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Detailed explanation of how JavaScript declares variables and data type instances

伊谢尔伦Original: 2017-07-18 16:43:191347browse

For a programming language, it must include variables and data types. Today we will take a look at the variables and data types of the JavaScript scripting language. Compared with other high-level programming languages such as Java and C++, JavaScript is very simple.

1. Variables
JavaScript variables are loosely typed. The so-called looseness is used to save any type of data. Variables are containers for storing information. When defining a variable, use the var operator (var is the keyword), followed by a variable name (the variable name is the identifier). A variable is a quantity that can be changed again after initialization.
Then let’s take a look at an example:

<span style="font-size:18px;">var x=2; 
var y=3; 
var z=2+3; 
document.write(x + "<br>"); 
document.write(y + "<br>"); 
document.write(z + "<br>");</span>

It’s like algebra: x=2, y=3, z=x+y in algebra , we use letters (such as x) to hold values (such as 2). Through the above expression z=x+y, we can calculate the value of z to be 5. In JavaScript, these letters are called variables. Therefore we can think of variables as containers for storing data.
(1) JavaScript variable name
Like algebra, JavaScript variables can be used to store values (such as x=2) and expressions (such as z=x+y ). Variables can have short names (such as x and y) or more descriptive names (such as age, sum, totalvolume).
It should be noted that:

1 Variables must start with letters
2 Variables can also start with $ and _ symbols (but we do not recommend this)
3 Variable names Case-sensitive (y and Y are different variables)
(2) JavaScript data types
JavaScript variables can also store other data types, such as text values ( name="Bill Gates"). In JavaScript, a piece of text like "Bill Gates" is called a string. There are many types of JavaScript variables, but for now, we'll just focus on numbers and strings. When assigning a text value
to a variable, the value should be surrounded by double or single quotes. When assigning a numeric value to a variable, do not use quotation marks. If you surround a numeric value with quotes, the value is
treated as text. There is a detailed introduction to data types later.
Example:

<span style="font-size:18px;">var pi=3.14; 
var name="Bill Gates"; 
var answer=&#39;Yes I am!&#39;; 
document.write(pi + "<br>"); 
document.write(name + "<br>"); 
document.write(answer + "<br>");</span>

(3) Declare (create) JavaScript variables In JavaScript Creating a variable is often called "declaring" the variable. A good programming habit is to declare the required variables uniformly at the beginning of the code. You can declare variables without using var, but this is not recommended.
We use the var keyword to declare a variable: var carname;
After a variable is declared, the variable is empty (it has no value). To assign a value to a variable, use the equal sign: carname="Volvo";
However, you can also assign a value to a variable when you declare it: var carname="Volvo";
Example: We create a variable named carname variable, assign it the value "Volvo", and then put it into the HTML paragraph with id="demo".

<span style="font-size:18px;"><!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> 
<html xmlns="http://www.w3.org/1999/xhtml"> 
<head> 
<meta http-equiv="Content-Type" content="text/html; charset=gb2312" /> 
<title>JS变量和数据类型</title> 
</head> 
 
<body> 
<p>点击这里来创建变量，并显示结果。</p> 
 
<button onclick="myFunction()">点击这里</button> 
 
<p id="demo"></p> 
 
<script type="text/javascript"> 
function myFunction() 
{ 
var carname="Volvo"; 
document.getElementById("demo").innerHTML=carname; 
} 
</script> 
</body> 
</html></span>

Click effect:

## (4) One statement, multiple Variables

You can declare many variables in one statement. The statement starts with var and uses commas to separate variables:

var name="Gates", age=56, job="CEO";

The statement can also span multiple lines:

<span style="font-size:18px;">var name="Gates", 
age=56, 
job="CEO";</span>

In computer programs, variables with no value are often declared. The value of a variable declared without a value is actually undefined. After executing the following statement
, the value of the variable carname will be undefined: var carname;

(5) Re-declare the JavaScript variable
If you re-declare JavaScript variable, the value of this variable will not be lost: after the execution of the following two statements, the value of the variable carname is still "Volvo":

<span style="font-size:18px;">var carname="Volvo"; 
var carname;</span>

(6) JavaScript arithmetic

You can do arithmetic through JavaScript variables, using operators such as and +: Example:

<span style="font-size:18px;"><!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> 
<html xmlns="http://www.w3.org/1999/xhtml"> 
<head> 
<meta http-equiv="Content-Type" content="text/html; charset=gb2312" /> 
<title>JS变量和数据类型</title> 
</head> 
 
<body> 
<p>假设 y=5，计算 x=y+2，并显示结果。</p> 
<button onclick="myFunction()">点击这里</button> 
 
<p id="demo"></p> 
 
<script type="text/javascript"> 
function myFunction() 
{ 
var y=5; 
var x=y+2; 
var demoP=document.getElementById("demo") 
demoP.innerHTML="x=" + x; 
} 
</script> 
</body> 
</html></span>

Click effect:

二、数据类型
       JavaScript的数据类型包括字符串、数字、布尔、数组、对象、Null、Undefined。在讲数据类型之前我们先讲一个操作符typeof。
       typeof操作符
       typeof操作符是用来检测变量的数据类型。对于值或变量使用typeof操作符会返回如下字符串：

