// Back Compat <1.8 extension point

jQuery.fx.step = {};

if ( jQuery.expr && jQuery.expr.filters ) {

    jQuery.expr.filters.animated = function( elem ) {

        return jQuery.grep(jQuery.timers, function( fn ) {

            return elem === fn.elem;

        }).length;

    };

}

jQuery.fn.offset = function( options ) {

    if ( arguments.length ) {//设置

        return options === undefined ?

            this :

            this.each(function( i ) {//每一个元素设置

                jQuery.offset.setOffset( this, options, i );

            });

    }

//获取

    var docElem, win,

        elem = this[ 0 ],

        box = { top: 0, left: 0 },

        doc = elem && elem.ownerDocument;//document

    if ( !doc ) {

        return;

    }

    docElem = doc.documentElement;//html标签

    // Make sure it's not a disconnected DOM node

    if ( !jQuery.contains( docElem, elem ) ) {

        return box;

    }

    // getBoundingClientRect()得到一些属性值:ClientRect {top: 8, right: 58, bottom: 58, left: 8, width: 50…},是到可视区的不包含滚动区域，

    if ( typeof elem.getBoundingClientRect !== core_strundefined ) {

        box = elem.getBoundingClientRect();

    }

    win = getWindow( doc );//获取window

    //pageYOffset是y轴的滚动距离，clientTop边框宽度，

    return {

        top: box.top + win.pageYOffset - docElem.clientTop,

        left: box.left + win.pageXOffset - docElem.clientLeft

    };

};

jQuery.offset = {

//设置offset

    setOffset: function( elem, options, i ) {

        var curPosition, curLeft, curCSSTop, curTop, curOffset, curCSSLeft, calculatePosition,

            position = jQuery.css( elem, "position" ),//获取元素的position属性，

            curElem = jQuery( elem ),

            props = {};

        // Set position first, in-case top/left are set even on static elem

        if ( position === "static" ) {//没有定位就加一个相对定位

            elem.style.position = "relative";

        }

        curOffset = curElem.offset();

        curCSSTop = jQuery.css( elem, "top" );

        curCSSLeft = jQuery.css( elem, "left" );

        calculatePosition = ( position === "absolute" || position === "fixed" ) && ( curCSSTop + curCSSLeft ).indexOf("auto") > -1;

        // Need to be able to calculate position if either top or left is auto and position is either absolute or fixed

        if ( calculatePosition ) {

            curPosition = curElem.position();

            curTop = curPosition.top;

            curLeft = curPosition.left;

        } else {

            curTop = parseFloat( curCSSTop ) || 0;

            curLeft = parseFloat( curCSSLeft ) || 0;

        }

        if ( jQuery.isFunction( options ) ) {

            options = options.call( elem, i, curOffset );

        }

        if ( options.top != null ) {

            props.top = ( options.top - curOffset.top ) + curTop;

        }

        if ( options.left != null ) {

            props.left = ( options.left - curOffset.left ) + curLeft;

        }

        if ( "using" in options ) {

            options.using.call( elem, props );

        } else {

            curElem.css( props );

        }

    }

};

jQuery.fn.extend({

    position: function() {//获取

        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 it's only offset parent

        if ( jQuery.css( elem, "position" ) === "fixed" ) {

            // We assume that getBoundingClientRect is available when computed position is fixed

            offset = elem.getBoundingClientRect();

        } else {

            // 原生方法找父级

            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 );

        }

        // 自己的top减去父级的top

        return {

            top: offset.top - parentOffset.top - jQuery.css( elem, "marginTop", true ),

            left: offset.left - parentOffset.left - jQuery.css( elem, "marginLeft", true )

        };

    },

    offsetParent: function() {

        return this.map(function() {

            var offsetParent = this.offsetParent || docElem;

            while ( offsetParent && ( !jQuery.nodeName( offsetParent, "html" ) && jQuery.css( offsetParent, "position") === "static" ) ) {

                offsetParent = offsetParent.offsetParent;

            }

            return offsetParent || docElem;

        });

    }

});

// Create scrollLeft and scrollTop methods

jQuery.each( {scrollLeft: "pageXOffset", scrollTop: "pageYOffset"}, function( method, prop ) {

    var top = "pageYOffset" === prop;

    jQuery.fn[ method ] = function( val ) {

        return jQuery.access( this, function( elem, method, val ) {

            var win = getWindow( elem );

            if ( val === undefined ) {

                return win ? win[ prop ] : elem[ method ];

