### 11.8 js
#### 11.8.1 js的引入方式
```html
<script type="text/javascript" src="js/index.js"></script>
放在哪个位置都生效
```
#### 11.8.2 字符类型
递增和递减运算符:
```html
<script>
var a = 4;
var c = a++;
console.log(a); 5
console.log(c); 4
//这是因为c=a++是先将a的值赋给c再进行++
var a = 4;
var c = ++a;
console.log(a); 5
console.log(c); 5
//这是因为c=++a是先进行++再讲a的值赋给c
</srcipt>
```
字符串的拼接：
```html
<script>
var name='liujia',age=18;
var srt = `${name}今年是${age}岁了`
console.log(str)
</script>
```
数组，就是python中的列表
```html
<script>
var arr =[1,'2','liujia'];
//var声明标量，会把var声明的变量提升到全局作用域
for(i=0;i<arr.length;i++);
console.log(arr[i]);
function fn(){
var a = 4;
}
</script>
```
数组的常用方法
```html
<script>
var num = 123;
var arr = ['red','green'.'yellow'];
console.log(Array.isArray(arr));
console.log(arr.toString());//red,green,yellow
console.log(num.toString());//将数字转化成字符串
console.log(typeof num.toString()) //检查类型
console.log(arr.join('#')) //将arr中的元素以#链接起来
console.log(arr.push('bule'));//返回了数组最新的长度
console.log(arr.pop());//返回删除的内容
console.log(arr.unshit('gray','black'));//添加内容
console.log(arr.shit('liuji/a'))；//添加内容
var.splice();//对数组进行添加，删除，替换
var.slice(1) //对数组进行分割
</script>
```
if 和switch
```html
<script>
var score = 100;
if(score > 80){
console.log('可以吃鸡')
}else if(score <60){
sonsole.log('在家呆着学习')
}
var weather = prompt('请输入今天的天气')；
switch(weather){
case'晴天':
console.log('可以出去打篮球')；
break;
case'下雨':
console.log('可以去上网')；
break;
default:
console.log('学习')；
break；
}
</script>
```
比较
```html
<script>
var a = 2;
var b = '2';
console.log(a==b); true//比较的是值
console.log(a===b);false//比较的是值和数据类型
</script>
```
循环
```html
<script>
var arr = [8,9,,0];
for(var i = 0;i<arr.length;i++)
console.log(arr[i]);
var a = 1;
while(a<=100){
console.log(a);
a+=1
}
</script>
```
函数
```html
<script>
function fn(){
switch(arguments.length);
case 2:
console.log('两个参数')；
break;
default:
break;
}
fn(2,3,4)
</script>
```
对象：
```html
<script>
//1.字面量创建方式
car obj = {};
obj.name = 'liujia';
obj.fav = function(){
sonsole.log(this);
}
obj.fav();
console.log(obj.name);
//2.构造函数
var obj2 = new Object();
console.log(obj2);
obj.name = 'liudanni';
function add(x,y){
console.log(this.name);
console.log(x);
console.log(y);
}
console.dir(add);
add();
add call(obj,1,2)
// 3.像python中的类
class Person{
constructor(name,age){
this.name = name;
this.age = age;
}
fav(){
console.log(this.name)
}
}
var p =new Person('liujia',18);
p.fav();
</script>
```
日期对象：
```html
<script>
var date = new Date();
console.log(date);
console.log(date.getDate());
console.log(date.getMonth()+1);
console.log(date.getFullYear());
consloe.log(date.getDay());
console.log(date.getHours);
console.log(date.getMinutes());
console.log(date.getSeconds());
console.log(date.toLocaleString());
</script>
```
数字时钟案例
```html
<body>
<h2 id = "time"></h2>
</body>
<script>
var timeObj = document.getElementById('time');
sonsole.log(time);
function getNowTime(){
var time = new Date();
var hour = time.getHours();
var minute = time.getMinutes();
var second = time.getSeconds();
var temp = "" + ((hour>12)?hour-12:hour);
if (hour == 0){
temp = "12";
}
temp += ((minute<10)?":0":":") + minute;
temp += ((second<10)?':0':":") + second;
temp += ((hour >= 12)?"PM":"AM");
timeObj.innerText = temp;
}
setInterval(getNowTime,20)
</script>
```
Math对象
```html
<script>
//求最大最小值
var values = [1,22,33,44,55,66,8];
var max = Math.max.apply(null,valuse);
console.log(max);
var a = 1.49999999999999
console.log(Math.ceil(a)); //天花板函数
console.log(Math.floor(a)); //地板函数
console.log(Math.round(a)); //四舍五入
console.log(Math.random()); 随机数
</script>
```
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