PageScrollTo.js

const SCROLL_TOP_OFFSET = -19;
class Scroll{
	static next(){
		//~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
		if( ! this.ary ){
			let elm, root = document.getElementById("ContentArea");
			this.ary = new Array();

			for( let i=0; i < root.childNodes.length; i++ ){
				if( root.childNodes[ i ].nodeType != 1 ) continue;
				elm = root.childNodes[ i ];
				this.ary.push( elm.offsetTop - elm.scrollTop + SCROLL_TOP_OFFSET );
			}
			this.ary[0] = 0;

			//console.log( this.ary );
		}

		//~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
		let i, pos=0, 
			cpos = window.pageYOffset;

		for( i=this.ary.length - 1; i >= 0; i-- ){
			if( cpos >= this.ary[i] ){
				pos = ( i+1 < this.ary.length )? this.ary[i+1] : 0;
				break;
			}
		}

		//~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
		//console.log("SCROLL TO", pos );
		this.to = pos;
		this.task.start();
	}

	static first(){
		this.to = 0;
		this.task.start();
	}
}

Scroll.idx	= 0;
Scroll.ary	= null;
Scroll.to	= 0;
Scroll.task	= new RunTillTrue( 15, function(){
	let py 	= window.pageYOffset,
		dif = Scroll.to - py;

	if( Math.abs(dif) <= 1 ){ window.scrollTo( 0, Scroll.to ); return true; }

	let offset = ( dif < 0 )? Math.floor( dif * 0.04 ) : Math.ceil( dif * 0.04 );
	window.scrollTo( 0, py + offset );
	return false;
});

class RunTillTrue{
	constructor( s, func = null ){
		this.sec	= s;
		this.thread = null;
		this.task 	= func;
		this.active	= false;
		
		this.run 	= ()=>{ 
			if( !this.active || this.task() == true ) this.stop();
		};
	}
	
	stop(){ 
		this.active = false; 
		clearInterval( this.thread ); 
		this.thread = null;
	}
	
	start(){
		if( !this.thread ) clearInterval( this.thread );
		this.active = true;
		this.thread = setInterval( this.run, this.sec );	
	}
}