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HTML 

              
                	<!DOCTYPE html>
	<html>
	<head>
		<meta charset="utf-8">
		<meta name="viewport" content="width = device-width, initial-scale = 1">
		<title></title>
		<link rel="stylesheet" type="text/css" href="slide.css">
		<link rel="stylesheet" href="https://use.fontawesome.com/releases/v5.5.0/css/all.css" integrity="sha384-B4dIYHKNBt8Bc12p+WXckhzcICo0wtJAoU8YZTY5qE0Id1GSseTk6S+L3BlXeVIU" crossorigin="anonymous">
	</head>
	<body>
		<div class="wraper">
			<div class="slides">
					<ul>
						<li style="background-image: url(https://images.wallpaperscraft.com/image/league_of_legends_poro_art_tongue_99916_1366x768.jpg">
						</li>
						<li style="background-image: url(https://images.wallpaperscraft.com/image/the_elder_scrolls_online_sword_of_the_night_warrior_assassin_95957_1366x768.jpg);">
						</li>
						<li class="hienlen" style="background-image: url(https://images.wallpaperscraft.com/image/league_of_legends_girl_face_art_108749_1366x768.jpg">
						</li>
						<li style="background-image: url(https://images.wallpaperscraft.com/image/league_of_legends_poro_teemo_105044_1366x768.jpg);">
						</li>
						<li style="background-image: url(https://images.wallpaperscraft.com/image/league_of_legends_malphite_art_107905_1366x768.jpg">
						</li>
						<div class="color" style="background-color:black; width: 100%; height: 100%">
				
						</div>
					</ul>
			</div>
			<div class="anhnho">
				<ul>
						<li style="background-image: url(https://images.wallpaperscraft.com/image/league_of_legends_poro_art_tongue_99916_1366x768.jpg">
						</li>
						<li style="background-image: url(https://images.wallpaperscraft.com/image/the_elder_scrolls_online_sword_of_the_night_warrior_assassin_95957_1366x768.jpg);">
						</li>
						<li class="duongvien" style="background-image: url(https://images.wallpaperscraft.com/image/league_of_legends_girl_face_art_108749_1366x768.jpg">
						</li>
						<li style="background-image: url(https://images.wallpaperscraft.com/image/league_of_legends_poro_teemo_105044_1366x768.jpg);">
						</li>
						<li style="background-image: url(https://images.wallpaperscraft.com/image/league_of_legends_malphite_art_107905_1366x768.jpg">
						</li>
					</ul>
			</div>
			<div class="nut">
				<ul>
					<li>
						<i class="far fa-caret-square-left"></i>
					</li>
					<li>
						<i class="far fa-caret-square-right"></i>
					</li>
				</ul>
			</div>
		</div>
		<script type="text/javascript" src="slide.js"></script>
	</body>
	</html>
!

CSS 

              
                *{
	padding:0;
	margin:0;
	border:none;
}
html,body,.wraper,.slides,ul,li{
	width: 100%;
	height: 100%;
}
.wraper {
    position: relative;
    overflow:hidden;
}
.nut ul li i {
    font-size: 40px;
}

.slides ul li {
    position: absolute;
    background-size: cover;
}
.nut {
    position: absolute;
    bottom: 0px;
    left: 50%;
    transform: translateX(-50%);
}

.nut ul li {
    display: inline;
    cursor: pointer;
}
.nut ul li i{
	transition: 0.5s;
}
.nut ul li:hover i{
	color:white;
	transition: 0.5s;
}
.slides ul li{
	opacity: 0 ;
}
ul li.hienlen{
	opacity: 1; 
	transition: 0.5s;
}
/*Viet hieu ung ra vao cho cac slide*/
div.anhnho{
	width: 100%;
	height: 100%;
	position: absolute;
	top: 50px;
}
div.anhnho li{
	width: 7%;
	height: 7%;
	background-size: cover;

}
.duongvien{
	border: 3px solid white;
}
!

JS 

              
                document.addEventListener("DOMContentLoaded",function(){
	//Truy xuất phần tử 2 nút trái phải và các slide
	var nut = document.querySelectorAll('div.nut ul li'),
		nuttrai = nut[0],
		nutphai = nut[1];
	var slides = document.querySelectorAll('div.slides ul li')
	// Tìm ra số phần tử cuối cùng vì là mảng nên chúng ta sẽ lấy độ dài của mảng - 1
	var cuoicung = slides.length - 1;
	//Khi click nút trái
	var anhnho = document.querySelectorAll('div.anhnho li');
	chuyenslide = function(){
		//1.Lấy vị trí slide hiện tại
		var vitrislideht = 0
		var slideht = document.querySelector('ul li.hienlen');
		var anhnhoht = document.querySelector('div.anhnho li');
		for(var i = 0; slideht = slideht.previousElementSibling; vitrislideht++){

		}// đã lấy được vị trí sau khi thực hiện vòng lặp for này.
		//2. Bỏ tất cả các class hienlen
		for(var i = 0; i < slides.length; i++){
			slides[i].classList.remove('hienlen');
			anhnho[i].classList.remove('duongvien');
		}
		//3.2 Khi slide chuyển đến 0 thì đặt vitrislide hien tại = vitricuoicung + 1 
		if(vitrislideht == 0){
			vitrislideht = cuoicung + 1;
			slides[vitrislideht-1].classList.add('hienlen');
			anhnho[vitrislideht-1].classList.add('duongvien');
		}
		else{
		//3.1 Chuyển sang slide phía sau 
		slides[vitrislideht-1].classList.add('hienlen');
		anhnho[vitrislideht-1].classList.add('duongvien');
		}

	}
	// Chỉ cần thực hiện 1 hàm r x2 lên thay đổi 1 tí là dc!!!
	chuyenslidep = function(){
		//1.Lấy vị trí slide hiện tại
		var vitrislideht = 0;
		var slideht = document.querySelector('ul li.hienlen');
		for(var i = 0; slideht = slideht.previousElementSibling; vitrislideht++){

		}// đã lấy được vị trí sau khi thực hiện vòng lặp for này.
		//2. Bỏ tất cả các class hienlen
		for(var i = 0; i < slides.length; i++){
			slides[i].classList.remove('hienlen');
			anhnho[i].classList.remove('duongvien');
		}
		if(vitrislideht == cuoicung){
			vitrislideht = -1;
		//ở đây mình k cần dùng else vì khi thực hiện xong lệnh if nó sẽ nhảy xuống thực hiện lệnh add class luôn. 
		}
		//3. Chuyển sang slide lên trước
		slides[vitrislideht+1].classList.add('hienlen');
		//4. Đến slide đầu tiên thì chuyển đến nó đến vị trí cuôi cùng.
		anhnho[vitrislideht+1].classList.add('duongvien');
	}

	nuttrai.addEventListener('click',chuyenslide);
	nutphai.addEventListener('click',chuyenslidep);
},false)
!
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