelector('#btn1').classList,
        document.querySelector('#btn2').classList,
        document.querySelector('#btn3').classList,
    ]
    
    let sections = [
        document.querySelector('#section_1').classList,
        document.querySelector('#section_2').classList,
        document.querySelector('#section_3').classList
    ]
    
    let seconds_dalay = 4

    let count = 0
    setInterval(() => {        
        switch (count) {
            case 0:                
                btns[2].remove('active')
                btns[count].add('active')
                
                sections[2].remove('active')
                sections[count].add('active')
                break;
            case 1:
                btns[0].remove('active')
                btns[count].add('active')
                
                sections[0].remove('active')
                sections[count].add('active')
                break;
            case 2:
                btns[1].remove('active')
                btns[count].add('active')
                
                sections[1].remove('active')
                sections[count].add('active')
                break;
        }

        if(count == 2)
            count = 0
        else
            count += 1

    }, (seconds_dalay * 1000))

})