数据类型：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
</head>
<body>
</body>
</html>
<script>
var valuel= "123"; //string
var value2= 0; //number
var value3= null; //null
var value4; //NaN
var value5= true; //boolean
var value6= "a"-1; console.log(typeof(value1)); //string
console.log(typeof(value2)); //number
console.log(typeof(value3)); //object
console.log(typeof(value4)); //undefined
console.log(typeof(value5)); //boolean
console.log(typeof(value6)); //number
</script>灯泡开关：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
<style>
div{
margin: 0 auto;
text-align: center;
}
</style>
</head>
<body>
<div>
<img src="img/pic_bulboff.gif"/>
<br>
<input type="button" id="kai" value="开"/>
<input type="button" id="guan" value="关"/>
</div>
</body>
</html>
<script>
var img = document.getElementsByTagName('img')[0];
var buttonKai = document.getElementById('kai');
buttonKai.onclick = function(){
img.src = 'img/pic_bulbon.gif';
}
var buttonGuan = document.getElementById('guan');
buttonGuan.onclick = function(){
img.src = 'img/pic_bulboff.gif';
}
</script>所需图片文件：
pic_bulboff.gif 关灯图片
pic_bulbon.gif 开灯图片
