js basic data types and conversion base operators
<!DOCTYPE html>
<html>
<head lang="en">
<meta charset="UTF-8">
<title>js 基础</title>
<script type="text/css" src="xxx.js"></script> <!--外链式 -->
</head>
<body>
<p onclick="alert('我是行内式')">你好</p>
<script type="text/javascript"> //内嵌式
// 输出语句
alert("我是内嵌式");
prompt("请输入：");
confirm("你好吗？");
console.log("我是控制台输出");
console.error("我是错误");//了解
console.warn("我是警告");//了解
document.write("<h1>我是h1标签</h1>");//页面输出内容而且识别标签
//数据类型
var str1 = "aa";
var str2 = "123";
var num = 123;
var bool = true; //true false
var kong =null;
var wu ; //undefined 未定义
//查看数据类型 (typeof 变量 )
var type = typeof(str1);
//类型转换 成数字number
var str = "123.2a";//字符串类型的数字
var bool = true;
var num1 = parseInt(str); //取第一部分数字 并取整 没有数字Nan 输出：123
var num2 = parseFloat(str); //取第一部分数字 保留小数 输出：123.2
var num3 = Number(str); //有非数字就会报 Nan 没有就全部转换成数字
console.log(typeof Number(bool)); //boolean 转成1
console.log(typeof (str-0)); //数子类型的字符串在处"+"外都会有隐士类型转换
console.log(typeof (bool-0));
console.log(typeof (str*1));
console.log(typeof (bool*1));
console.log(typeof (str/1));
console.log(typeof (bool/1));
console.log(typeof typeof (bool/1)); //数据类型是用string定义的
//类型转换 成字符串String
var num4 = 123;
var num5 = num+""; //"+"在有字符串在于运算的时候表示拼接 而非相加
var num6 = num.toString();
var num7 = String(num2);
console.log(typeof num5);
//进制转换
var num8 = parseInt(111,2); //任意进制转换成十进制。111：值,2：进制 输出：7
var num = 10;
var renyi = num.toString(16); //十进制转换成任意进制 输出：a
//特殊的 undefined 和 null
console.log(undefined == null); //true
console.log(undefined === null); //false
console.log(undefined+10); // Nan 不是一个数字
console.log(null+10); //10
//操作符种类
一,算数运算符（+—*/...）
a) 一元运算符：正号,负号,++,--,平方等一个变量就能运算
b) 二元运算符：+-*/%等两个变量才能运算
c) 三元运算符： 值1？值2：值3；
二,逻辑运算符（ ||&& ! ）（或且非）
三,比较运算符（<,>,==,>=...）
四,赋值运算符（=,+=,-=,*=,/=,%=）
//优先级
1 ()
2 !,-（负数）,++,-- （正数省略+）（一元运算）
3 *,/,%
4 +,- （加,减）（二元运算）
5 <,<=,<,>= （一级逻辑运算）
6 ==,!=,===,!==, （二级逻辑运算）
7 && （三级级逻辑运算）
8 ||
9 ?: （三元运算）
10 =,+=,-=,*=,/=,%= （赋值运算）
// && 和 || 运算
一 && 链接两个boolean类型,有一个是false结果就是false, A与B
都是true取后面,都是false取前面
二 || 链接两个boolean类型,有一个是true结果就是true, A或B
都是true取前面,都是false取后面
</script>
</body>
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