Smooth Scroll behavior polyfill

smooth-scrolling-polyfill.md

The Scroll Behavior specification has been introduced as an extension of the Window interface to allow for the developer to opt in to native smooth scrolling. To date this has only been implemented in Chrome, Firefox and Opera.

There's a complete polyfill here (3.3KB minified). But most of the times, the following is enough for me (641 bytes minified):

smooth-scrolling-poyfill.js

Use as: scrollToElem('#elem-selector');

smooth-scrolling-poyfill.js

// native smooth scrolling for Chrome, Firefox & Opera
// @see: https://caniuse.com/#feat=css-scroll-behavior
const nativeSmoothScrollTo = elem => {
    window.scroll({
        behavior: 'smooth',
        left: 0,
        top: elem.getBoundingClientRect().top + window.pageYOffset
    });
};

// polyfilled smooth scrolling for IE, Edge & Safari
const smoothScrollTo = (to, duration) => {
    const element = document.scrollingElement || document.documentElement,
        start = element.scrollTop,
        change = to - start,
        startDate = +new Date();

    // t = current time
    // b = start value
    // c = change in value
    // d = duration
    const easeInOutQuad = (t, b, c, d) => {
        t /= d/2;
        if (t < 1) return c/2*t*t + b;
        t--;
        return -c/2 * (t*(t-2) - 1) + b;
    };

    const animateScroll = _ => {
        const currentDate = +new Date();
        const currentTime = currentDate - startDate;
        element.scrollTop = parseInt(easeInOutQuad(currentTime, start, change, duration));
        if(currentTime < duration) {
            requestAnimationFrame(animateScroll);
        }
        else {
            element.scrollTop = to;
        }
    };
    animateScroll();
};

// detect support for the behavior property in ScrollOptions
const supportsNativeSmoothScroll = 'scrollBehavior' in document.documentElement.style;

// smooth scrolling stub
const scrollToElem = elemSelector => {
    if (!elemSelector) {
        return;
    }

    const elem = document.querySelector(elemSelector);
    if (elem) {
        if (supportsNativeSmoothScroll) {
            nativeSmoothScrollTo(elem);
        } else {
            smoothScrollTo(elem.offsetTop, 600);
        }
    }
};

smooth-scrolling-poyfill.min.js

var nativeSmoothScrollTo=function(a){window.scroll({behavior:"smooth",left:0,top:a.getBoundingClientRect().top+window.pageYOffset})},smoothScrollTo=function(a,f){var c=document.scrollingElement||document.documentElement,d=c.scrollTop,g=a-d,k=+new Date,h=function(e){e=+new Date-k;var l=parseInt;var b=e/(f/2);1>b?b=g/2*b*b+d:(b--,b=-g/2*(b*(b-2)-1)+d);c.scrollTop=l(b);e<f?requestAnimationFrame(h):c.scrollTop=a};h()},supportsNativeSmoothScroll="scrollBehavior"in document.documentElement.style,
scrollToElem=function(a){a&&(a=document.querySelector(a))&&(supportsNativeSmoothScroll?nativeSmoothScrollTo(a):smoothScrollTo(a.offsetTop,600))};

arrow function expression in IE11:
const nativeSmoothScrollTo = elem => {
... and other
replaced by:
const nativeSmoothScrollTo = function(elem) {
and it works.
TNX!