• JS数据类型以及运算符

    1.JS的六种基本类型与typeof操作符

    1. Undefined类型
    2. Null类型
    3. Boolean类型
    4. Number类型
    5. String类型
    6. Object类型
    typeof 
       1:  <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
       2:  <html xmlns="http://www.w3.org/1999/xhtml">
       3:  <head>
       4:  <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
       5:  <title>typeof 使用</title>
       6:  <script type="text/javascript">
       7:  /*//1.Undefined
       8:  var box;
       9:  alert(box);
      10:  alert(typeof box);//box为Undefined类型,值为undefined,typeof返回值为字符串'undefined'
      11:  
      12:  //2.Boolean
      13:  var boxBoolean=true;
      14:  alert(typeof box);//boxBoolean为Boolean类型,typeof返回值为:'boolean'
      15:  
      16:  //3.String
      17:  var boxString='abc';
      18:  alert(typeof boxString);//boxString为String类型,typeof返回值为:'string'
      19:  
      20:  //4.空的对象:对象已创建,但是里面没有东西
      21:  var boxObject={};
      22:  alert(boxObject);//[object Object]
      23:  alert(typeof boxObject);//obj是Object类型,值为[object Object],typeof返回值为:'object'
      24:  
      25:  //5.空对象:表示没有创建,就是一个null
      26:  var boxNull=null;
      27:  alert(boxNull);//null
      28:  alert(typeof boxNull);//'object' //Null类型派生自Object
      29:  
      30:  //6.Number
      31:  var boxNumber=300;
      32:  alert(typeof boxNumber);//boxNumber为Number类型,值为300,typeof返回值为:'number'
      33:  */
      34:   
      35:  //7.function
      36:  function boxFunction(){
      37:   
      38:  }
      39:  alert(boxFunction);//值为:function boxFunction(){}
      40:  alert(typeof boxFunction);//'function'
      41:   
      42:  //直接测试
      43:  alert(typeof 'hehe');//'string'
      44:  </script>
      45:  </head>
      46:   
      47:  <body>
      48:    <ol>
      49:     <li>typeof操作符</li>
      50:     <li>Undefined类型</li>
      51:     <li>Null类型</li>
      52:     <li>Boolean类型</li>
      53:     <li>Number类型</li>
      54:     <li>String类型</li>
      55:     <li>Object类型</li>
      56:    </ol>
      57:  </body>
      58:  </html>
       1:  <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
       2:  <html xmlns="http://www.w3.org/1999/xhtml">
       3:  <head>
       4:  <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
       5:  <title>六种基本类型</title>
       6:  <script type="text/javascript">
       7:  //Unidefined
       8:   var box;
       9:   alert(box);//值被隐式赋值为undefined 相当于 var box=undefined;
      10:   alert(typeof box);//类型为'undefined'
      11:   //alert(age);//age is not defined
      12:   alert(typeof age);//'undefined' 注意age的类型也为Undefined
      13:   /*声明一个变量必须初始化,以避免这种问题*/
      14:   
      15:  //Null
      16:  var boxObject=null;//我留着以后创建对象,只声明了一个对象引用boxObject,初始化为null
      17:  boxObject={};
      18:  alert(boxObject);//[object Object]
      19:   
      20:  //Null与Undefined
      21:  alert(undefined==null);//true
      22:  alert(undefined===null)//false 恒等:数据类型也必须相等 Undefined!=Object
      23:   
      24:  //Boolean
      25:  var boxBoolean=Boolean('嘿嘿');//显式转换:String->Boolean
      26:  alert(boxBoolean);//true
      27:  var str='嘿嘿';
      28:  if(str)//隐式转换:String->Boolean
      29:    alert('真');//真
      30:  else
      31:    alert('假');
      32:  //转换规则详见图
      33:  </script>
      34:  </head>
      35:   
      36:  <body>
      37:  </body>
      38:  </html>
    其它类型转换成Boolean规则
     
     

    2.JS中的进制:

