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offset That's the offset , Use offset The position of the element can be obtained dynamically by series related attributes （ The offset ）、 Size, etc .

（1） Get element distance With positioning The position of the parent element ;

（2） Get the size of the element itself （ Width and height ）.

Be careful ： None of the returned values have units .

offset Common properties ：

（1）offsetWidth The calculation formula of is ：border+ width + padding;

（2）offsetHeight Calculation formula ：border+ height + padding;

（3）offsetTop： It is based on its own margin-top Value + With the inner margin of the positioning parent ;

（4）offsetLeft： It is based on its own margin-left Value + With the inner margin of the positioning parent .

1.1、offsetParent

1.1.1、 The element itself has fixed location ,offsetParent yes null

    <div id="box" style="position: fixed;"></div>
    <script>
        var box = document.querySelector('#box');
        console.log(box.offsetParent);
    </script>

1.1.2、 The element itself has no position or is relative or absolute ,offsetParent yes body

   <div id="box">aaa</div>
    <script>
        var box = document.querySelector('#box');
        console.log(box.offsetParent);
    </script>

1.1.3、 The element itself is not positioned , But the parent element has a location ,offsetParent Is the nearest parent positioning element

    <div id="father" style="position: relative;">
        <div>
            <div id="box"></div>
        </div>
    </div>
    <script>
        var box = document.querySelector('#box');
        console.log(box.offsetParent);
    </script>

1.1.4、body Elemental offsetParent yes null

console.log(document.body.offsetParent);

1.2、offsetTop and offsetLeft

No positioning ：

    <style>
        * {
            margin: 0;
            padding: 0;
        }
        
        .grandfather {
            width: 400px;
            height: 400px;
            background-color: skyblue;
            margin: 100px;
        }
        
        .father {
            width: 200px;
            height: 200px;
            background-color: pink;
            margin-left: 100px;
        }
        
        .son {
            width: 100px;
            height: 100px;
            background-color: red;
            margin-left: 50px;
        }
    </style>



    <div class="grandfather">
        <div class="father">
            <div class="son"></div>
        </div>
    </div>
    <script>
        var gf = document.querySelector('.grandfather');
        console.log(gf.offsetTop); //100
        console.log(gf.offsetLeft); //100

        var f = document.querySelector('.father');
        console.log(f.offsetTop); //100
        console.log(f.offsetLeft); //200


        var s = document.querySelector('.son');
        console.log(s.offsetTop); //100
        console.log(s.offsetLeft); //250
    </script>

to grandfather After positioning ：

   <style>
        * {
            margin: 0;
            padding: 0;
        }
        
        .grandfather {
            width: 400px;
            height: 400px;
            background-color: skyblue;
            margin: 100px;
            position: relative;
        }
        
        .father {
            width: 200px;
            height: 200px;
            background-color: pink;
            margin-left: 100px;
        }
        
        .son {
            width: 100px;
            height: 100px;
            background-color: red;
            margin-left: 50px;
        }
    </style>



    <div class="grandfather">
        <div class="father">
            <div class="son"></div>
        </div>
    </div>
    <script>
        var gf = document.querySelector('.grandfather');
        console.log(gf.offsetTop); //100
        console.log(gf.offsetLeft); //100

        var f = document.querySelector('.father');
        console.log(f.offsetTop); //0
        console.log(f.offsetLeft); //100


        var s = document.querySelector('.son');
        console.log(s.offsetTop); //0
        console.log(s.offsetLeft); //150
    </script>

summary ：

（1）offsetTop and offsetLeft You can get the offset position of the element , Returns a numeric value without units ;

（2）offsetTop and offsetLeft It is based on the nearest father with a location , If you have a father , But there is no positioning , with body For father .

1.3、offsetWidth and offsetHeight

    <style>
        * {
            margin: 0;
            padding: 0;
        }
        
        .son {
            width: 100px;
            height: 100px;
            background-color: red;
            margin: 50px;
            padding: 50px;
            border: 50px;
        }
    </style>

    <div class="son"></div>
    <script>
        var s = document.querySelector('.son');

        console.log(s.offsetWidth);//200
        console.log(s.offsetHeight);//200
    </script>

summary ：offsetWidth and offsetHeight The size of is only related to border、height perhaps width、padding of , And magin irrelevant .

