JavaScript学习笔记（3）
JavaScript的数据类型
字符型、数字、布尔、数组、对象、Null、Undefined
数组的定义：
var cars=new Array();
cars[0]="Audi";
cras[1]="BMW";
cars[3]="Benze";
或者：
var cars = new Array("Audi","BMW","Benze");
或者：
var cars =["Audi","Audi","Audi","Audi","Audi"];
如：
<!DOCTYPE html>
<html>
<body>
<script>
var i;
//方式1
var cars1 = new Array();
cars1[0] = "Audi";
cars1[1] = "BMW";
cars1[2] = "Volvo";
//方式2：
var cars2 = new Array("Audi","BMW","Volvo","Audi","BMW","Volvo");
//方式3
var cars3=["Audi","Audi","Audi","Audi","Audi"];
for (i=0;i<cars3.length;i++)
{
document.write(cars3[i] + "<br>");
}
</script>
</body>
</html>
JavaScript对象
对象由花括号分隔。在括号内部，对象的属性以名称和值对的形式 (name : value) 来定义。属性由逗号分隔：
var person={firstname:"Bill", lastname:"Gates", id:5566};
上面例子中的对象 (person) 有三个属性：firstname、lastname 以及 id。
空格和折行无关紧要。声明可横跨多行：
var person={
firstname : "Bill",
lastname : "Gates",
id : 5566
};
对象属性有两种寻址方式：
name=person.lastname;
name=person["lastname"];
如：
<!DOCTYPE html>
<html>
<body>
<script>
var person={
firstname : "Bill",
lastname : "Gates",
id : 5566
};
document.write(person.firstname+ " "+person.lastname +" " +person.id+"<br />");
document.write(person["firstname"]+" "+person["lastname"]+" "+person["id"] + "<br />");
</script>
</body>
</html>
Undefined 和 Null
undefined这个值表示变量不会含有值
Null可以用来清空变量
如：
<!DOCTYPE html>
<html>
<body>
<script>
var person;
var car="Volvo";
document.write(person + "<br />");
document.write(car + "<br />"+ "<br />");
var car=null
var person="Steve Jobs";
document.write(person + "<br />");
document.write(car + "<br />");
</script>
</body>
</html>
输出结果：
undefined
Volvo
Steve Jobs
null
声明变量类型
可以使用new关键字声明变量的类型
如：
<!DOCTYPE html>
<html>
<body>
<button onclick="myFunction()">点击这里</button>
<p id="test1"></p>
<p id="test2"></p>
<p id="test3"></p>
<p id="test4"></p>
<script>
function myFunction()
{
var numVal = new Number;
numVal = 123.123;
var strVal = new String;
strVal = "THIS IS A STRING.";
var bVal = new Boolean;
bVal = true;
var arrVal = new Array;
arrVal[0]=100;
arrVal[2]=200;
arrVal[3]=300;
arrVal[4]=400;
arrVal[5]=500;
document.getElementById("test1").innerHTML=numVal;
document.getElementById("test2").innerHTML=strVal;
document.getElementById("test3").innerHTML=bVal;
document.getElementById("test4").innerHTML=arrVal[3];
}
</script>
</body>
</html>
输出结果：
<!DOCTYPE html>
<html>
<body>
<button onclick="myFunction()">点击这里</button>
<p id="test1"></p>
<p id="test2"></p>
<p id="test3"></p>
<p id="test4"></p>
<script>
function myFunction()
{
var numVal = new Number;
numVal = 123.123;
var strVal = new String;
strVal = "THIS IS A STRING.";
var bVal = new Boolean;
bVal = true;
var arrVal = new Array;
arrVal[0]=100;
arrVal[2]=200;
arrVal[3]=300;
arrVal[4]=400;
arrVal[5]=500;
document.getElementById("test1").innerHTML=numVal;
document.getElementById("test2").innerHTML=strVal;
document.getElementById("test3").innerHTML=bVal;
document.getElementById("test4").innerHTML=arrVal[3];
}
</script>
</body>
</html>
点击按钮后输出结果：
123.123
THIS IS A STRING.
true
300
