var carname="Volvo";
var carname;
重新声明变量后变量的值不变，在这里还是volvo
var carname="Volvo";
document.getElementById("demo").innerHTML=carname;
改变段落元素的值
<!DOCTYPE html>
<html>
<body>
<script>
function myFunction()
{
document.write("糟糕！文档消失了。");
}
</script>
<h1>My First Web Page</h1> <p>My First Paragraph.</p> <button onclick="myFunction()">点击这里</button> </body>
</html>
如果在文档已完成加载后执行 document.write，整个 HTML 页面将被覆盖：
var str
如果一个元素只声明没有定义的话，默认值为undefined
<!DOCTYPE html>
<html>
<body> <script>
var person=new String;
var person1;
var car="Volvo";
var shifei=new Boolean;
var x= new Number;
document.write(person + "<br />");
document.write(person1 + "<br />");
document.write(car + "<br />");
document.write(shifei + "<br />");
var car=null
document.write(car + "<br />");
</script> </body>
</html>
声明变量类型及默认值了解
var carname1="Bill Gates";
var carname2='Bill Gates';
var answer1="Nice to meet you!";
var answer2="He is called 'Bill'";
创建字符变量的时候可以使用单引号或双引号
并且可以在引号内可以有引号 只要不与开头结尾引号对应即可
function myFunction(a,b)
{c=8;
if (a>b)
{
return;
}
x=a+b
}
给未声明的变量赋值 即使是在function中 也默认c为全局变量
greeting=(visitor=="主席")?"Dear President ":"Dear ";
条件赋值法 如果visitor是主席 赋值为dear president 否则赋值为 Dear
var day=new Date().getDay();
switch (day)
{
case 0:
x="Today it's Sunday";
break;
case 1:
x="Today it's Monday";
break;
case 2:
x="Today it's Tuesday";
break;
case 3:
x="Today it's Wednesday";
break;
case 4:
x="Today it's Thursday";
break;
case 5:
x="Today it's Friday";
break;
case 6:
x="Today it's Saturday";
break;
}
使用case语句输出今天星期几
var day=new Date().getDay();
switch (day)
{
case 6:
x="Today it's Saturday";
break;
case 0:
x="Today it's Sunday";
break;
default:
x="Looking forward to the Weekend";
}
默认关键词
<!DOCTYPE html>
<html>
<body>
<script>
function changeImage()
{
element=document.getElementById('myimage')
if (element.src.match("bulbon"))
{
element.src="/i/eg_bulboff.gif";
}
else
{
element.src="/i/eg_bulbon.gif";
}
}
</script> <img id="myimage" onclick="changeImage()" src="/i/eg_bulboff.gif"> <p>点击灯泡来点亮或熄灭这盏灯</p> </body>
</html>
点击更换图片
<p id="demo">
JavaScript 能改变 HTML 元素的内容。
</p> <script>
function myFunction()
{
x=document.getElementById("demo"); // 找到元素
x.innerHTML="Hello JavaScript!"; // 改变内容
}
</script>
<p>请输入数字。如果输入值不是数字，浏览器会弹出提示框。</p> <input id="demo" type="text"> <script>
function myFunction()
{
var x=document.getElementById("demo").value;
if(x==""||isNaN(x))
{
alert("Not Numeric");
}
}
</script>