            }

            if ( win ) {

                win.scrollTo(

                    !top ? val : window.pageXOffset,

                    top ? val : window.pageYOffset

                );

            } else {

                elem[ method ] = val;//原生的方法

            }

        }, method, val, arguments.length, null );

    };

});

function getWindow( elem ) {

    return jQuery.isWindow( elem ) ? elem : elem.nodeType === 9 && elem.defaultView;

}

// Create innerHeight, innerWidth, height, width, outerHeight and outerWidth methods

jQuery.each( { Height: "height", Width: "width" }, function( name, type ) {

    jQuery.each( { padding: "inner" + name, content: type, "": "outer" + name }, function( defaultExtra, funcName ) {

        // margin is only for outerHeight, outerWidth

        jQuery.fn[ funcName ] = function( margin, value ) {

            var chainable = arguments.length && ( defaultExtra || typeof margin !== "boolean" ),

                extra = defaultExtra || ( margin === true || value === true ? "margin" : "border" );

            return jQuery.access( this, function( elem, type, value ) {

                var doc;

                //是window就得到可视区的宽高

                if ( jQuery.isWindow( elem ) ) {

                    // As of 5/8/2012 this will yield incorrect results for Mobile Safari, but there

                    // isn't a whole lot we can do. See pull request at this URL for discussion:

                    // https://github.com/jquery/jquery/pull/764

                    return elem.document.documentElement[ "client" + name ];

                }

                //是document就得到页面的宽高

                if ( elem.nodeType === 9 ) {

                    doc = elem.documentElement;

                    // Either scroll[Width/Height] or offset[Width/Height] or client[Width/Height],

                    // whichever is greatest

                    return Math.max(

                        elem.body[ "scroll" + name ], doc[ "scroll" + name ],

                        elem.body[ "offset" + name ], doc[ "offset" + name ],

                        doc[ "client" + name ]

                    );

                }

                return value === undefined ?

                    jQuery.css( elem, type, extra ) ://css方式获取

                    jQuery.style( elem, type, value, extra );//style方式获取

            }, type, chainable ? margin : undefined, chainable, null );

        };

    });

});

// Limit scope pollution from any deprecated API

// (function() {

// The number of elements contained in the matched element set

jQuery.fn.size = function() {

    return this.length;

};

jQuery.fn.andSelf = jQuery.fn.addBack;

// })();

if ( typeof module === "object" && module && typeof module.exports === "object" ) {

    // Expose jQuery as module.exports in loaders that implement the Node

    // module pattern (including browserify). Do not create the global, since

    // the user will be storing it themselves locally, and globals are frowned

    // upon in the Node module world.

    module.exports = jQuery;

} else {

    // Register as a named AMD module, since jQuery can be concatenated with other

    // files that may use define, but not via a proper concatenation script that

    // understands anonymous AMD modules. A named AMD is safest and most robust

    // way to register. Lowercase jquery is used because AMD module names are

    // derived from file names, and jQuery is normally delivered in a lowercase

    // file name. Do this after creating the global so that if an AMD module wants

    // to call noConflict to hide this version of jQuery, it will work.

    if ( typeof define === "function" && define.amd ) {

        define( "jquery", [], function () { return jQuery; } );

    }

}

// If there is a window object, that at least has a document property,

// define jQuery and $ identifiers

if ( typeof window === "object" && typeof window.document === "object" ) {

    window.jQuery = window.$ = jQuery;

}

})( window );

<!DOCTYPE HTML>

<html>

<head>

<meta http-equiv="Content-Type" content="text/html; charset=utf-8">

<title>sss</title>

<script src="jquery-203.js"></script>

</head>

<body>

<script>

window.onload = function(){

    console.log( $('#div1').offset().left );//到屏幕左边的距离。不看父节点有没有定位

    console.log( $('#div2').position().left );//到屏幕左边的距离。相对于有定位的父级,不计算margin值

    $('#div2').position( {left:100,top:100} );//会自动加一个相对定位

    console.log($('#div2').get(0).getBoundingClientRect());//ClientRect {top: 8, right: 58, bottom: 58, left: 8, width: 50…},是到可视区的不包含滚动区域，

}

</script>

</head>

<body>

<div id="div1" style="width:100px;height:100px;background-color:yellow;posotion:relative;">

    <div id='div2' style="width:50px;height:50px;background-color:red;position:left;left:10;top:10" >

    </div>

</div>

</body>

</html>