Edit in JSFiddle 

function after_scroll_display_top()
{
	"use strict";
	
	try{
		window.onscroll=function(){
			console.log(window.pageYOffset);
			
			var scrollpoint=window.pageYOffset;
			var toTopElement=document.getElementById('toTop');
			
			if(scrollpoint>100){toTopElement.style.display='block';}
			else{toTopElement.style.display='none';}
			return true;
		};
	}catch(e){
		return e.message;
	}
}


function main()
{
	"use strict";
	var result=after_scroll_display_top();
	if(result!==true){
		console.log(result);
	}
}

main();
<!doctype html>
<body>
    <h1 class="header fixed">header</h1>
    
    <p class="contents" id="top">contents data</p>
    
    <p class="toTop" id="toTop"><a href="#top">toTop</a></p>
    
    <script src="js/03_if_scroll.js"></script>
</body>
/* 
html5doctor.com Reset Stylesheet
v1.6.1
Last Updated: 2010-09-17
Author: Richard Clark - http://richclarkdesign.com 
Twitter: @rich_clark
*/

html, body, div, span, object, iframe,
h1, h2, h3, h4, h5, h6, p, blockquote, pre,
abbr, address, cite, code,
del, dfn, em, img, ins, kbd, q, samp,
small, strong, sub, sup, var,
b, i,
dl, dt, dd, ol, ul, li,
fieldset, form, label, legend,
table, caption, tbody, tfoot, thead, tr, th, td,
article, aside, canvas, details, figcaption, figure, 
footer, header, hgroup, menu, nav, section, summary,
time, mark, audio, video {
    margin:0;
    padding:0;
    border:0;
    outline:0;
    font-size:100%;
    vertical-align:baseline;
    background:transparent;
}

body {
    line-height:1;
}

article,aside,details,figcaption,figure,
footer,header,hgroup,menu,nav,section { 
	display:block;
}

nav ul {
    list-style:none;
}

blockquote, q {
    quotes:none;
}

blockquote:before, blockquote:after,
q:before, q:after {
    content:'';
    content:none;
}

a {
    margin:0;
    padding:0;
    font-size:100%;
    vertical-align:baseline;
    background:transparent;
}

/* change colours to suit your needs */
ins {
    background-color:#ff9;
    color:#000;
    text-decoration:none;
}

/* change colours to suit your needs */
mark {
    background-color:#ff9;
    color:#000; 
    font-style:italic;
    font-weight:bold;
}

del {
    text-decoration: line-through;
}

abbr[title], dfn[title] {
    border-bottom:1px dotted;
    cursor:help;
}

table {
    border-collapse:collapse;
    border-spacing:0;
}

/* change border colour to suit your needs */
hr {
    display:block;
    height:1px;
    border:0;   
    border-top:1px solid #cccccc;
    margin:1em 0;
    padding:0;
}

input, select {
    vertical-align:middle;
}

.header{
    background: #fc0;
    color: #fff;
    height: 50px;
    width: 100%;
    line-height: 50px;
    box-sizing: border-box;
    text-align: center;
}

.fixed{
    position: fixed;
    top: 0;
}

.contents{
    padding: 10px;
    padding-top:50px;
    text-align: center;
    height: 2000px;
	background: -webkit-gradient(linear, 0 0, 0 100%, from(#3fd950), color-stop(0.44,#fafa2e), to(#ff2035));
	background: -webkit-linear-gradient(#3fd950 0%, #fafa2e 44%, #ff2035 100%);
	background: -moz-linear-gradient(#3fd950 0%, #fafa2e 44%, #ff2035 100%);
	background: -o-linear-gradient(#3fd950 0%, #fafa2e 44%, #ff2035 100%);
	background: linear-gradient(#3fd950 0%, #fafa2e 44%, #ff2035 100%);
}

.toTop{
    position: fixed;
    right: 10px;
    bottom: 10px;
    height: 50px;
    width: 50px;
    text-decoration: none;
    display: none;
}

.toTop a{
    display: block;
    background: #fc0;
    color: #fff;
    height: 50px;
    line-height: 50px;
    text-align: center;
    width: 50px;
    text-decoration: none;
}