document.querySelectorAll('a[href^="#"]').forEach(elem => {
    elem.addEventListener('click', e => {
        e.preventDefault();
        let block = document.querySelector(elem.getAttribute('href')),
            offset = elem.dataset.offset ? parseInt(elem.dataset.offset) : 0,
            bodyOffset = document.body.getBoundingClientRect().top;
        window.scrollTo({
            top: block.getBoundingClientRect().top - bodyOffset + offset,
            behavior: 'smooth'
        }); 
    });
});

Browser support -> https://caniuse.com/#feat=scrollintoview