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Detailed explanation of javaScript basics

高洛峰Original: 2017-01-21 09:12:531328browse

First let’s talk about the placement of JavaScript: <script> and </script> can be placed between the head and the body, or in the body or in the head.

The six major functions of JavaScript:

1 Directly output in script

document.write("<h1>This is a heading</h1>");//document.write表输出，括号中h1标签同样有效

2React to events

<button type="button" onclick="alert(&#39;Welcome!&#39;)"> //点击这里</button>//onclick代表当点击是触发事件，alert会弹框

3Change HTML content

x=document.getElementById("demo") //查找元素 ，通过id得到某一个元素，就像我之前做的笔记说id只能用一次所以是唯一的。
x.innerHTML="Hello JavaScript"; //改变内容，通过触发事件会是这个元素的值发生改变

4Change HTML image

element=document.getElementById(&#39;myimage&#39;) //得到img这个元素
element.src="/i/eg_bulboff.gif";   //改变Src属性，也就相当于给它换了张图片

5 Change HTML style

x=document.getElementById("demo") //同样通过id找到元素
x.style.color="#ff0000";   //改变样式

6 Validate input

If( isNaN(x)) {alert("Not Numeric")}; // isNaN( ) 这是一个方法，专门用来判断括号中是否全为数字，是为true

Three commonly used methods for javaScript to find elements (excluding jquery to find elements)

1: Search by id;

x=document.getElementById("demo") //通过id查询是唯一的，x也是代表唯一一个元素。

2: Search by name:

X= document.getElementsByName("myInput");//这里是ments说明他返回的是一个集合，因为name属性可以重复定义
X[0].innerHTML="Hello JavaScript";//因为是数组，所以要指定第几个，就算只有一个也要写零

3: Search by element name:

var x=document.getElementsByTagName("input"); //同样也是返回一个数组
alert(x.length); //这里有几个input标签就输出几
JavaScript的注释和java是一样的有//和/* */

JavaScript variable

javaScript data has a string , number, Boolean, array, object, Null, Undefined, but use var

var cars=new Array(); cars[0]="Audi"; //数组表达式
var cars=["Audi","BMW","Volvo"]; //同样也是数组表达式
person=new Object(); //对象表达式

JavaScript function

function getElements(){  } //可以带参数那就往里面传参（var1,var2），如果要返回值return，函数类型不变

JavaScript operator, arithmetic operation operator, assignment operator, comparison operator, logical operator, conditional operator, IF statement, Switch statement, For loop, while do-while, Break and Continue are the same as java rules (the only difference is for (x in person) )

Next we will do a few small cases:

1: With account name verification, check whether the two passwords are consistent

<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
<!DOCTYPE HTML >
<html>
 <head>
 <title>雨点的名字</title>
 </head>
 <script>
 function checkForm(){
 var form=document.forms["form1"]; //根据表单的名字 获取指定的表单，这算第四种获得元素方法
 //根据表单获得username的value值，因为下面这个username只有一个那就默认就取这一个，不用form.username[0].value这样反而报错，因为不是集合
 var username =form.username.value;
 if(username.length<3 || username.length>12){
  alert("输入的用户名不合法..."); //.length是属性所以不能括号，这和String不同
  return false;
 }
 var password1 =form.password[0].value;//因为password下面有两个所以是一个集合所以要[0]；
 var password2=document.getElementsByName("password")[1].value;//这是通过另一种方法获得
  if(password1!=password2){
  
 alert("两次密码不一致..");
 return false();
 }
 return true;
 }
 </script>
<body>
 <!--onsubmit只在form中使用，如果返回为false则不提交数据，否则提交数据-->
 <form action="" name="form1" onsubmit="checkForm()">
 用 户 名:<input type="text" name="username"/><br><br>
 密 码:<input type="password" name="password"/><br><br>
 确认密码:<input type="password" name="password"/><br><br>
 <input type="submit" value="提交"/> <!-- 这里定义了为submit类型，当点提交时，自动触发onsubmit方法 -->
 </form>
</body>
</html>

The running results are as follows:

Detailed explanation of javaScript basics

2: Implement a small calculator for addition

<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
<!DOCTYPE HTML >
<html>
 <head>
 <title>雨点的名字</title>
 <style type="text/css">
 input{
 width:50px;
 }
 </style>
 </head>
 <script>
 function calc(){
  var form1 =document.forms["form1"];
  var x=form1["x"].value; //获取form1表单中name属性为x的值,又一种方法（其实[]和.是有点不同的这里不细讲）
  var y=form1["y"].value;
  var z=parseInt(x)+parseInt(y); //parseInt(x)类型转换，把String转成int
  var result=form1["result"]; //得到元素
  result.value=z; //value这里插入value值，不是用innerHTML，他们的使用类型不同
 }
 </script>
 
<body>
 <form action=" " name="form1" method="post">
 <input type="text"name="x"/>+<input type="text" name="y"/>=<input type="text" name="result"/><br/>
 <input type="button" value="计算" onclick="calc()" /> <!-- onclick监听事件改编属性 -->
 </form>
</body>

The running results are as follows:

Detailed explanation of javaScript basics

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