CODEWITHSUNDEEP
javascript
roth514
roth514

Reputation: 49

Using JavaScript utilize "Back to Top"

I found a "Back to Top" effect written in JavaScript. But there's something in the code I don't understand.Thanks a lot!

window.onload = function () {
    var oBtn = document.getElementById('btn1');
    var timer=null;

    oBtn.onclick= function () {
       timer=setInterval(function () {
            var scrollTop = document.documentElement.scrolltop || document.body.scrollTop;
            var iSpeed = Math.floor(-scrollTop/8);//what does this mean?
           if(scrollTop===0)
           {
               clearInterval(timer);
           }

            document.body.scrollTop = document.documentElement.scrollTop = scrollTop + iSpeed;
        }, 30);
    };
};

Upvotes: 1

Views: 57

Answers (2)

Ayrit
Ayrit

Reputation: 103

iSpeed is best thought of as a velocity. As the document scrolls back upwards, it gets set to that velocity which gradually becomes smaller as it gets closer to the targeted element.

Math.floor(-scrollTop/8) In this case, you're constantly scrolling up an 1/8th of the distance between your position and that element.

Upvotes: 1

Duncan Thacker
Duncan Thacker

Reputation: 5188

-scrollTop means 0-scrollTop.

-scrollTop/8 means 0-scrollTop divided by 8.

Math.floor() rounds down to the nearest whole number.

Upvotes: 1

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