Text
The concept of closure:
A closure is an expression (usually a function) that has many variables and an environment bound to these variables, so these variables are also the expression part of.
The most common closures
function a() {
var i=0;
return function(){
alert(i );
}
}
var b=a( );
for(var i=0;i<3;i ){
b();
}
Before explaining the above code, first accept a The creation and execution process of the function
The first step: define the function, set the environment, and create the scope chain. Now a is a global variable, then the scope chain of a only has window
Step 2: Execute a, first create the scope (a.scope=a), then create the active object (callObject), and put the callObject at the top of the scope chain of a, so the scope chain of a contains two objects (a and window)
Step 3: Add an arguments attribute on the active object to save the parameter value when calling a
Step 4: Assign formal parameters and internal variables Go to active object a
javascriptGC principle: If an object is no longer referenced, then the object will be recycled by GC. If two objects reference each other without interference, then both objects will also be recycled.
Summary:
1. When defining a first, the scope chain of a is created
2. (var b=a()) executes a When creating the scope a.scope=a, create a callObject object and add it to the scope of a
3. Add the arguments attribute to the a object, and assign the i and return functions to the active object
4. When a is executed, b points to the ruturn function value of a, and b refers to the local variable i in a. Therefore, it does not meet the GC recycling standards. The active object a is not recycled, so b accesses i. is the object accessed for the first time and can only be accessed in b
![[Web front-end] Node.js quick start](https://img.php.cn/upload/course/000/000/067/662b5d34ba7c0227.png)