<span style="font-size:18px;">var box=&#39;English&#39;; 
alert(typeof box); 
alert(typeof English);</span>

上述两种方式都是可行的。
typeof操作符可以操作变量，也可以操作字面量。虽然可以这样使用，typeof(box),但，typeof是操作符而非内置函数。函数是对象，不是一种数据类型，所以，使用typeof来区分function和object是非常有必要的。
返回值是函数的例子：

<span style="font-size:18px;">function box(){ 
} 
alert(typeof box);//box是Function函数，值是function box(){}，类型返回的字符串是function。</span>

(1)JavaScript拥有动态类型
JavaScript拥有动态类型。这意味着相同的变量可用作不同的类型：
实例：

<span style="font-size:18px;">var x //x为undefined 
var x = 6; //x为数字 
var x = "Bill"; //x为字符串</span>

(2)JavaScript字符串String类型
字符串是存储字符的变量。字符串可以是引号中的任意文本。您可以使用单引号或双引号：;
实例:可以在字符串中使用引号，只要不匹配包围字符串的引号即可

<span style="font-size:18px;">var carname1="Bill Gates"; 
var carname2=&#39;Bill Gates&#39;; 
var answer1="Nice to meet you!"; 
var answer2="He is called &#39;Bill&#39;"; 
var answer3=&#39;He is called "Bill"&#39;; 
document.write(carname1 + "<br>") 
document.write(carname2 + "<br>") 
document.write(answer1 + "<br>") 
document.write(answer2 + "<br>") 
document.write(answer3 + "<br>")</span>

(3)JavaScript数字

JavaScript只有一种数字类型。数字可以带小数点，也可以不带。Number类型包含两种数值：整型和浮点型。输出的格式均按照十进制数输出。最基本的数值字面量是十进制。也包括八进制数值字面量，前导必须是0，八进制序列(0到7，以8为基数)；十六进制字面量前面两位必须是0x，后面的是(0到9及A到F)；浮点类型，就是该数值中必须包含一个小数点，并且小数点后面必须至少有一位数字。
1对于那些过大或过小的数值，我们可以采用科学计数法(e表示法)，用e表示该数值的前面10的指数次幂。例如：

<span style="font-size:18px;"><span style="font-size:18px;">var box=4.12e-9;</span></span>

2要想确定一个数值到底是否超过了规定范围，可以使用isFinite()函数，如果没有超过，返回true，超过了返回false。
3isNaN()函数用来判断这个值到底是不是NaN。isNaN()函数在接收到一个值后，会尝试将这个值转换为数值。
isNaN()函数也适用于对象。在调用isNaN()函数过程中，首先会调用value()方法，然后确定返回值是否能够转换为数值。如果不能，则基于这个返回值再调用toString()方法，再测试返回值。
实例：

<span style="font-size:18px;">var x1=36.00; 
var x2=36; 
var y=123e5; 
var z=123e-5; 
document.write(x1 + "<br />") 
document.write(x2 + "<br />") 
document.write(y + "<br />") 
document.write(z + "<br />")</span> 
 (4)JavaScript布尔
 布尔(逻辑)只能有两个值:true或false。例如：
var x=true； 
var y=false；

(4)JavaScript数组
数组下标是基于零的，所以第一个项目是[0]，第二个是[1]，以此类推。下面的代码创建名为cars的数组：

<span style="font-size:18px;">var cars=new Array(); 
cars[0]="Audi"; 
cars[1]="BMW"; 
cars[2]="Volvo";</span> 
 或者：
<span style="font-size:18px;">var cars=new Array("Audi","BMW","Volvo"); </span>

实例

<span style="font-size:18px;">var i; 
var cars = new Array(); 
cars[0] = "Audi"; 
cars[1] = "BMW"; 
cars[2] = "Volvo"; 
for (i=0;i<cars.length;i++) 
{ 
document.write(cars[i] + "<br>"); 
}</span>

输出的结果很容易知道。
(5)JavaScript对象
对象由花括号分隔。在括号内部，对象的属性以名称和值对的形式 (name : value) 来定义。属性由逗号分隔：
var person={firstname:"Bill", lastname:"Gates", id:5566};
上面例子中的对象(person)有三个属性：firstname，lastname以及id。空格和折行无关紧要。声明可横跨多行：

var person={ 
firstname : "Bill", 
lastname : "Gates", 
id: 5566 
};

对象属性有两种寻址方式：
实例

var person={ 
firstname : "Bill", 
lastname : "Gates", 
id: 5566 
}; 
document.write(person.lastname + "
"); 
document.write(person["lastname"] + "
");

(6)Undefined和Null
Undefined这个值表示变量不含有值。可以通过将变量的值设置为null来清空变量。
Undefined类型

var box; 
alert(typeof box);//box是Undefined类型，值是undefined，类型返回的字符串是undefined。

Null类型

var box=null; 
alert(typeof box);//box是Null类型，值是null，类型返回的字符串是object。

(7)声明变量类型
JavaScript变量均为对象。当您声明一个变量时，就创建了一个新的对象。当声明新变量时，可以使用关键词"new"来声明其类型：

var carname=new String; 
var x= new Number; 
var y= new Boolean; 
var cars= new Array; 
var person= new Object;

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