What are anonymous functions, nested functions, and closures?

Anonymous function: It was also mentioned in the previous article, which refers to a function without a function name

function (){ 
console.log('匿名函数') 
}

What about nested functions? Please look at the code:

function test (){
   var  sum = 20;
   //内嵌函数
   demo = function(){
        alert(sum);
        console.log('我是嵌套函数');
    }
}

As shown in the code, the function is embedded inside the function, which is called a nested function.

What is closure?

Not much to say, look at the code

function demo(){
   var num = 0;
   
   // 返回一个函数 
   return function(){
      alert( num+1 );
   }
} 
//将返回的函数赋值给 add 变量
var add = demo();
// add就是一个闭包
add();

Looking at it this way, it feels like as long as it is a nested function and can access variables in the upper scope, it is a closure. is that so?

We know that in js, it is divided into global scope and local scope. Each function is equivalent to a local scope.

Similarly, variables are also divided into global variables and local variables. what differences are there?

In the browser, the global scope object is window, which means that the window object exists as soon as the page is opened.

In js, each function is a local scope, and local variables will be created with the execution of the function and destroyed after execution.

Global variables will always exist as long as the page is not closed. It will not be destroyed when the function is executed.

So what does it have to do with closure?

In the book "JavaScript Advanced Programming", the concept of "scope chain" is mentioned. The special thing is that global variables can be read directly inside the function.

The variables inside the function cannot be read outside the function.

In other words, the scope chain is like a ladder that can only go up but not down. Let’s look at the code snippet to understand

var  name = "window";
var  age    =  20;
dmeo();
function demo(){
     var age = 21;
     console.log(name);  // window
     console.log(age);     //21
}

When the demo() function is executed, a chain leading to the global scope will be created to save the variables of the current scope for search and return.

When executing the code console.log(name), it will search whether the name variable exists in the current scope (demo function). Since the current scope does not exist, the global variable name is found upwards. Therefore, return to window;

When executing the code console.log(age), it will also search whether the age variable exists in the current scope (demo function). Because it exists, 21 is returned.

Since the mechanism can only read upward, consider a question, how to read the variables of the internal function externally?

There is no way, just make some changes. This requires the use of the concept of closure.

function  f1(){
   var num = 0 ;
   //定义内部函数
   function f2(){
      return  num + 1;
   }
   //返回 f2函数引用
   retufn  f2;
}

// bar 变量也指向 f2 函数，在此也是一个闭包
var bar = f1();

//执行
bar();   // 1;

We know that the variables in the function will be destroyed after the execution of the function is completed. As for the above code, after the f1() function is executed, the f2 function is assigned to a global variable, and the variable of the f2 function depends on the num variable of f1. Therefore, the num variable in f1

will not change accordingly. It is destroyed after the function of f1 is executed.

Here we borrow the title of "Ruan Yifeng" to analyze and deepen our understanding.

Link: Learning javaScript closure

var name = "the window";
  
 var obj = {
      name : 'the obj',
      getName : function(){
          return funciton(){ 
               return this.name;
          }
      }
}

//执行 getName返回的函数
alert(obj.getName()());

Let’s look at the call function analysis and divide it into two parts for execution. Let’s first look at obj.getName(); At this time, the getName function is called by the obj object, so the value of this is obj. But this time it does not output but returns a function.

Add a (); to execute the returned function, but at this time the returned function is not called by any object. When it is not called by the object itself, the value of this will be promoted to the window object. So the output is "the window"

var name = "the Window";
var obj = {
    name : "the obj",
    getName : funciton(){
        var that = this;
        return function(){
             return that.name;
         }
    }
}
//执行 getName 返回的函数
alert(obj.getName()());

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