放在函数里面
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>dem</title>
<style type="text/css">
h1 {
color: red;
}
</style>
</head>
<body>
<p id="dem">改变内容</p>
<button type="button" onclick="dd()">点击</button>
<script type="text/javascript">
function dd(){
document.getElementById('dem').innerHTML='<h1>Hello Javascript</h1>';
}
</script>
</body>
</html>
(一)输入输出
语句 | 功能 |
---|---|
window.alert(); | 弹出警告框 |
document.write(); | 写入HTML,重置文档 |
innerHTML(); | 写入HTML,不会重置文档 |
console.log(); | 写入控制台 |
1.alert();
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>dem</title>
</head>
<body>
<button type="button" onclick="alert('hello word!');">点击</button>
</body>
</html>
2.innerHTML
在被选中元素的内部添加新的HTML代码;不会使页面重置
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>dem</title>
<style type="text/css">
h1 {
color: red;
}
</style>
</head>
<body>
<p id="dem">改变内容</p>
<button type="button" onclick='document.getElementById("dem").innerHTML="<h1>Hello Javascript</h1>"'>点击</button>
</body>
</html>
3.document.write();
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>dem</title>
</head>
<body>
<p>Frist</p>
<button type="button" onclick='document.write(5+6);'>点击</button>
</body>
</html>
4.console.log();
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>dem</title>
</head>
<body>
<p>Frist</p>
<button type="button" onclick='console.log("sssaa")'>点击</button>
</body>
</html>
常用输入
1.prompt对话框
prompt(text,defaultText);
text:可选,在对话框中显示的纯文本
default:可选,默认的输入文本
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>dem</title>
</head>
<body>
<button type="button" onclick="f_prompt()">点击</button>
<script type="text/javascript">
function f_prompt(){
var name=prompt("请输入姓名!","admin");
if(name!=null&&name!=""){
document.write("你好!"+name+"!");
}
else{
alert("Wrong!");
}
}
</script>
</body>
</html>
(二)变量,常量,字面量
常量:const q=10;不可改
变量:可以使用var关键字声明变量,变量的值可以是任意类型
var name="sss";
let与var类似,但是只是声明的变量只在其所在代码块内有效
let name="sss";
变量提升:
浏览器会先解析代码
例子:
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>dem</title>
</head>
<body>
<script type="text/javascript">
console.log(content);
</script>
</body>
</html>
以上代码会报错: content is not defined
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>dem</title>
</head>
<body>
<script type="text/javascript">
console.log(content);
var content="sss";
</script>
</body>
</html>
以上代码不会报错:undefined(为content的值)
因为浏览器将 var content=“sss”;拆分为var content;content=“sss”;然后将var content;提到代码最前面
所以上面代码实际为:
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>dem</title>
</head>
<body>
<script type="text/javascript">
var content;
console.log(content);
content="sss";
</script>
</body>
</html>
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>dem</title>
</head>
<body>
<script type="text/javascript">
var content;
console.log(content);
content="sss";
console.log(content);
</script>
</body>
</html>
1.例题:
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>dem</title>
</head>
<body>
<button onclick="foo();">点解</button>
<script type="text/javascript">
function foo(){
if(false){
//var content="sss";
}
console.log(content);
}
</script>
</body>
</html>
var content=“sss”;
注释掉前:undfined
注释掉后:报错
2.例子
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>dem</title>
</head>
<body>
<script type="text/javascript">
console.log(vl);
var vl=100;
function foo(){
console.log(vl);
var vl=200;
console.log(vl);
}
foo();
</script>
</body>
</html>
结果为:
undfined
undfined
200