JavaScript实现电灯开关效果的三种方法

<!DOCTYPE html>
<html lang="en">
<head>
 <meta charset="UTF-8">
 <title>Title</title>
</head>
<body>
<img src="imgs/on.gif" alt="" id="img"> <br><!--图片-->
<input type="button" id="bt1" value="开" onclick="f1()"><!--按钮-->
<button id="bt2" onclick="f2()">关</button>
</body>
<script>
function f1() {//开
 let bt1=document.getElementById("bt1");//获取按钮id
 let img=document.getElementById("img");//获取图片id
 img.src="imgs/on.gif";//修改图片
}
function f2() {//关
 let bt2=document.getElementById("bt2");
 let img=document.getElementById("img");
 img.src="imgs/off.gif";
}
</script>
</html>

<!DOCTYPE html>
<html lang="en">
<head>
 <meta charset="UTF-8">
 <title>Title</title>
</head>
<body>
<img src="imgs/off.gif" alt="" id="img"> <br>
</body>
<script>
 let img=document.getElementById("img");
 img.onclick=f;
 let flag=false;
 function f() {
 if (flag==true){
  img.src="imgs/off.gif"
  flag=false;
 }else {
  img.src="imgs/on.gif"
  flag=true;
 }
 }
</script>
</html>

<!DOCTYPE html>
<html lang="en">
<head>
 <meta charset="UTF-8">
 <title>Title</title>
</head>
<body>
<img src="imgs/on.gif" height="180" width="109" id="img" onclick="f()">
</body>
<script>
 let img = document.getElementById("img");
 let i=0;
 function f() {
 img.src='imgs/'+(++i%2==0?'off':'on')+'.gif';
 }
</script>
</html>