43.js -- scope chain

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List of articles

• Preface
• One 、pandas What is it? ？
• Two 、 Use steps
  • 1. Import and stock in
  • 2. Read in the data
• summary

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1. Scope chain concept

         stay JavaScript in , Functions are also objects , actually ,JavaScript It's all about objects . Function objects are just like other objects , Has properties that can be accessed by code and a series of properties JavaScript Internal properties accessed by the engine .

   [[scope]] A collection of runtime context objects stored in , This set is linked in a chain , We call this chain connection Scope chain of function , It determines which data can be accessed by functions .

The purpose of the scope chain is Ensure orderly access to all variables and functions that the execution environment has access to .

2. Execution context

         When the function executes , An internal object called the execution context is created . An execution time context defines the environment in which a function executes , Each time a function executes, the corresponding execution context is unique , So calling a function multiple times can result in multiple execution contexts being created , When the function is finished executing , The execution context it generates is " The destruction ".

<pre> analysis 1： Execution context refers to the context generated immediately before the function is executed AO object </pre>
<pre> analysis 2： The function execution environment refers to those obtained by the variable promotion function AO Object properties </pre>
<pre> analysis 3：
function test() {    }

Function called many times , Produce different AO object ：
test();            ---->AO{}
test();            ---->AO{}
After the function is executed, the corresponding AO Destruction of objects .
</pre>
[[scope]] It refers to what we call the scope , A collection in which the runtime context is stored .

js Object has two kinds of members

            One is the above member (js Members whose syntax can be accessed directly )

            One is the following members ( Members accessed by the underlying syntax )  

            [[scopes]] Enclosed member names Is the following member          

          function fn () {          

            }
            console.dir(fn)

              This " object " What is saved internally is the scope of the function

              The function is defining / When making a statement And then there is [[scopes]] The upper layer is preserved inside AO object              

              Function call will generate AO object AO Save in scopes Inside the object

for example ：

            function  fn (b) {
			 	var a=20
				function fm () {
					
				}
			 }			 
			 fn(100)
			 fn(200)

When the function is generated, there will be an attribute [[scopes]] Scope " Array "( Engine only )

Generated when the function is called AO object   Will be able to AO The object is placed in scopes

Each call will be placed in scopes front ( Top )    

Each function scopes There is one in the array AO object It is the upper layer of this function AO

Case study 1：

           function fn() {
				var a = 30
				function fm() {
					var b = 20
					console.log(a)
				}
				fm()
				fm()
			}
			fn()
			fn()

Running results ：

 

Case study 2：

         function fn(a) {				
				function fm() {
					var b = 20
					console.log(a)
				}
				fm()
				fm()
			}
			fn(100)
			fn(200)

Running results ：

 

Program ：

            function a() {
				var aaa = 123;
				function b() {
					var bbb = 234;
					console.log(aaa);
				}
				return b;
			}

			var glob = 100;
			var demo = a();
			demo();

Running results ：

  Execution process ：

        Go:{a function ,glob:100,demo:a.[[scopes In the object ,b function ]]}

            a.[[scopes]]==>[AO:{aaa:123,b function ,},Go]

            b function .[[scopes]]==>[,a.[[scopes]]]<====>[AO(b):{bbb:234, Print aaa},AO(a):{aaa:123,b function ,},Go]