Javascript：变量与数组

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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>

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