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js basic knowledge

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2020-06-08 16:26:522276browse

Basic concepts of js

Local variables and global variables of js

js basic knowledge

js The data type

var is a weak data type, but js can recognize its data type

<head>
  <meta charset="utf-8">
  <title></title>
  <script type="text/javascript">
   function abc(){
    var a=1;
    var b="张三";
    var c=true;
    var d=new Date();
    
    alert("a的数据类型："+typeof(a));
    alert("b的数据类型："+typeof(b));
    alert("c的数据类型："+typeof(c));
    alert("d的数据类型："+typeof(d));
   }
  </script>
 </head>
 <body>
  <input type="button" name="" id="" value="js的数据类型" onclick="abc()"/>
 </body>

About js methods

Writing of methods

<script type="text/javascript">
  function test(){
   console.log("不传参数");
  }
  function test1(a){
   console.log("传1个参数"+a);
  }
  function test2(a,b){
   console.log("传2个参数：" +a+"第二个参数:"+b);
  }
  function abc(a){
   console.log("这是在abc的方法的值："+a);
   return a;
  }
  function test3(a){
   var m=abc(a);
   console.log("调用了别人的返回值的方法"+m)
  }
  
 </script>
 <body>
  <input type="button" name="" id="" value="不传参数的按钮" onclick="test()" /><br />
  <input type="button" name="" id="" value="传1个参数" onclick="test1(12)" /><br />
  <input type="button" name="" id="" value="传2个参数" onclick="test2(1,&#39;张三&#39;)" /><br />
  <input type="button" name="" id="" value="调用了一个有返回值的按钮" onclick="test3(&#39;张三&#39;)" /><br />
 </body>

Method coverage

Unlike Java, there is no duplication of methods in js Load, only method override

as long as the method name is the same. No matter how many parameters there are, js will only recognize the last method (method override) 

<head>
  <meta charset="utf-8">
  <title></title>
  
  <script type="text/javascript">
   function abc(a){
    //var name=&#39;张三&#39;;//在方法体内部的局部变量，只能自己用
    alert(&#39;这是第一个方法&#39;+a);
   }
   
   function abc(){
    alert(&#39;这是第二个方法&#39;);
   }
   
   function abc(){
    alert(&#39;这是真的&#39;);
   }
  </script>
 </head>
 
 <body>
  <!-- js中不会重载，只有方法覆盖啊 -->
  <input type="button"  value="方法重载和多态" onclick="abc(44)"/> 
  <!-- 输出：这是真的 -->
 </body>

js data type conversion

Although js only A var is used to describe a variable (weak data type), but the system can identify its data type and can also perform data type conversion

<script type="text/javascript">
  function test(){
   var x=&#39;12.3&#39;;
   console.log("x的数据类型是："+typeof(x));
   var m=parseInt(x);
   console.log("x转换后的数据类型是："+typeof(m)+"值是："+m);
   
   var y=&#39;12.111&#39;;
   console.log("y的数据类型是："+typeof(x));
   var m1=parseFloat(y);
   console.log("y转换后的数据类型是："+typeof(m1)+"值是："+m1);
   
   var z=&#39;3*4&#39;;
   console.log("z的数据类型是："+typeof(z)+"z的值是："+z);
   var m2=eval(z);
   console.log("z计算后的数据类型是："+typeof(m2)+"值是："+m2);
   
   var l=true;
   console.log("l的数据类型是："+typeof(l)+"l的值是："+l);
   var m3=l.toString();
   console.log("l转换后的数据类型是："+typeof(m3)+"值是："+m3);
  }
 </script>
 <body>
  <input type="button" name="" id="" value="数据类型的转换"  onclick="test()"/>
  </body>

