The concept, function, problem and solution of closure

Closure (closure)

** Concept ：** Functions that have access to variables in the scope of another function
** effect ：** Extend the scope of the variable
Special point ：
There are two kinds of variables in the code ： Global and local variables , Global variables can be provided to the functions used globally , Local variables can only be supplied to the current function , And only when the function is called , Destroy immediately after the function is executed . Closure is a special function , It can access local variables in another function
Explanation examples ：
function fn(){
var num = 10;
function fun(){
num++
console.log(num)
}
fun()
}
fn()
The function is called fn(), The result will be through the function fun() Get the return value 10;
reflection ：fun() There is no variable defined in the function num,num The definition of is written in its upper function fn() Inside , It can be understood as fn() Local variables of , however fun() Function is called , This is irregular in terms of the characteristics of local variables
Explain and analyze ：
js The chain index rule of the interpreter States , When an internal undefined variable is used in a function , Will send letters to the upper level in turn Number to index values , The destruction time of the local variables of the upper function is after the execution of the function , So when its embedded function fun() Execution time , The local variables of its upper function have not been destroyed , In this way, the principle of closure can be explained . also , In such closures ,fun() The variable num The modification of the value will affect the variables in the upper function num Value
Another way of writing ：
function fn(){
var num = 10;
return function fun(){
console.log(num)
}
}
var f = fn()
f()
var f2 = fn()
The problem is ：
The above said , Because the upper function fn() Not completed , So local variables are not destroyed , The embedded function in it fn() Can continue to use through the chain index fn() Local variables of , And here when f = fn(),fn() It is obvious that All right, it's over , Logically speaking num It has been destroyed , But the fact is that we get the inner function body fun() The variable of f You can still use variables num
Explain and analyze ：
When executed f=fn() when , because fun() Its upper function is used in fn() Local variable in , This leads to fn() It's not done , But be fun() Get stuck , So the variables in it num Not destroyed , therefore f() Can be called many times ,num But will not be destroyed , You can even record cumulative changes to values ( Only in fn() Internal records , Does not affect fn(), This is a difference from the previous writing , The reason is var f = fn() when , take fun() The function body is stored in a new space ), For example, add up ; The problem is ： because fn() stuck , Cause it to occupy memory all the time .
The whole can be understood as ：
num about fn() And the code as a whole , It's a local variable ( Cannot be called by an external function ), But for f() for , It's from fn() inherited , Can cumulatively record changes , Local variables that will not be destroyed , This value is only in f() For internal use , The update of the value will not affect the upper function fn() The corresponding value , So no matter f() How did the value change ,f2() The initial value inside is still from fn() It's got 10