How do I get the offset().top value of an element without using jQuery?

问题

I'm programming a single-page application using the Angular framework. I'm new to it. I've read this guide to help me understand the fundamental differences between jQuery and Angular and I'd like to follow this guidance as much as possible and NOT use jQuery.

Except that jQuery helps get around some of the browser incompatibilities and provides a useful library of functions, like being able to know the top position of an element from the top of the window, as in $('element').offset().top. No plain Javascript seems to be able to come close without rewriting this function, at which point wouldn't it be a better idea to use a jQuery or jQuery like library?

Specifically, what I'm trying to do is set up a directive that fixes an element in place once the top of it is scrolled to a certain position in the window. Here's what it looks like:

directives.scrollfix = function () {
    return {
        restrict: 'C',
        link: function (scope, element, $window) {

            var $page = angular.element(window)
            var $el   = element[0]
            var elScrollTopOriginal = $($el).offset().top - 40

            $page.bind('scroll', function () {

                var windowScrollTop = $page[0].pageYOffset
                var elScrollTop     = $($el).offset().top

                if ( windowScrollTop > elScrollTop - 40) {
                    elScrollTopOriginal = elScrollTop - 40
                    element.css('position', 'fixed').css('top', '40px').css('margin-left', '3px');
                }
                else if ( windowScrollTop < elScrollTopOriginal) {
                    element.css('position', 'relative').css('top', '0').css('margin-left', '0');
                }
            })

        }
    }
}

If there's a much better way to achieve this in Angular that I'm still just not getting, I'd appreciate the advice.

回答1:

use getBoundingClientRect if $el is the actual DOM object:

var top = $el.getBoundingClientRect().top;

JSFiddle

Fiddle will show that this will get the same value that jquery's offset top will give you

Edit: as mentioned in comments this does not account for scrolled content, below is the code that jQuery uses

https://github.com/jquery/jquery/blob/master/src/offset.js (5/13/2015)

offset: function( options ) {
    //...

    var docElem, win, rect, doc,
        elem = this[ 0 ];

    if ( !elem ) {
        return;
    }

    rect = elem.getBoundingClientRect();

    // Make sure element is not hidden (display: none) or disconnected
    if ( rect.width || rect.height || elem.getClientRects().length ) {
        doc = elem.ownerDocument;
        win = getWindow( doc );
        docElem = doc.documentElement;

        return {
            top: rect.top + win.pageYOffset - docElem.clientTop,
            left: rect.left + win.pageXOffset - docElem.clientLeft
        };
    }
}

回答2:

Here is a function that will do it without jQuery:

function getElementOffset(element)
{
    var de = document.documentElement;
    var box = element.getBoundingClientRect();
    var top = box.top + window.pageYOffset - de.clientTop;
    var left = box.left + window.pageXOffset - de.clientLeft;
    return { top: top, left: left };
}

回答3:

Seems you can just use the prop method on the angular element:

var top = $el.prop('offsetTop');

Works for me. Does anyone know any downside to this?

回答4:

the accepted solution by Patrick Evans doesn't take scrolling into account. i've slightly changed his jsfiddle to demonstrate this:

css: add some random height to make sure we got some space to scroll

body{height:3000px;} 

js: set some scroll position

jQuery(window).scrollTop(100);

as a result the two reported values differ now: http://jsfiddle.net/sNLMe/66/

UPDATE Feb. 14 2015

there is a pull request for jqLite waiting, including its own offset method (taking care of current scroll position). have a look at the source in case you want to implement it yourself: https://github.com/angular/angular.js/pull/3799/files

回答5:

You can try element[0].scrollTop, in my opinion this solution is faster.

Here you have bigger example - http://cvmlrobotics.blogspot.de/2013/03/angularjs-get-element-offset-position.html

回答6:

For Angular 2+ to get the offset of the current element (this.el.nativeElement is equvalent of $(this) in jquery):

export class MyComponent implements  OnInit {

constructor(private el: ElementRef) {}

    ngOnInit() {
      //This is the important line you can use in your function in the code
      //-------------------------------------------------------------------------- 
        let offset = this.el.nativeElement.getBoundingClientRect().top;
      //--------------------------------------------------------------------------    

        console.log(offset);
    }

}

来源：https://stackoverflow.com/questions/18953144/how-do-i-get-the-offset-top-value-of-an-element-without-using-jquery