What are variables in js and what types are they

In js, variables are containers for storing information; there are two types of variables in JavaScript: local variables and global variables.

#How to declare variables in js?

In js, you can use the keyword "var" or "let" and add the "variable name" to declare a variable. [Recommended related video tutorials: JavaScript Tutorial]

The names of js variables, also called identifiers, need to follow some standards:

1. The name must start with letters ( a to z or A to Z), begin with an underscore (_) or dollar ($) sign, but it is not recommended to use an underscore (_) or dollar ($) sign at the beginning.

2. After the first letter, you can also use numbers (0 to 9), such as value1.

3. JS variables are case-sensitive. For example: x and X are different variables.

Let’s take a look at the declaration of js variables through examples:

Correct variable declaration:

var  x  =  10 ;  
var  _value = "sonoo" ;

Incorrect variable declaration:

var   123 = 30 ;  
var * aa = 320 ;

JavaScript local variables

Variables declared within a block or function are called local variables; local variables can only be accessed within the declared block or function. The declared block or function has no effect and cannot be accessed. Example:

<div class="demo">
    <p id="p1"></p>
    <p id="p2"></p>
</div>
<script type="text/javascript">
	function abc(){  
		var x= 10 ; //局部变量  
		var y= 10 ; //局部变量  
		document.getElementById("p1").innerHTML=x;
	}  
	abc();
	document.getElementById("p2").innerHTML=y;
</script>

Rendering:

This is because the variable x and variable y are both local variables; the variable x is in the function abc( ) is called and output internally, but variable y cannot be called and output outside function abc(), and an error will be reported:

JavaScript global variable

Variables declared outside a block or function, or declared using a window object, are called global variables. Global variables can be accessed by any function (or block) throughout the code. Example:

<div class="demo">
    <p id="p1"></p>
    <p id="p2"></p>
    <p id="p3"></p>
    <p id="p4"></p>
</div>
<script type="text/javascript">
	var x= "x=10" ; //局部变量  
	function abc(){  
		var y= "y=10" ; //局部变量  
		document.getElementById("p1").innerHTML=x;
		document.getElementById("p2").innerHTML=y;
	}  
	abc();
	document.getElementById("p3").innerHTML=x;
	document.getElementById("p4").innerHTML=y;
</script>

Rendering:

It can be seen that the local variable y cannot be accessed and output outside the function abc(), and an error will be reported:

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