Pure js to achieve the effect of carousel

 /*Clear the default inner and outer margins of an element*/
* {
    margin: 0;
    padding: 0
}
body{
    width: 1000px;
    margin: 0 auto;
}
/*Remove the dot in front of the list*/
li {
    list-style: none;
}
/*The picture has no border and removes the blank gap at the bottom of the picture*/
img {
    border: 0; /*ie6*/
    vertical-align: middle;
}
/*Remove the underline from the link*/
a {
    color: #666;
    text-decoration: none;
}

a:hover {
    color: #e33333;
}
.fl {
    float: left;
}
.fr {
    float: right;
}
.focus {
    position: relative;
    width: 721px;
    height: 455px;
    background-color: purple;
    overflow: hidden;
    margin-top: 20px;
}

.focus ul {
    position: absolute;
    top: 0;
    left: 0;
    width: 600%;
}

.focus ul li {
    float: left;
}

.arrow-l,
.arrow-r {
    display: none;
    position: absolute;
    top: 50%;
    margin-top: -20px;
    width: 24px;
    height: 40px;
    background: rgba(0, 0, 0, .3);
    text-align: center;
    line-height: 40px;
    color: #fff;
    font-family: 'icomoon';
    font-size: 18px;
    z-index: 2;
}

.arrow-r {
    right: 0;
}

.circle {
    position: absolute;
    bottom: 10px;
    left: 50px;
}

.circle li {
    float: left;
    width: 8px;
    height: 8px;
    /*background-color: #fff;*/
    border: 2px solid rgba(255, 255, 255, 0.5);
    margin: 0 3px;
    border-radius: 50%;
    /*Show the little hand when the mouse passes by*/
    cursor: pointer;
}

.current {
    background-color: #fff;
}

<div class="focus fl">
    <!-- Left button -->
    <a href="javascript:;" class="arrow-l arrow"> < </a>
    <!-- Right button -->
    <a href="javascript:;" class="arrow-r arrow"> > </a>
    <!-- Core scrolling area -->
    <ul>
        <li>
            <a href="#" ><img src="images/focus.jpg" alt=""></a>
        </li>
        <li>
            <a href="#" ><img src="images/focus1.jpg" alt=""></a>
        </li>
        <li>
            <a href="#" ><img src="images/focus2.jpg" alt=""></a>
        </li>
        <li>
            <a href="#" ><img src="images/focus3.jpg" alt=""></a>
        </li>
    </ul>
    <!-- Small circle -->
    <ol class="circle">

    </ol>
</div>

window.addEventListener('load', function() {
    // 1. Get the element var arrow_l = document.querySelector('.arrow-l');
    var arrow_r = document.querySelector('.arrow-r');
    var focus = document.querySelector('.focus');
    var focusWidth = focus.offsetWidth;
    // 2. When the mouse passes through focus, the left and right buttons are displayed and hidden focus.addEventListener('mouseenter', function() {
        arrow_l.style.display = 'block';
        arrow_r.style.display = 'block';
        clearInterval(timer);
        timer = null; // clear the timer variable });
    focus.addEventListener('mouseleave', function() {
        arrow_l.style.display = 'none';
        arrow_r.style.display = 'none';
        timer = setInterval(function() {
            //Manually call the click event arrow_r.click();
        }, 2000);
    });
    // 3. Dynamically generate small circles. There are several pictures, so I will generate several small circles. var ul = focus.querySelector('ul');
    var ol = focus.querySelector('.circle');
    // console.log(ul.children.length);
    for (var i = 0; i < ul.children.length; i++) {
        // Create a small li 
        var li = document.createElement('li');
        // Record the index number of the current small circle through custom attributes li.setAttribute('index', i);
        //Insert the small li into ol ol.appendChild(li);
        // 4. Exclusive idea of ​​small circles We can directly bind the click event while generating the small circle li.addEventListener('click', function() {
            // Kill everyone and clear all small li current class name for (var i = 0; i < ol.children.length; i++) {
                ol.children[i].className = '';
            }
            //Leave my own current small li Set current class name this.className = 'current';
            // 5. Click on the small circle to move the image. Of course, the ul is moved. 
            // The moving distance of ul is the index number of the small circle multiplied by the width of the image. Note that it is a negative value. // When we click on a small li, we get the index number of the current small li. var index = this.getAttribute('index');
            // When we click on a small li, we need to give the index number of this li to num  
            num = index;
            // When we click on a small li, we need to give the index number of this li to circle  
            circle = index;
            // num = circle = index;
            console.log(focusWidth);
            console.log(index);

            animate(ul, -index * focusWidth);
        })
    }
    // Set the class name of the first small li in ol to current
    ol.children[0].className = 'current';
    // 6. Clone the first image (li) and put it at the end of ul var first = ul.children[0].cloneNode(true);
    ul.appendChild(first);
    // 7. Click the button on the right to scroll one picture var num = 0;
    // circle controls the playback of the small circle var circle = 0;
    // flag throttle var flag = true;
    arrow_r.addEventListener('click', function() {
        if (flag) {
            flag = false; // Close the throttle // If we reach the last copied image, our ul needs to quickly restore left to 0
            if (num == ul.children.length - 1) {
                ul.style.left = 0;
                num = 0;
            }
            num++;
            animate(ul, -num * focusWidth, function() {
                flag = true; // open throttle });
            // 8. Click the button on the right, and the small circle will change accordingly. You can declare another variable to control the play of the small circle circle++;
            // If circle == 4, it means we have reached the end of the cloned image and we will restore if (circle == ol.children.length) {
                circle = 0;
            }
            //Call function circleChange();
        }
    });

    // 9. Left button method arrow_l.addEventListener('click', function() {
        if (flag) {
            flag = false;
            if (num == 0) {
                num = ul.children.length - 1;
                ul.style.left = -num * focusWidth + 'px';

            }
            num--;
            animate(ul, -num * focusWidth, function() {
                flag = true;
            });
            // Click the button on the left, and the small circle will change accordingly. You can declare another variable to control the play of the small circle circle--;
            // If circle < 0, indicating the first picture, the small circle should be changed to the fourth small circle (3)
            // if (circle < 0) {
            // circle = ol.children.length - 1;
            // }
            circle = circle < 0 ? ol.children.length - 1 : circle;
            //Call function circleChange();
        }
    });

    function circleChange() {
        // Clear the current class names of the remaining small circles first for (var i = 0; i < ol.children.length; i++) {
            ol.children[i].className = '';
        }
        //Leave the current class name of the current small circle ol.children[circle].className = 'current';
    }
    // 10. Automatically play the carousel var timer = setInterval(function() {
        //Manually call the click event arrow_r.click();
    }, 2000);

})

function animate(obj, target, callback) {
    // console.log(callback); callback = function() {} when calling callback()

    // Clear the previous timer first, and keep only the current timer to execute clearInterval(obj.timer);
    obj.timer = setInterval(function() {
        // Write the step value to the timer // Change our step value to an integer to avoid decimal problems // var step = Math.ceil((target - obj.offsetLeft) / 10);
        var step = (target - obj.offsetLeft) / 10;
        step = step > 0 ? Math.ceil(step) : Math.floor(step);
        if (obj.offsetLeft == target) {
            // Stopping the animation essentially stops the timer clearInterval(obj.timer);
            //Write the callback function to the end of the timer // if (callback) {
            // // Call function // callback();
            // }
            callback && callback();
        }
        // Change the step value of increasing by 1 each time to a gradually decreasing value. Step formula: (target value - current position) / 10
        obj.style.left = obj.offsetLeft + step + 'px';

    }, 15);
}