https://www.nakajima-jibika.jp/js/smoothscroll.js

1var today = new Date();
2var nyr = today.getFullYear(); 
3/* Smooth scrolling
4   Changes links that link to other parts of this page to scroll
5   smoothly to those links rather than jump to them directly, which
6   can be a little disorienting.
7   
8   sil, http://www.kryogenix.org/
9   
10   Wordpress Plug-in Smooth Scroll Links [SSL] by Chetan Gole [http://chetangole.com/]
11   
12*/
13
14var ss = {
15  fixAllLinks: function() {
16    // Get a list of all links in the page
17    var allLinks = document.getElementsByTagName('a');
18    // Walk through the list
19    for (var i=0;i<allLinks.length;i++) {
20      var lnk = allLinks[i];
21      if ((lnk.href && lnk.href.indexOf('#') != -1) && 
22          ( (lnk.pathname == location.pathname) ||
23	    ('/'+lnk.pathname == location.pathname) ) && 
24          (lnk.search == location.search)) {
25        // If the link is internal to the page (begins in #)
26        // then attach the smoothScroll function as an onclick
27        // event handler
28        ss.addEvent(lnk,'click',ss.smoothScroll);
29      }
30    }
31  },
32
33  smoothScroll: function(e) {
34    // This is an event handler; get the clicked on element,
35    // in a cross-browser fashion
36    if (window.event) {
37      target = window.event.srcElement;
38    } else if (e) {
39      target = e.target;
40    } else return;
41
42    // Make sure that the target is an element, not a text node
43    // within an element
44    if (target.nodeName.toLowerCase() != 'a') {
45      target = target.parentNode;
46    }
47  
48    // Paranoia; check this is an A tag
49    if (target.nodeName.toLowerCase() != 'a') return;
50  
51    // Find the <a name> tag corresponding to this href
52    // First strip off the hash (first character)
53    anchor = target.hash.substr(1);
54    // Now loop all A tags until we find one with that name
55    var allLinks = document.getElementsByTagName('a');
56    var destinationLink = null;
57    for (var i=0;i<allLinks.length;i++) {
58      var lnk = allLinks[i];
59      if (lnk.name && (lnk.name == anchor)) {
60        destinationLink = lnk;
61        break;
62      }
63    }
64    if (!destinationLink) destinationLink = document.getElementById(anchor);
65
66    // If we didn't find a destination, give up and let the browser do
67    // its thing
68    if (!destinationLink) return true;
69  
70    // Find the destination's position
71    var destx = destinationLink.offsetLeft; 
72    var desty = destinationLink.offsetTop;
73    var thisNode = destinationLink;
74    while (thisNode.offsetParent && 
75          (thisNode.offsetParent != document.body)) {
76      thisNode = thisNode.offsetParent;
77      destx += thisNode.offsetLeft;
78      desty += thisNode.offsetTop;
79    }
80  
81    // Stop any current scrolling
82    clearInterval(ss.INTERVAL);
83  
84    cypos = ss.getCurrentYPos();
85  
86    ss_stepsize = parseInt((desty-cypos)/ss.STEPS);
87    ss.INTERVAL =
88setInterval('ss.scrollWindow('+ss_stepsize+','+desty+',"'+anchor+'")',10);
89  
90    // And stop the actual click happening
91    if (window.event) {
92      window.event.cancelBubble = true;
93      window.event.returnValue = false;
94    }
95    if (e && e.preventDefault && e.stopPropagation) {
96      e.preventDefault();
97      e.stopPropagation();
98    }
99  },
100
101  scrollWindow: function(scramount,dest,anchor) {
102    wascypos = ss.getCurrentYPos();
103    isAbove = (wascypos < dest);
104    window.scrollTo(0,wascypos + scramount);
105    iscypos = ss.getCurrentYPos();
106    isAboveNow = (iscypos < dest);
107    if ((isAbove != isAboveNow) || (wascypos == iscypos)) {
108      // if we've just scrolled past the destination, or
109      // we haven't moved from the last scroll (i.e., we're at the
110      // bottom of the page) then scroll exactly to the link
111      window.scrollTo(0,dest);
112      // cancel the repeating timer
113      clearInterval(ss.INTERVAL);
114      // and jump to the link directly so the URL's right
115      location.hash = anchor;
116    }
117  },
118
119  getCurrentYPos: function() {
120    if (document.body && document.body.scrollTop)
121      return document.body.scrollTop;
122    if (document.documentElement && document.documentElement.scrollTop)
123      return document.documentElement.scrollTop;
124    if (window.pageYOffset)
125      return window.pageYOffset;
126    return 0;
127  },
128
129  addEvent: function(elm, evType, fn, useCapture) {
130    // addEvent and removeEvent
131    // cross-browser event handling for IE5+,  NS6 and Mozilla
132    // By Scott Andrew
133    if (elm.addEventListener){
134      elm.addEventListener(evType, fn, useCapture);
135      return true;
136    } else if (elm.attachEvent){

137      var r = elm.attachEvent("on"+evType, fn);
138      return r;
139    } else {
140      alert("Handler could not be removed");
141    }
142  } 
143}
144
145ss.STEPS = 25;
146
147ss.addEvent(window,"load",ss.fixAllLinks);