JavaScript基础语法

声明一个变量，没有赋值，就是undefined类型。

除了空字符以外，包括空格在内转换为布尔值都是true。

<!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 type="text/javascript">
        // 声明保存用户名的变量
        /*var name_01;
        name_01="marry";*/
        // 声明保存年龄的变量
        var age = 18;//声明的同时赋值
        //声明保存邮箱的变量
        var email="marry@sina.com.cn";
        //一次声明多个变量

        var name_01="marry",age=18,email="marry@sohu.com",address="北京";

        var distance=12.67980;

        var id="16";

        console.log(typeof(distance));    //在控制台中打印    

        console.log(typeof(age-"abc"));

        console.log(isNaN(email));

        console.log(isNaN(id));

        id=Number(id);

        name_01=Number(name_01);

        console.log(typeof id);

        console.log(name_01);    //NaN

        var topval=parseInt("28px");    //保留字windows.top

        var c="abc58";

        console.log(topval);

        console.log(parseInt(c));

        console.log(parseInt("0xf",16));

        var d=parseFloat("12.34.56px");

        var e=parseFloat("0.123abc");

        console.log(e);

        var mag='hello world';

        var ids=78965;

        var idstr=ids.toString();

        var m;

        var isStudent=true;

        var isChild=false;

        console.log(typeof(idstr));

        console.log(String(m));

        console.log(isChild.toString());

        var x=0;

        var strings=" ";

        var y;

        var timer=null;

        console.log(Boolean(x));

        console.log(Boolean(timer));
    </script>
</body>
</html>

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>赋值操作符</title>
</head>
<body>
    <script type="text/javascript">
        var a=10;
        var b=20;
        var str="hello "
        a+=5;//a=a+5;
        b%=4;//b=b%4;
        str+="world";//str=str+"world";
        console.log(a);
        console.log(b);
        console.log(str);

        var x=10,
            y="10";
            z=x==y;        //值是否相等
            q=x===y;    //全等
            n=x!=y;
            m=x!==y;
        console.log(z);
        console.log(q);
        console.log(n);
        console.log(m);

        var soce=85;
        var result=(soce>=60)?"及格":"不及格";

        console.log(result);
    </script>
</body>
</html>

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>算数操作符</title>
</head>
<body>
    <script type="text/javascript">
        var num1=10,
            num2=5,
            num3=num1++-num2,    //++num1  num1=num1+1
            x1=20,
            x2=30,
            x3=--x1+x2--;

        console.log(num1);    //11
        console.log(num3);    //16


        console.log(x1);    //19
        console.log(x2);    //29
        console.log(x3);    //19+30=49
    </script>
</body>
</html>

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>逻辑运算符</title>
</head>
<body>
    <script type="text/javascript">
        var num1=10,
            num2=20,
            num3=30,
            str="welcome",
            bool=true,
            n=null,
            m;

        // console.log(num1<num2 && num2<num3);

        // console.log(str && num3);
        //所有的字符串除了""转换成布尔型都是true，所有的数字除了0转换为布尔值都是true

        // console.log(99 && "" && 88);

        console.log(n && num3);
        console.log(55 && true && 33*"abc");
        console.log(m&&true);

        console.log("hello" || 0);//短路操作，第一个条件符合就不往下执行
        console.log("" || 0 || null || NaN || "abc");//abc
        console.log("" || m);//undefined
        console.log(30*"abc"||55-"def");//NaN

        console.log(!false);//true
        console.log(!88);//false
        console.log(!0);//true
        console.log(!"red");//false
        console.log(!NaN);//true
        console.log(!null);//true
        console.log(!undefined);//true
        console.log(!!"");//false
        console.log(!!"blue");//true
    </script>
</body>
</html>