Operation calculations in js

The operation rules of js are the same as Java (but special attention: x= y)

function abc(){
    var a=&#39;10&#39;;
    var b=&#39;8&#39;;
    console.log("b的值 "+b+"  b的数据类型转换成 "+typeof(b)+"  "+a)
    /* =+ a先转换成number 再给a的值复制给b */
    /* += 等价与 b+=a == b=b+a */
   }

js basic knowledge

Select statement and loop statement

Omitted: Same as java

js main object

window object

Time intervaler

<script type="text/javascript">
   function test(){  
    console.log(&#39;test方法开始执行了&#39;);
    /* 参数： 执行的方法， 等待的时间（毫秒单位） */
    window.setTimeout("hello()",1000);
   }
   function hello(){
    console.log(&#39;hello&#39;);
   }
   function test1(){
    console.log(&#39;test1方法开始执行了&#39;);
    window.setInterval("hello()",1000);
   }
  </script>
 </head>
 <body>
  <input type="button" value="等待一定时间,再执行" onclick="test()" /><br />
  <input type="button" name="" id="" value="每间隔一定时间,反复执行" onclick="test1()"/>
 </body>

js basic knowledge

Use of array

<script type="text/javascript">
  function test(){
   /* 第一种声明方式 */
   var a=[1,2,3,4,5,6,7,8,9];
   for (var i = 0; i < a.length; i++) {
    console.log("当前数组的角标："+i+"当前的值："+a[i]);
   }
  }
  function test1(){
   /* 第二种声明方式 */
   var a=new Array();
   a[0]=[1,2,3];
   a[1]=[&#39;张三&#39;,&#39;李四&#39;,&#39;王五&#39;];
   a[2]=[2,5,1,3,6];
   for (var i = 0; i < a.length; i++) {
    for (var j = 0; j < a[i].length; j++) {
     console.log("当前数组的角标："+i+", "+j+"当前位置的值："+a[i][j]);
    }
   }
  }
  function test2(){
   /* join(分隔符) 将数组元素中加分割符号后串接并返回一个字符串 */
   var a=[1,3,2,9,7,8,5];
   console.log(a.join("*"));
   /* reverse() 将数组元素按照原先相反位置存放 */
   console.log("数组的取反："+a.reverse());
   /* slice(始[,终) 返回一个子数组  （前包后不包)*/
   console.log(a.slice(1,4));
   /* sort() 按照字母排序 */
   console.log(a.sort());
  }
 </script>
 <body>
  <input type="button"  value="一维数组的遍历" onclick="test()"/>
  <input type="button"  value="二维数组的遍历" onclick="test1()"/>
  <input type="button"  value="数组的操作" onclick="test2()"/>
 </body>

js basic knowledge

Basic operations on strings

<script type="text/javascript">
  function test(){
   var a="hello world";
   var index_a=a.indexOf("o");//第一个字母的位置
   var index_b=a.indexOf("p");//没有就返回-1
   console.log("o的角标位置："+index_a);
   /* 字符截取（前包后不包） */
   var new_a=a.substring(1,3);
   console.log(new_a);
   /* 根据特定字符，格式化字符串 */
   var ip=&#39;192.168.0.1&#39;;
   var ip_array=ip.split(".");
   for (var i = 0; i < ip_array.length; i++) {
    console.log(ip_array[i]);
   }
   /* 大小写转换 */
   var b=&#39;abc&#39;;
   console.log(b.toUpperCase());
   var c=&#39;ABC&#39;;
   console.log(c.toLowerCase());
  }
 </script>
 <body>
  <input type="button" value="字符串处理" onclick="test()"/>
 </body>

js basic knowledge

Formatting of js time

<script src="../js/dateFormat.js" type="text/javascript" charset="utf-8"></script>
 <script type="text/javascript">
  function test(){
   var date=new Date();
   console.log(date);
   //方法1：引入控件
   var sdate=date.format(&#39;yyyy-MM-dd&#39;);
   var stime=date.format(&#39;yyyy-MM-dd HH:mm:ss&#39;)
   console.log(sdate);
   console.log(stime);
   
   //方法2：
   var y=date.getFullYear();//年
   var mon=date.getMonth()+1;//月
   var d=date.getDate();//日
   var h=date.getHours();//时
   var m=date.getMinutes();//分
   var s=date.getSeconds();//秒
   var weeks=date.getDay();
   var weekday=["星期日","星期一","星期二","星期三","星期四","星期五","星期六"];
   console.log(y+"年 "+mon+"月 "+d+"日  "+h+":"+m+":"+s+"  "+weekday[weeks])
  }
 </script>
 <body>
  <input type="button" value="日期处理" onclick="test()" />
 </body>

js basic knowledge

Recommended tutorial: "JS Tutorial"

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