Javascript - window.scroll({ behavior: 'smooth' }) not working in Safari

Use smootscroll polyfill (solution for all browsers), easy applicable and lightweight dependency: https://github.com/iamdustan/smoothscroll

Once you install it via npm or yarn, add it to your main .js, .ts file (one which executes first)

import smoothscroll from 'smoothscroll-polyfill';
// or if linting/typescript complains
import * as smoothscroll from 'smoothscroll-polyfill';

// kick off the polyfill!
smoothscroll.polyfill();

Behavior options aren't fully supported in IE/Edge/Safari, so you'd have to implement something on your own. I believe jQuery has something already, but if you're not using jQuery, here's a pure JavaScript implementation:

function SmoothVerticalScrolling(e, time, where) {
    var eTop = e.getBoundingClientRect().top;
    var eAmt = eTop / 100;
    var curTime = 0;
    while (curTime <= time) {
        window.setTimeout(SVS_B, curTime, eAmt, where);
        curTime += time / 100;
    }
}

function SVS_B(eAmt, where) {
    if(where == "center" || where == "")
        window.scrollBy(0, eAmt / 2);
    if (where == "top")
        window.scrollBy(0, eAmt);
}

And if you need horizontal scrolling:

function SmoothHorizontalScrolling(e, time, amount, start) {
    var eAmt = amount / 100;
    var curTime = 0;
    var scrollCounter = 0;
    while (curTime <= time) {
        window.setTimeout(SHS_B, curTime, e, scrollCounter, eAmt, start);
        curTime += time / 100;
        scrollCounter++;
    }
}

function SHS_B(e, sc, eAmt, start) {
    e.scrollLeft = (eAmt * sc) + start;
}

And an example call is:

SmoothVerticalScrolling(myelement, 275, "center");

For a more comprehensive list of methods for smooth scrolling, see my answer here.

window.requestAnimationFrame can be used to perform smooth scrolling in an exact amount of time.

For smooth vertical scrolling, the following function can be used. Note that horizontal scrolling can be done in much the same manner.

/*
   @param time: the exact amount of time the scrolling will take (in milliseconds)
   @param pos: the y-position to scroll to (in pixels)
*/
function scrollToSmoothly(pos, time) {
    var currentPos = window.pageYOffset;
    var start = null;
    if(time == null) time = 500;
    pos = +pos, time = +time;
    window.requestAnimationFrame(function step(currentTime) {
        start = !start ? currentTime : start;
        var progress = currentTime - start;
        if (currentPos < pos) {
            window.scrollTo(0, ((pos - currentPos) * progress / time) + currentPos);
        } else {
            window.scrollTo(0, currentPos - ((currentPos - pos) * progress / time));
        }
        if (progress < time) {
            window.requestAnimationFrame(step);
        } else {
            window.scrollTo(0, pos);
        }
    });
}

Demo:

/*
   @param time: the exact amount of time the scrolling will take (in milliseconds)
   @param pos: the y-position to scroll to (in pixels)
*/
function scrollToSmoothly(pos, time) {
    var currentPos = window.pageYOffset;
    var start = null;
    if(time == null) time = 500;
    pos = +pos, time = +time;
    window.requestAnimationFrame(function step(currentTime) {
        start = !start ? currentTime : start;
        var progress = currentTime - start;
        if (currentPos < pos) {
            window.scrollTo(0, ((pos - currentPos) * progress / time) + currentPos);
        } else {
            window.scrollTo(0, currentPos - ((currentPos - pos) * progress / time));
        }
        if (progress < time) {
            window.requestAnimationFrame(step);
        } else {
            window.scrollTo(0, pos);
        }
    });
}

document.querySelector('button').addEventListener('click', function(e){
  scrollToSmoothly(500, 1500);
});

html, body {
  height: 1000px;
}

<button>Scroll to y-position 500px in 1500ms</button>

For more complex cases, the SmoothScroll.js library can be used, which handles smooth scrolling both vertically and horizontally, scrolling inside other container elements, different easing behaviors, scrolling relatively from the current position, and more. It also supports most browsers that do not have native smooth scrolling.

var easings = document.getElementById("easings");
for(var key in smoothScroll.easing){
    if(smoothScroll.easing.hasOwnProperty(key)){
        var option = document.createElement('option');
        option.text = option.value = key;
        easings.add(option);
    }
}
document.getElementById('to-bottom').addEventListener('click', function(e){
    smoothScroll({yPos: 'end', easing: easings.value, duration: 2000});
});
document.getElementById('to-top').addEventListener('click', function(e){
    smoothScroll({yPos: 'start', easing: easings.value, duration: 2000});
});

<script src="https://cdn.jsdelivr.net/gh/LieutenantPeacock/[email protected]/src/smoothscroll.min.js" integrity="sha384-UdJHYJK9eDBy7vML0TvJGlCpvrJhCuOPGTc7tHbA+jHEgCgjWpPbmMvmd/2bzdXU" crossorigin="anonymous"></script>
<!-- Taken from one of the library examples -->
Easing: <select id="easings"></select>
<button id="to-bottom">Scroll To Bottom</button>
<br>
<button id="to-top" style="margin-top: 5000px;">Scroll To Top</button>