javascript 学习随笔

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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javascript 学习随笔

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