       1:  <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
       2:  <html xmlns="http://www.w3.org/1999/xhtml">
       3:  <head>
       4:  <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
       5:  <title>JS的进制</title>
       6:  <script type="text/javascript">
       7:  /*//八进制
       8:  var box=070;
       9:  alert(box);//56
      10:  
      11:  //十六进制
      12:  var box2=0xA;
      13:  alert(box2);
      14:  
      15:  //科学计数法
      16:  var box3=0.000000003;
      17:  alert(box3);//3e-9 自动转成科学计数法
      18:  
      19:  //当超出浮点数的范围:
      20:  var box4=100e1000;
      21:  alert(box4);//超出Number.MAX_VALUE的范围 //Infinity 正无穷;
      22:  box4=-100e1000;
      23:  alert(box4);//超出Number.MIN_VALUE的范围 //-Infinity 负无穷
      24:  alert(isFinite(box4));//判断一个数值是否超出规定范围,超出false,没有true
      25:  */
      26:   
      27:  //NaN
      28:  /*var box5=0/0;
      29:  alert(box5);//NaN,但是不影响下面语句的执行
      30:  box5=1/0;
      31:  alert(box5);//Infinity
      32:  alert(NaN==NaN);//false,造成NaN的原因可能不同*/
      33:   
      34:  //isNaN:将该值能否转换成数值,能->非NaN->则返回false,否则返回true
      35:  alert(isNaN('12'));//false 转成 12
      36:  alert(isNaN('abc'));//true 不能转成数值
      37:  alert(isNaN(true));//false 可以转成1
      38:   
      39:  //**有3个函数可以把非数值转换成数值
      40:  //Number(),parseInt(),parseFloat();
      41:  //Number():任何数据类型->数值
      42:  //剩余两个:String->数值
      43:  alert(Number(true)+","
      44:  +Number(null)+","+Number(undefined));//1,0,NaN
      45:  alert(Number('')+","+Number('0xA')+","+Number('060'));//0,10,60
      46:  alert(Number('abc'));//NaN
      47:   
      48:  alert(parseInt('12abc')+","+parseInt('abc12'));//12,NaN //只获取第一个数值
      49:  alert(parseInt('AF',16)+","+parseInt('1001',2));//175,9 //十六进制转十进制,二进制转十进制
      50:   
      51:  alert(parseFloat('012')+","+parseFloat('1.23')+","+parseFloat('0x13'));//12,1.23,0 //八进制和十六进制不能识别
      52:  </script>
      53:  </head>
      54:   
      55:  <body>
      56:  </body>
      57:  </html>

    3.String类型:

       1:  <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
       2:  <html xmlns="http://www.w3.org/1999/xhtml">
       3:  <head>
       4:  <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
       5:  <title>String类型</title>
       6:  <script type="text/javascript">
       7:  //1.JS中单引号与双引号均可表示字符串
       8:  var str='abc';
       9:  var str2="abc";
      10:  alert(str+","+str2);//abc,abc
      11:  alert('u4f60');//你,UTF-16编码
      12:   
      13:  //2.ECMAscript中的字符串是不可变的,也就是说,字符串一旦创建它们的值就不能改变
      14:  //要改变某个变量保存的字符串,系统会销毁原来的字符串,然后再用另一个包含新值得字符串
      15:  //来填充该变量(类似Java的字符串常量)
      16:  var str3='zhang';
      17:  str3 += 'hong';//'zhang'会被销毁,str3指向新的字符串'zhanghong'
      18:  alert(str3);
      19:   
      20:  //3.toString方法:任意类型数据转换成字符串
      21:  var box=12;
      22:  var boxBoolean=true;
      23:  alert(box.toString()+","+boxBoolean.toString());//'12','true'
      24:  alert(box.toString(2));//'1100'
      25:  box=0xA;
      26:  alert(box.toString(2));//'1010' //将任意进制转成指定进制的字符串
      27:   
      28:  //4.对于undefined和null不能使用toString转换
      29:  //var boxUndefined
      30:  //alert(boxUndefined.toString());//报错
      31:  //var boxNull=null;
      32:  //alert(boxNull.toString());//报错:无法获取未定义或 null 引用的属性"toString"
      33:   
      34:  alert(String(null)+","+String(undefined));//'null','undefined'
      35:                                            //String()将任意类型转换成字符串
      36:                                            //如果非null 或 非 undefined则会调用toString()返回字符串
      37:                                            //否则返回'null'和'undefined'
      38:  </script>
      39:  </head>
      40:   
      41:  <body>
      42:  </body>
      43:  </html>

    4.Object类型:

