39.js-- scope

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        1. Definition and rules of scope

        2. Scope classification

        （1） Global scope

        （2） Function scope （ Local scope ）

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1. Definition and rules of scope

        var                  keyword

        var fn=;          fn It's an identifier

Scope : identifier ( Variable and function names ) Where can I be visited   Those places are the scope of this identifier

These places refer to   Inside the code block of a function and outside the code block of a function

       （1） The rules 1: The code inside the function can access the identifier inside the function , You can also access the identifier outside the function ( side effect )

       （2） The rules 2: The identifier outside the function can be accessed outside the function , But you cannot access the identifier inside the function

          Generally, the internal identifier of a function is a local identifier

2. Scope classification

Scope is divided into global scope and function scope ( Also called local scope ) Two kinds of

（1） Global scope

         All global variables and functions are used as window Object's properties and methods create . Global variables have global scope , stay javascript Everywhere in the code is defined . The global scope will not be destroyed until the application exits, such as closing the web page or browser .

– JS The global variables declared in are the properties of the global object ;

– Function body Don't use var Variables declared and assigned directly are treated as global variables ;

（2） Function scope （ Local scope ）

Within the function (var Statement ) The variable of is only defined in the function body . They are local variables , The scope is local . Function arguments are also local variables , They are only defined in the function body . After all the code in the function scope is executed , The scope is destroyed , All the variables and function definitions stored in it are destroyed .

In the body of the function , Local variables take precedence over global variables with the same name , If A local variable declared in a function or a variable with the same name as a global variable in a function parameter , Then the global variable is covered by the local variable .

Case study 1：

        function fn () {	   	
		    a=100
		}

		var a=200;
		fn()
		console.log(a)

Running results ：

  Case study 1 notes ： Function body does not use var Variables declared and assigned directly are treated as global variables

Case study 2：

        function fn (a) {
		    // var a=300
			a=100
		}

		var a=200;
		fn(300)
		console.log(a)

Running results ：

 

Case study 3：fn(),fm()

         function fn (a) {
		                   // Implicitly declare ：var a=200
				  function fm (a) {
		
		 console.log(a,122)
				  	 a=90
		 console.log(a,123)
				  }
		 fm(a)             //fm(200)
		 console.log(a,124)
		}
		var a=200;
		fn(a)             // Implicitly declare ：fn(200)
		console.log(a,125)

Running results ：

 

Case study 4： increment (){  }

        var total=0;
		function  increment () {
		 var total=0
			  total=total+2
		}
		function  decrease() {
			  total=total-2
		}
		increment()
		increment()
		decrease()	  
		console.log(total)

Running results ：

 

Case study 5： increment (){  }

      	var total = 0;
		function increment() {
			// Implicitly declare  var total = 0
			total = total + 2
			console.log(total)
		}
		increment()    // Implicitly declare   {var total=0;  total = total + 2}
		increment()    // Implicitly declare   {var total=0;  total = total + 2}

Running results ：

 

  Case study 6：fn (){  }

      	var a = 10
		function fn() {
			console.log(a)
			a = 40
			var a = 20
			console.log(a)
		}
		console.log(a)
		fn()
		console.log(a)

Running results ：