1.4、offset and style The difference between

    <style>
        .box {
            width: 200px;
            height: 100px;
            background-color: red;
            padding: 10px;
        }
    </style>

<div class="box" style="width: 100px;"></div>
<script>
    var div = document.querySelector('div')
    console.log(div.offsetWidth); // 220 = 10 * 2 + 200
    // div.offsetWidth = 300; //  Invalid assignment 

    console.log(div.style.width); // 200px
    // div.style.width = '300px'; //  Assignment valid 
</script>

summary ： want Get the element size use offset More appropriate , To give Element change value use style.

1.5、offsetParent and parentNode difference

    <style>
        * {
            margin: 0;
            padding: 0;
        }
        
        .father {
            width: 400px;
            height: 400px;
            background-color: pink;
        }
        
        .son {
            width: 100px;
            height: 100px;
            background-color: red;
            margin-left: 50px;
            padding: 50px;
            border: 50px;
        }
    </style>


    <div class="father">
        <div class="son"></div>
    </div>
    <script>
        var s = document.querySelector('.son');

        console.log(s.offsetParent);//body
        console.log(s.parentNode);//father
    </script>

difference ：

（1）offsetParent Positioning is required , If there is a father , But there is no positioning , Then return to body;

（2）parentNode No need to locate , Return to the nearest father .

1.6、 Case study

1.6.1、 Get the mouse coordinates in the box

As shown in the figure ：

The code is as follows ：

<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <title> Get the mouse coordinates in the box </title>
    <style>
        .box {
            width: 200px;
            height: 200px;
            border: 1px solid #000;
            margin: 100px auto;
            background-color: pink;
        }
    </style>
</head>

<body>
    <div class="box"></div>
    <script>
        // 1.  Get the box 
        var box = document.querySelector('.box')
        box.onclick = function(e) {
            // e.pageX  Its value is the distance from the mouse position to the left of the page 
            // offsetLeft  Its value is the distance from the current object to the left of the page 
            //  When you want to calculate the mouse in  div  In the position of , Need to use  e.pageX - offsetLeft  Value 
            // console.log(e.pageX , this.offsetLeft)
            var cursorPlaceX = e.pageX - this.offsetLeft;
            //console.log(cursorPlaceX);

            var cursorPlaceY = e.pageY - this.offsetTop;
            this.innerHTML = 'X Coordinates are ' + cursorPlaceX + ',' + "Y Coordinates are " + cursorPlaceY;
            // console.log(' The current position of the mouse ：' + cursorPlaceX + ', ' + cursorPlaceY);
        };
    </script>
</body>

</html>

1.6.2、 Drag the modal box

pop-up , We also call it modal box .

demand ：

（1） Click on the pop-up layer , The modal box will pop up , And show the gray translucent cover .

（2） Click the close button , You can close the modal box , And turn off the gray translucent occlusion at the same time .

（3） Mouse over the top line of the modal box , You can hold down the mouse and drag the modal box to move through the page .

（4） Release the mouse , You can stop dragging the modal box to move .

As shown in the figure ：

The code is as follows ：

<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <title> Drag the modal box </title>
    <style>
        * {
            padding: 0;
            margin: 0;
        }
        
        .login-header {
            width: 100%;
            text-align: center;
            height: 30px;
            font-size: 24px;
            line-height: 30px;
        }
        
        .login {
            display: none;
            width: 512px;
            height: 280px;
            position: fixed;
            border: 1px solid #ebebeb;
            left: 50%;
            top: 50%;
            background: #ffffff;
            box-shadow: 0 0 20px #ddd;
            z-index: 9999;
            transform: translate(-50%, -50%);
        }
        
        .login-title {
            width: 100%;
            margin-top: 10px;
            text-align: center;
            line-height: 40px;
            height: 40px;
            font-size: 18px;
            position: relative;
            cursor: move;
        }
        
        .login-input-content {
            margin-top: 20px;
        }
        
        .login-button {
            width: 50%;
            margin: 30px auto 0 auto;
            line-height: 40px;
            font-size: 14px;
            border: 1px solid #ebebeb;
            text-align: center;
        }
        
