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javascript 学习随笔

2015-02-09 17:04 302 查看

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>

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