Closures in JavaScript

All variables inside the function body can be stored in the function scope. ----- Closures (function variables can be hidden inside the scope chain, so it looks like the function "wraps" the variable around)

When a function nests another function, the outer function returns the nested object as a return value. Often times closed. Understanding closures starts with understanding the lexical scoping rules for nested functions.

var scope = 'global scope';
 function checkscope(){
     var scope = 'local scope';
     function f(){return scope};
     return f;
 }
 checkscope()()
//"local scope"

Feature: Local variables (and parameters) can be captured and persisted as if they were bound to the external function in which they were defined.

var unique = (function(){
     var conter = 0;
     return function(){return conter++}
 }());
 unique()

Use closures to implement private property accessor methods (a common practice for private state sharing):

 function addPrivateProperty(o,name,predicate){
     var value;
     o[`get${name}`] = function(){return value;};
     o[`set${name}`] = function(v){
         if(predicate && !predicate(v))
             throw Error(`set ${name}:invalid value ${v}`)
         else 
             value = v;
     }
 }
 var o = {};
 addPrivateProperty(o,'Name',function(x){return typeof x == "string"});
 o.setName("Frank");
 console.log(o.getName());

Defining two closures in the same scope chain that share the same private variables or variables is a very important technique.
Important: The parameter array of the outer function cannot be directly accessed from within the closure, unless the outer function stores the parameter array in another variable:

var outerArguments = arguments ；

var that = this;