Question

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JavaScript OR (||) variable assignment explanation

Can someone help explain what this line does, and how?

var scrollTop = html.scrollTop || body && body.scrollTop || 0;

Answer 1

You can think of the logic a bit like this…

if (html.scrollTop > 0) {
    scrollTop = html.scrollTop;
    return;
}

if (body != undefined) {
    if (body.scrollTop > 0) {
         scrollTop = body.scrollTop;
         return;
    }

}

scrollTop = 0;
return;

It is setting the scrollTop variable using a list of priorities.

1. First, try to use html.scrollTop if it exists and is greater than zero.
2. If not, then make sure that body exists and use body.scrollTop if it is greater than zero.
3. If all else fails, just set it to 0