jQuery position() 源码解读

    position: function() {
        if ( !this[ 0 ] ) {
            return;
        }

        var offsetParent, offset,
            elem = this[ 0 ],
            parentOffset = { top: 0, left: 0 };

        // Fixed elements are offset from window (parentOffset = {top:0, left: 0}, because it is its only offset parent
        if ( jQuery.css( elem, "position" ) === "fixed" ) {
            // Assume getBoundingClientRect is there when computed position is fixed
            offset = elem.getBoundingClientRect();

        } else {
            // Get *real* offsetParent
            offsetParent = this.offsetParent();

            // Get correct offsets
            offset = this.offset();
            if ( !jQuery.nodeName( offsetParent[ 0 ], "html" ) ) {
                parentOffset = offsetParent.offset();
            }

            // Add offsetParent borders
            parentOffset.top += jQuery.css( offsetParent[ 0 ], "borderTopWidth", true );
            parentOffset.left += jQuery.css( offsetParent[ 0 ], "borderLeftWidth", true );
        }

        // Subtract parent offsets and element margins
        return {
            top: offset.top - parentOffset.top - jQuery.css( elem, "marginTop", true ),
            left: offset.left - parentOffset.left - jQuery.css( elem, "marginLeft", true )
        };
    }