        .login-bg {
            display: none;
            width: 100%;
            height: 100%;
            position: fixed;
            top: 0;
            left: 0;
            background: rgba(0, 0, 0, .3);
        }
        
        a {
            text-decoration: none;
            color: #000000;
        }
        
        .login-button a {
            display: block;
        }
        
        .login-input input.list-input {
            float: left;
            line-height: 35px;
            height: 35px;
            width: 350px;
            border: 1px solid #ebebeb;
            text-indent: 5px;
        }
        
        .login-input {
            overflow: hidden;
            margin: 0 0 20px 0;
        }
        
        .login-input label {
            float: left;
            width: 90px;
            padding-right: 10px;
            text-align: right;
            line-height: 35px;
            height: 35px;
            font-size: 14px;
        }
        
        .login-title span {
            position: absolute;
            font-size: 12px;
            right: -20px;
            top: -30px;
            background: #ffffff;
            border: 1px solid #ebebeb;
            width: 40px;
            height: 40px;
            border-radius: 20px;
        }
    </style>
</head>

<body>
    <div class="login-header">
        <a id="link" href="javascript:;"> Click to pop up the login box </a>
    </div>


    <div id="login" class="login">
        <div id="title" class="login-title"> Login member 
            <span><a id="closeBtn" href="javascript:void(0);" class="close-login"> close </a></span>
        </div>

        <div class="login-input-content">
            <div class="login-input">
                <label> user name ：</label>
                <input type="text" placeholder=" Please enter a user name " name="info[username]" id="username" class="list-input">
            </div>


            <div class="login-input">
                <label> The login password ：</label>
                <input type="password" placeholder=" Please enter the login password " name="info[password]" id="password" class="list-input">
            </div>
        </div>

        <div id="loginBtn" class="login-button"><a href="javascript:void(0);" id="login-button-submit"> Login member </a></div>
    </div>




    <!--  Covering layer  -->
    <div id="bg" class="login-bg"></div>
    <script>
        // 1.  Get elements 
        var link = document.querySelector('#link')
        var login = document.querySelector('#login')
        var title = document.querySelector('#title')
        var closeBtn = document.querySelector('#closeBtn')
        var mask = document.querySelector('#bg')

        // 2.  Click the box 
        link.onclick = function() {
            login.style.display = 'block';
            mask.style.display = 'block';
        };

        // 3.  Click Close 
        closeBtn.onclick = function() {
            login.style.display = 'none';
            mask.style.display = 'none';
        };

        // 4.  Drag the bullet box 
        title.onmousedown = function(e) {
            e = e || window.event;
            //var x = e.pageX - this.parentNode.offsetLeft;
            //  Get the mouse in  login  Window  x  Coordinates and  y  Position of coordinates 
            var x = e.pageX - login.offsetLeft;
            var y = e.pageY - login.offsetTop;

            //  Add a move event to the document object 
            document.onmousemove = function(e) {
                //  modify  login  The location of the object 
                // e.pageX - x  It's calculation  login  The distance from the left of the box to the left of the page 
                // e.pageY - y  It's calculation  login  The distance between the top edge of the box and the top edge of the page 
                login.style.left = e.pageX - x + 'px';
                login.style.top = e.pageY - y + 'px';
            };

            //  Release the mouse 
            document.onmouseup = function(e) {
                //  Unbind event 
                document.onmousemove = null;
            };
        };
    </script>
</body>

</html>

Two 、client

client Refers to the client , It refers to the space occupied by the current element content to the inner margin .

Properties and functions are shown in the following table ：

Example ：

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>client attribute </title>
    <style>
        div {
            width: 200px;
            height: 200px;
            border: 10px solid red;
            padding: 10px;
        }
    </style>
</head>
<body>
<div></div>
<script>
    /**
     * client  Represents a client , It has four properties ：
     * 1. clientTop： Returns the top border of the element 
     * 2. clientLeft： Returns the left border of the element 
     * 3. clientWidth： Returns the inner margin of an element  +  The value of the content width 
     * 4. clientHeight： Returns the inner margin of an element  +  Value of content height 
     */
    var div = document.querySelector('div');
    console.log(div.clientLeft); // 10
    console.log(div.clientTop); // 10
    console.log(div.clientWidth); // 220 = 10 + 200 + 20
    console.log(div.clientHeight); // 220 = 10 + 200 + 20

    //  have access to  clientWidth  and  clientHeight  To get the size of the page 
    window.onresize = function () {
        var doc = document.documentElement;
        console.log(doc.clientWidth, doc.clientHeight);
    };
</script>
</body>
</html>

