JS scope and scope chain

Scope

Local scope

Generally, it can be accessed only in fixed code fragments

function add(){
 var name = " test "
 console.log(name)
}
add()
// console.log(name) name is not defined


Global scope

var name = " external "
function add(){
 var name = " test "
 console.log(name)
}
add()
 console.log(name) 


Scope chain

var name = "Out"
function out(){
 var inOut = "In";
 function inTao(){
 console.log("inTao---------------"+inOut) 
 //undefined ,  Why would it be undefinded Well , 
 // Because it will first find out whether there is a inOut, If there is , also inOut  State in advance , Then the value will be displayed correctly ; Otherwise, it will show undefinded, Because it only knows that there is one in the current environment inOut
 var ss = name;
 var inOut = ss;
 console.log(" After the assignment ss-----------"+ss) //Out
 console.log("inTao The modified inOut-----------"+inOut) // Out
 }
 inTao()
}
console.log(" External name---------------"+name) // Out
out()


• Summary of relevant knowledge of scope chain ：

 1. The execution environment determines the life cycle of the variable , And which part of the code can access the variables

 2, The execution environment has a global execution environment （ Global environment ） And local execution environment .

 3. Enter a new execution environment every time , Will create a scope chain for searching variables and functions

 4. The local environment of a function can access variables and functions in the function scope , You can also access its parent environment , And even variables and environments in the global environment .

 5. The global environment can only access variables and functions defined in the global environment , No direct access to any data in the local environment .

 6. The execution environment of variables helps determine that memory should be properly Freed .

The scope chain always follows   Look from the inside out layer by layer

Own variable object -----> Variable external () Variable object of local environment  -----> Variable object of global environment .