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//第一種是正常的點(diǎn)擊返回頂部
// 滾動(dòng)條
$('#back_to_top').click(function(){
    // $(window).scrollTop(0);
})

//第二種是加了滾動(dòng)條變化的動(dòng)畫(huà)效果
// 滾動(dòng)條
$('#back_to_top').click(function(){
    $('html,body').stop().animate({
        scrollTop:"0px"
    },"slow")
})

The first time is $(window) and the second time is $('html,body')
What are the differences between these three? When to use window, when to use html, and when to use body?
Also why sometimes both html and body are written in.