js to achieve sliding carousel effect

<div class="banner">
        <ul>
            <li>
                <a href="#" >
                    <img src="images/1.jpeg">
                </a>
            </li>
            <li>
                <a href="#" >
                    <img src="./images/2.jpg">
                </a>
            </li>
            <li>
                <a href="#" >
                    <img src="./images/3.jpg">
                </a>
            </li>
            <li>
                <a href="#" >
                    <img src="./images/4.jpg">
                </a>
            </li>
            <li>
                <a href="#" >
                    <img src="./images/5.jpg">
                </a>
            </li>
        </ul>
        <ol>
            
        </ol>
        <a href="#" class="left">&lt;</a>
        <a href="#" class="right">&gt;</a>
</div>

<style>
    *{
        padding: 0;
        margin: 0;
        list-style: none;
        text-decoration: none;
        color:#000;
    }
    .banner{
        width: 500px;
        height: 300px;
        margin:50px auto;
        position: relative;
        border:1px solid #000;
        overflow:hidden;
    }
    .banner ul{
        position: absolute;
        left: 0;
        top: 0;
        height: 300px;
        /* width: 2500px; */
    }
    .banner ul li{
        width: 500px;
        height: 300px;
        float:left;
    }
    .banner ul li a img{
        width: 500px;
        height: 300px;
    }
    .banner ol{
        /* width: 100px; */
        height: 20px;
        position:absolute;
        bottom:10px;
        background-color: rgba(255,255,255,.7);
        left:50%;
        transform: translateX(-50%);
        border-radius:10px;
        display: flex;
        justify-content: space-evenly;
        align-items: center;
    }
    /* .banner ol li{
        width: 10px;
        height: 10px;
        border-radius:50%;
        background-color: skyblue;
    } */
    .banner>a{
        width: 20px;
        height: 40px;
        position:absolute;
        top:50%;
        transform: translateY(-50%);
        background-color: rgba(10,10,10,.5);
        font-size:20px;
        color:#fff;
        line-height: 2;
        text-align: center;
    }
    .banner>a.left{
        left: 0;
    }
    .banner>a.right{
        right: 0;
    }

