Javascript scrollTop gets the offset of the scroll bar relative to its top (for example, making a button that automatically shows hidden “back to top”). The following problems are often encountered in practical applications: Document. The documentElement. ScrollTop total of 0 in the Chrome Document.body.scrolltop is always 0 in IE and firefox

1, each browser window. PageYOffset/document. The documentElement. The scrollTop/document. The body. The scrollTop difference Example:

window.scroll(0,100)
console.log( " window.pageYOffset: " +window.pageYOffset)
console.log( " document.documentElement.scrollTop: " +document.documentElement.scrollTop)
console.log( " document.body.scrollTop: " +document.body.scrollTop)

6/7/8: Doctype: Window. PageYOffset: undefined Document. The documentElement. ScrollTop: 100 Document. The body. The scrollTop: 0

No doctype: Window. PageYOffset: undefined Document. The documentElement. ScrollTop: 0 Document. The body. The scrollTop: 100 Safari/Chrome: Window. PageYOffset: 100 Document. The documentElement. ScrollTop: 0 Document. The body. The scrollTop: 100

Firefox/Opera:

Doctype: Window. PageYOffset: 100 Document. The documentElement. ScrollTop: 100 Document. The body. The scrollTop: 0

No doctype: Window. PageYOffset: 100 Document. The documentElement. ScrollTop: 0 Document. The body. The scrollTop: 100

**2. Get the scrollTop value **Perfect scrollTop assignment shorthand:

var scrollTop = window.pageYOffset|| document.documentElement.scrollTop || document.body.scrollTop;