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HomeWeb Front-endJS Tutorialjs 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(&#39;我是行内式&#39;)">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 id="我是h-标签">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>
</html>

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