var div = document.querySelector('.banner');
var ul = document.querySelector('ul');
var ol = document.querySelector('ol');
var left = document.querySelector('a.left');
var right = document.querySelector('a.right');
    // Set a variable index to use as the subscript var index = 1;
    // Traverse the li below ul
for(var i=0;i<ul.children.length;i++){
    // Each li under a ul should correspond to a small dot button below // Create the same number of small dots var li = document.createElement('li');
    setStyle(li,{
        width: "10px",
        height: "10px",
        "border-radius":"50%",
        "background-color": "skyblue"
    })
    // Put all created li into the big box ol.appendChild(li);
}
    // The ol box itself has no width. We need to set the width of the ol box according to the number of small dots inside. ol.style.width = ol.firstElementChild.offsetWidth*ul.children.length*2 + 'px';
    // Set a variable that represents the dot with style var that = ol.children[index-1];
//Set the color red for the first son of ol that.style.background = 'red';
// To achieve better connection between sliding carousel, add a li before and after ul.
var lastLi = ul.lastElementChild.cloneNode(true);
var firstLi = ul.firstElementChild.cloneNode(true);
// Place the copied labels in front and behind the ul box ul.appendChild(firstLi);
ul.insertBefore(lastLi,ul.firstElementChild);
// Because the child elements under ul have changed, we need to set the corresponding width for ul ul.style.width = ul.children.length*lastLi.offsetWidth + 'px';
// Because we slide from right to left, we need to negative the left value of ul ul.style.left = -ul.firstElementChild.offsetWidth + 'px';
// Set the variable and assign it to the timer var timeId;
// Define a switch var flag = true;
//Right click right.onclick = function(){
    // If the switch is not on, return false
    if(!flag){
        return false;
    }
    // And assign the return value to the switch to indicate that if the switch is closed, clicking it again will have no effect flag = false;
    // Earlier we defined an index and every time we click on it, index++
    index++;
    // Call move function move(ul,{left:-index*ul.children[0].offsetWidth},function(){
        // Put what needs to be done in the function after the exercise is completed // First, we need to judge the index. There are a total of seven child elements in ul. The index corresponds to the subscript of the ul child element, so the index cannot exceed ul.children.length-1;
        if(index == ul.children.length - 1){
            // If index is equal to ul.children.length-1
            // Reassign index to 1
            index = 1;
            // And also reassign the left value of ul ul.style.left = -index*ul.children[0].offsetWidth + 'px';
        }
        // The dot must also move with the image // We set the dot corresponding to the image to the variable that because the dot itself has a style, first set its style to the default that.style.backgroundColor = 'skyblue';
        // The style of the corresponding small dot has changed ol.children[index-1].style.backgroundColor = 'red';
        // After the style conversion is successful, assign the dot containing the style to the variable that
        that = ol.children[index-1];
        // At the end of the movement, turn on the switch so that you can right-click to turn it on again flag = true;
    })
}
//Left click left.onclick = function(){
    if(!flag){
        return false;
    }
    flag = false;
    // Left click is sliding from left to right and becomes index--
    index--;
    move(ul,{left:-index*ul.children[0].offsetWidth},function(){
        if(index == ul.children.length - 1){
            index = 1;
            ul.style.left = -index*ul.children[0].offsetWidth + 'px';
        }
         //After entering the event, first determine the value of index if(index==0){
            // If the value of index becomes 0, reassign index index = ul.children.length-2;
            // And reassign the left value of ul to the value of the second picture ul.style.left = -index * firstLi.offsetWidth + "px"
        }
        that.style.backgroundColor = 'skyblue';
        ol.children[index-1].style.backgroundColor = 'red';
        that = ol.children[index-1];
        flag = true;
    })
}
// Traverse all li under ol
for(let i=0;i<ol.children.length;i++){
    // Click on the small dot event ol.children[i].onclick = function(){
    if(!flag){
        return false;
    }
    flag = false;
    // Because the bottom of the dot is 1 less than the subscript of the image, we need to reassign the index in the dot click event // so that the dot and the image can correspond to index = i+1;
    move(ul,{left:-index*firstLi.offsetWidth},function(){
            // if(index == 0){
            // index = ul.children.length - 2
            // ul.style.left = -index.ul.children[0].offsetWidth + 'px'
            // }
            that.style.backgroundColor = 'skyblue';
            ol.children[index-1].style.backgroundColor = 'red';
            that = ol.children[index-1];
            flag = true;
        })
    }
};
    // Automatic rotation timeId = setInterval(function(){
        if(!flag){
            return false;
        }
        flag = false;
        index++;
        move(ul,{left:-index*firstLi.offsetWidth},function(){
            if(index == ul.children.length - 1){
                index = 1
                ul.style.left = -index*ul.children[0].offsetWidth + 'px'
            }
            that.style.backgroundColor = 'skyblue';
            ol.children[index-1].style.backgroundColor = 'red';
            that = ol.children[index-1];
            flag = true;
        })
    },2000);

    // Stop the carousel when the mouse passes over itdiv.onmouseover = function(){
        clearInterval(timeId);
    }

    //Automatically rotate after the mouse leaves div.onmouseout = function(){
        timeId = setInterval(function(){
            if(!flag){
                return false;
            }
            flag = false;
            index++;
            move(ul,{left:-index*firstLi.offsetWidth},function(){
                if(index == ul.children.length - 1){
                    index = 1
                    ul.style.left = -index*ul.children[0].offsetWidth + 'px'
                }
                that.style.backgroundColor = 'skyblue';
                ol.children[index-1].style.backgroundColor = 'red';
                that = ol.children[index-1];
                flag = true;
            })
        },1000);
    };
//Packaged multi-motion function function move(ele,obj,fn){
            let timeObj = {};
            for(let attr in obj){
                timeObj[attr] = setInterval(function(){
                    var target = parseFloat(obj[attr]);
                    if (attr === 'opacity') {
                        target*=100
                    }
                    var t = parseFloat(getStyle(ele,attr));
                    if (attr === 'opacity') {
                        t*=100
                    }
                    console.log(t)
                    if((target-t)/100>0){
                        var percent = Math.ceil((target-t)/100);
                    }else{
                        var percent = Math.floor((target-t)/100);
                    }
                    t += percent;
                    if (attr === 'opacity') {
                        ele.style[attr] = t/100 
                    }else{
                        ele.style[attr] = t + 'px';
                    }
                    if(t == target){
                        clearInterval(timeObj[attr])
                        delete timeObj[attr]
                        let k = 0;
                        for(let i in timeObj){
                            k++;
                        }
                        if( k==0){
                            fn();
                            console.log(123)
                        }
                    }
                },10)
            }
        }

// Encapsulated function to get style function getStyle(ele,attr){
    if (window.getComputedStyle) {
        return window.getComputedStyle(ele)[attr];
    }else{
        return ele.currentStyle[attr];
    }
}
// Encapsulate the function of setting style function setStyle(ele,styleObj){
    for(var attr in styleObj){
        ele.style[attr] = styleObj[attr];
    }
};