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window.scrollTo({ top: 900, behavior: 'smooth' })
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const btn = document.getElementById('elem');
btn.addEventListener('click', () => window.scrollTo({
top: 400,
behavior: 'smooth',
}));
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<script type="text/javascript" src="https://cdnjs.cloudflare.com/ajax/libs/smooth-scroll/16.1.0/smooth-scroll.min.js"></script>