Javascript变量属于弱类型，即声明一个变量的时候无需指定类型，并且之后可以将任何类型的数值复制给它。
代码整理自w3school：http://www.w3school.com.cn
效果图：
示例代码：
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<meta http-equiv="Content-Language" content="zh-cn" />
<title>Javascript 变量</title>
<head>
<style>
body {background-color:#e5e5e5}
span.red {color:red}
</style>
</head>
<body>
<h3>（一）js语句会按照它在html中的位置进行执行。</h3>
<p>This is a paragraph <span class="red">before</span> script.</p>
<script>
document.write("Javascript!");
</script>
<p>This is a paragraph <span class="red">after</span> script.</p>
<h3>（二）js是弱类型</h3>
<script>
var x;
document.write("x="+x);
document.write("<br/>")
x=2;
document.write("x="+x);
document.write("<br/>")
x="hello"
document.write("x="+x);
document.write("<br/>")
</script>
<h3>（三）js定义数组</h3>
<script>
var arr = new Array();
arr[0]=1;
arr[1]=2;
arr[2]=3;
document.write("arr[0]="+arr[0]+" arr[1]="+arr[1]+" arr[2]="+arr[2]+" arr[4]="+arr[4]);
//其他创建数组的方式：
document.write("<br/>");
var arr2 = new Array("Tom","Smith","John");
var arr3 = ["Tom","Smith","John"];
document.write("arr2[0]="+arr2[0]);
document.write("<br/>");
document.write("arr3[0]="+arr3[0]);
</script>
<p>JS中数据不存在“下标越界”的问题。（超过数组大小取值会得到undefine）
<h3>（四）js对象</h3>
<script>
//创建对象
var person = {"firstName":"Will","lastName":"Smith"}
//取得对象的属性值有两种方式：
var name1 = person.firstName
var name2 = person["firstName"]
document.write("name1:"+name1+",name2:"+name2)
</script>
<h3>（五）Undefined 和 Null</h3>
<script>
var var1;
var var2 = null;
var var3 = "hello"
document.write("var1="+var1+", var2="+var2+", var3="+var3);
document.write("<br/>");
var3 = null;
document.write("var3="+var3+"<br/>");
</script>
<h3>（六）声明变量类型</h3>
<script>
var x = new String;
var y = new Number;
var z = new Array;
x="hello"
y=6
z=["Tom","John","Smith"]
//输出变量的类型
document.write("x:"+typeof x+"<br/>");
document.write("y:"+typeof y+"<br/>");
document.write("z:"+typeof z+"<br/>");
x = 2;
document.write("set x=2, x:"+typeof x+"<br/>");
</script>
</body>
</html>