<div class="img">
 <img src="images/1.jpg" id="myImg" class="myImg" alt="這里是1.jpg">
 <p>
 <input type="button" id="pre" class="btn" value="上一張">
 <input type="button" id="next" class="btn" value="下一張">
 </p> 
</div>

*{
 margin: 0;
 padding: 0;
}
img{
 boder:none;
}
button{
 outline: none;
 vertical-align: middle;
}
.img{
 width: 100%;
 margin-left: auto;
 margin-right: auto;
 margin-top: 20px;
 text-align: center;
}
.myImg{
 width: 500px;
 height: 300px;
}
p{
 text-align: center;
}
p .btn{
 width: 100px;
 height: 30px;
 background: #306bbf;
 color: #fff;
 margin-top: 20px;
 margin-bottom: 20px;
}

//找標簽
let myImg = document.getElementById("myImg");
let pre=document.getElementById("pre");
let next=document.getElementById("next");

//創(chuàng)建一個保存圖片的數(shù)組
let arrImg = ["images/1.jpg", "images/1-1.png", "images/3.jpg" ];
//數(shù)組的索引下標
let index=0;
//定義事件函數(shù)
function preImg(event){
 index--;
 //實現(xiàn)循環(huán)切換
 if (index<0)
 {
 index=arrImg.length-1;
 }
 myImg.src = arrImg[index];
}
function nextImg(event){
 index++;
 //實現(xiàn)循環(huán)切換
 if (index>arrImg.length-1)
 {
 index=0;
 }
 myImg.src = arrImg[index];
}

pre.addEventListener('click',preImg);
next.addEventListener('click',nextImg);