3、 ... and 、scroll

3.1、scroll Use

Example ：

<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <title> Rolling height </title>
    <style>
        #test {
            width: 100px;
            height: 100px;
            background-color: pink;
            border: 510x solid red;
            padding: 50px;
            margin: 100px auto;
            overflow: auto;
        }
    </style>
</head>
<div id="test"> I am content, I am content, I am content, I am content, I am content, I am content, I am content, I am content, I am content, I am content, I am content, I am content, I am content, I am content, I am content, I am content, I am content, I am content, I am content, I am content, I am content, I am content </div>

<body>
    <script>
        var test = document.querySelector('#test')

        console.log(test.clientHeight); //height+padding 200
        console.log(test.scrollHeight); // Actual content height   495

        test.addEventListener('scroll', function() {
            console.log(test.scrollTop);
        })
    </script>
</body>

</html>

3.2、 Page scrolling case

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title> Page scrolling </title>
    <style>
        body {
            height: 2000px;
        }
        .box {
            width: 100px;
            height: 100px;
            border: 1px solid #000000;
            padding: 10px;
            margin: 10px;
            overflow: scroll;
        }
        #btn {
            position: fixed;
            bottom: 30px;
            right: 30px;
            display: none;
        }
    </style>
</head>
<body>
<div id="box">
    <p> Content </p>
    <p> Content </p>
    <p> Content </p>
    <p> Content </p>
    <p> Content </p>
    <p> Content </p>
    <p> Content </p>
    <p> Content </p>
    <p> Content </p>
    <p> Content </p>
    <p> Content </p>
    <p> Content </p>
    <p> Content </p>
    <p> Content </p>
    <p> Content </p>
    <p> Content </p>
    <p> Content </p>
    <p> Content </p>
    <p> Content </p>
</div>
<button id="btn"> Back to the top </button>
<script>
    window.onscroll = function () {
        var btn = document.querySelector('#btn')
        //  The height of the roll 
        var scrollVal = document.documentElement.scrollTop;
        //  The height of the visualization area 
        var showHeight = document.documentElement.clientHeight;
        //  Determine whether the scroll height exceeds the height of the visualization area （ Whether the distance of one screen is reached ）
        if (scrollVal <= showHeight) {
            btn.style.display = 'none';
        } else {
            btn.style.display = 'block';
        }
        btn.onclick = function () {
             document.documentElement.scrollTop = 0;
            btn.style.display = 'none';
        };
    };
</script>
</body>
</html>

3.3、 Rolling method scrollTo(x,y)

scrollTo(x,y) Method to scroll through the current window The document shown in , Let the coordinates in the document be x,y The point of is in the upper left corner .

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title> Page scrolling </title>
    <style>
        body {
            height: 2000px;
        }
        .box {
            width: 100px;
            height: 100px;
            border: 1px solid #000000;
            padding: 10px;
            margin: 10px;
            overflow: scroll;
        }
        #btn {
            position: fixed;
            bottom: 30px;
            right: 30px;
            display: none;
        }
    </style>
</head>
<body>
<div id="box">
    <p> Content </p>
    <p> Content </p>
    <p> Content </p>
    <p> Content </p>
    <p> Content </p>
    <p> Content </p>
    <p> Content </p>
    <p> Content </p>
    <p> Content </p>
    <p> Content </p>
    <p> Content </p>
    <p> Content </p>
    <p> Content </p>
    <p> Content </p>
    <p> Content </p>
    <p> Content </p>
    <p> Content </p>
    <p> Content </p>
    <p> Content </p>
</div>
<button id="btn"> Back to the top </button>
<script>
    window.onscroll = function () {
        var btn = document.querySelector('#btn')
        //  The height of the roll 
        var scrollVal = document.documentElement.scrollTop;
        //  The height of the visualization area 
        var showHeight = document.documentElement.clientHeight;
        //  Determine whether the scroll height exceeds the height of the visualization area （ Whether the distance of one screen is reached ）
        if (scrollVal <= showHeight) {
            btn.style.display = 'none';
        } else {
            btn.style.display = 'block';
        }
        btn.onclick = function () {
            // document.documentElement.scrollTop = 0;
            scrollTo(0, 0)
            btn.style.display = 'none';
        };
    };
</script>
</body>
</html>

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