JavaScript（简称：JS） 是一门跨平台、面向对象的脚本语言，是用来控制网页行为的，实现人机交互效果。JavaScript 和 Java 是完全不同的语言，不论是概念还是设计。但是基础语法类似。
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
</head>
<body>
<script>
alert("hello world");
输出语句
window.alert('第一种输出语句')
document.write('第二种输出语句')
console.log('第三种输出语句')
变量
let a = 10;
a = 'abc';
alert(a);
常量
const PI = 3.14;
PI = 3.15;//会报错，常量不可修改
console.log(PI);
变量声明
var name = "张三";
name = "李四";
console.log(name);
name = "王五";
console.log(name);
tyypeof' 检测变量类型
console.log(typeof 3);
console.log(typeof 3.14);
console.log(typeof 'a');
console.log(typeof "hello");
console.log(typeof true);
console.log(typeof null);
let a;
console.log(typeof a);
反引号` `
let name = "张三";
let age = 18;
console.log("姓名" + name + " ,年龄" + age);//老式拼接
console.log(`姓名${name},年龄${age}`);//模板字符串。内容拼接时用${}来引用变量名
//运算符
console.log(1+1);
console.log(1-1);
console.log(1*1);
console.log(6/3);
console.log(5%2);
let a = 1;
console.log(++a);
console.log(a);
let b = 1;
console.log(b++);
console.log(b);
//关系运算符
console.log(1>2);
console.log(1<2);
console.log(1>=2);
console.log(1<=2);
console.log(1==2);
console.log(1==='1');
//逻辑运算符
console.log(true&&false);
console.log(true||false);
console.log(!true);
//赋值运算符
let c = 1;
c += 1;
console.log(c);
三目运算符
console.log(1>2?true:false);
//数据类型转换
console.log(10+"1");
console.log(10-"1");
console.log(Number("123"));
console.log(String(123));
console.log(Boolean(0));//0为false，其他为true
console.log(Boolean(""));//空字符串为false
console.log(Boolean(null));//null为false
console.log(Boolean(undefined));//undefined为false
console.log(Boolean(NaN));//NaN为false
console.log(Boolean(1));
console.log(Boolean("abc"));
console.log(Boolean(-1));
console.log(Boolean(0.1));
console.log(Number("123"));
let d ='123a4'
let e ="a1234"
console.log(parseInt(d));
console.log(parseInt(e));
console.log(parseInt("123.5"));//转换为整数
console.log(parseFloat("123.5"));//转换为浮点型
//if-else语句
let age =8;
if(age>=18){
alert("已成年可以进网吧")
}else{
alert("未成年不可以进网吧")
};
//if-else if-else语句
let grade = 60;
if (grade>=90){
alert("优秀");
}else if(grade>=80){
alert("良好");
}else if(grade>=70){
alert("及格");
}else{
alert("不及格");
}
//for 循环
for(let i=1;i<=10;i++){
console.log("hello world");
console.log(`打印次数：${i}`);
if(i == 10){
console.log("打印结束");
}
}
</script>
<!-- <script src="js/demo.js"></script> -->
</body>
</html>
