JavaScript基础深入

一、typeof 方法

        console.log(typeof (123));  //number
        console.log(typeof (typeof (123)));  //string
        console.log(typeof ("123"));  //string
        console.log(typeof (true));  //boolean
        console.log(typeof ({}));  //object
        console.log(typeof ([]));  //object
        console.log(typeof (null));  //object   null 是一个bug,是空对象的指针和占位符
        console.log(typeof (undefined));  //undefined
        console.log(typeof (function () { }));  //function
        // console.log(a); //a is not defined
        console.log(typeof (a));  //undefined
        console.log(typeof (typeof (a)));  //string
        console.log(typeof (undefined)); //undefined
        console.log(typeof (1 - "1"));  //number
        console.log(typeof ("1" - "1"));  //number
        console.log(typeof (2 - "1"));  //number

二、显示类型转换

 // 转换成数值类型
        console.log(Number('123'));  //123
        console.log(Number('123') + '-' + typeof (Number('123')));  //123-number
        console.log(Number("true") + '-' + typeof (Number("true")));  //NaN-number
        console.log(Number(true) + '-' + typeof (Number(true)));  //1-number
        console.log(Number(null) + '-' + typeof (Number(null)));  //0-number
        console.log(Number(undefined) + '-' + typeof (Number(undefined)));  //NaN-number
        console.log(Number('a') + '-' + typeof (Number('a')));  //NaN-number

        // 转换成整型
        console.log(parseInt('12312') + '-' + typeof (parseInt('123')));  //12312-number
        console.log(parseInt('true') + '-' + typeof (parseInt('true')));  //NaN-number
        console.log(parseInt(true) + '-' + typeof (parseInt(true)));  //NaN-number
        console.log(parseInt(null) + '-' + typeof (parseInt(null)));  //NaN-number
        console.log(parseInt(undefined) + '-' + typeof (parseInt(undefined)));  //NaN-number
        console.log(parseInt('3.14') + '-' + typeof (parseInt('3.14')));  //NaN-number
        // 以十六进制为基础转换成十进制
        console.log(parseInt(11, 16));  //17
        console.log(parseInt('b', 16));  //11
        console.log(parseInt('123abc'));  //123
        console.log(parseInt('abc123'));  //NaN
        console.log(parseInt('5bc123'));  //5
        console.log(parseFloat('3.1415956'));  //3.1415956
        console.log(parseFloat('3.1465956').toFixed(2));  //3.15   四舍五入

        console.log(typeof (String(123)));   //string

        // undefined null 没有 toString() 这个方法
        console.log(parseInt('100', 2));  //4
        console.log(parseInt('100', 2).toString(16));  //4  以 2进制为基础转换成10进制，再转换成16进制

        console.log(Boolean(1));  //true

三、隐示类型转换

       var a = '123'
        a++
        console.log(a);  //124

      var a = '1' + 1
       console.log(a);  //11

       var a = '3' * 2
        console.log(a);  //6    * /  -  %  都是str 转 number

        var a = '1' > 2
        console.log(a);  //false

        var a = 1 > '2'
        console.log(a);  //false

        var a = 'c' > 'd'
        console.log(a);  //false  
        var a = 'c' < 'd'
        console.log(a);  //true

     var a = 1 != '2'
        console.log(a);  //true

        var c = 1 === '1'
        console.log(c);  //false

        var d = NaN === NaN
        console.log(d);

      var a = 1 != '2'
        console.log(a);  //true

        var c = 1 === '1'
        console.log(c);  //false

        var d = NaN == NaN
        console.log(d);  //false  NaN不等于任何

        var a1 = undefined == 0
        var a2 = null == 0
        var a3 = null == undefined
        console.log(a1, a2, a3);  //false false true

        console.log(isNaN('a'));  //true
        console.log(isNaN(null));  //false
        console.log(isNaN(NaN));  //true
        console.log(isNaN(undefined));  //true

四、

ASCII码 表一 0-127 表二 128-255 都是一个字节
UNICODE码 涵盖 ASCII码 256 开始是两个字节

       var str = 'A'
        var pos = str.charCodeAt(0)
        console.log(pos);  //65

        var str1 = 'abc'
        var pos1 = str1.charCodeAt(2)
        console.log(pos1);  //99

//求字符串的字节
 var getBytes = function (str) {
            var bytes = 0
            for (var i = 0; i < str.length; i++) {
                if (str.charCodeAt(i) <= 255) {
                    bytes++
                } else {
                    bytes += 2
                }
            }
            return bytes
        }
        console.log(getBytes('124hello你好！'));  //14

        var getBytes = function (str) {
            // 获取字符串的长度
            var strBytes = str.length
            for (var i = 0; i < str.length; i++) {
                var pos = str.charCodeAt(i)
                // 英文字符是一个字节，中文字符是两个字节
                // 如果是有一个汉字，那长度就要加一
                if (pos > 255) {
                    strBytes++
                }
            }
            return strBytes
        }
        console.log(getBytes('124hello你好！'));  //14