javascript closure

Concept:

A closure is a function that can read the internal variables of other functions.

Create a closure method:

Create another function inside a function, and access the local variables of this function through another function.

function box(){
  var user ='Zhu';
  return function(){
    return user;
  }
}
var b = box();
console.log(b())
console.log(box()())

Advantages of using closures:
You can store local variables in memory to avoid global variable pollution.
Accumulate local variables through closures:

function func(){
  var num=100;
  incr=function(){// 未使用关键字var ， 该函数为全局
     num+=1;
    return num;
  }
  var m1= function(){
    console.log(num)
  }
  return m1;
}
var m1 = func();
m1();//100
console.log(incr())//101。。通过全局函数操作局部变量
m1();//101

Disadvantages:
Since the local variable resources returned by the scope in the closure will not be destroyed and recycled immediately, excessive use of closures can easily lead to performance degradation.