       1:  <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
       2:  <html xmlns="http://www.w3.org/1999/xhtml">
       3:  <head>
       4:  <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
       5:  <title>Object类型</title>
       6:  <script type="text/javascript">
       7:  var obj1={},obj2=new Object();
       8:  alert(obj1+","+obj2);//[object Object],[object Object]
       9:                       //第一个object表示它是一个对象(而不是值),后面表示它的类（构造函数名）。
      10:   
      11:  //当向Object()传值
      12:  var obj3=new Object(3);
      13:  alert(obj3);//3 但是类型为Object
      14:  alert(12+obj3);//15 底层经过一系列转换
      15:   
      16:  //new也可以创建其它类型对象
      17:  var obj4=new Number(),obj5=new Boolean(),obj6=new String();
      18:  alert(obj4+","+obj5+","+obj6)//0,false,'' 类型依然为Object
      19:   
      20:  </script>
      21:  </head>
      22:   
      23:  <body>
      24:   
      25:  </body>
      26:  </html>

    5.JS 中的运算符:

    运算符优先级:

    运算符优先级1

    运算符优先级2

       1:  <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
       2:  <html xmlns="http://www.w3.org/1999/xhtml">
       3:  <head>
       4:  <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
       5:  <title>JS中的比较运算符</title>
       6:  <script type="text/javascript">
       7:   /*
       8:    1.如果其中一个是数值字符串则转换成数值再比较.
       9:    2.如果两个都是字符串字符按字典顺序比较
      10:    3.两个操作其中一个是对象,则调用对象的valueOf()或toString()方法,再用结果比较
      11:   */
      12:   /*alert((3>'22')+","+(3>true)+","+('67'>'ab'));//false,true,false //3>22 3>1 6与a的ASCII比较
      13:   var obj={
      14:    toString:function(){
      15:       return '4';      
      16:     }     
      17:   }
      18:   alert('3'>obj);//false //'3'>'4'
      19:   
      20:   alert(2=='2');//true //2==2
      21:   alert(null==undefined)//true;
      22:   alert({}=={});//false //比较的是两个对象的地址,两个对象地址不同
      23:   alert((null==0)+","+(undefined==0))//false,false //虽然Number(null)为0,但是null在比较时不会自动转换
      24:   */
      25:   
      26:  //逻辑运算符(&&(短路与),||)
      27:  //注意在前面说过的的Boolean(任意类型)中:
      28:  //非null对象:true ,null:false ,undefined:false
      29:   alert(true&&5);//5
      30:   alert(({}&&1)+","+(0&&{})+","+(1&&{})+","+({}&&{}));//1,0,[object,Object],[object,Object]
      31:                                           //第一个操作数是对象,则返回第二个操作数
      32:                                           //第二个操作数是对象,第一个操作数为false,则返回第一个操作数,否则返回对象.
      33:   alert((undefined&&1)+","+(0&&undefined)+","+(1&&undefined));//undefined,0,undefined
      34:   alert((null&&1)+","+(0&&null)+","+(1&&null));//null,0,null
      35:   
      36:   alert(true&&5);//true
      37:   alert(({}||1)+","+(0||{})+","+(1||{})+","+({}||{}));     
      38:                                               //[object,Object],[object,Object],1,[object,Object]  
      39:   alert((undefined||1)+","+(0||undefined)+","+(1||undefined));//1,undefined,1
      40:   alert((null||1)+","+(0||null)+","+(1||null));//1,null,1     
      41:   //逻辑!(表达式),先求出表达式的boolean值(会Boolean()强转),再取反                                   
      42:  </script>
      43:  </head>
      44:   
      45:  <body>
      46:      
      47:  </body>
      48:  </html>
  • 相关阅读:
    [转]SQL Server中的执行引擎入门
    [转]C# 导入导出Excel通用类(SamWang)
    【转】TSQL查询进阶—理解SQL Server中的锁
    [转载]使用分页方式读取超大文件的性能试验
    【总结】读《WEB三层架构的束缚》后的感想
    【转】jQuery使用 $.post提交json数据
    【原创】关于索引的总结
    【转】.net 匿名函数的变化
    【转】List<T>的Sort,Find,Exists等的使用
    【转】Http中Get/Post请求区别
  • 原文地址：https://www.cnblogs.com/yiqiu2324/p/3528746.html
Copyright © 2020-2023  润新知