Detailed explanation of js variables, scope and memory_Basic knowledge

Basic type values ​​include: undefined, NUll, Boolean, Number and String. These types occupy a fixed size space in the memory. Their values ​​are stored in the stack space and we access them by value.

(1) Value type: numeric value, Boolean value, null, undefined.
(2) Reference type: object, array, function.

If the value assigned is a reference type, space must be allocated for this value in heap memory. Since the size of such values ​​is not fixed (objects have many properties and methods), they cannot be saved to stack memory. But the memory address size is fixed, so the memory address can be saved in stack memory.

<script type="text/javascript”>
var box = new Object(); //创建一个引用类型
var box = "lee";  //基本类型值是字符串
box.age = 23;  //基本类型值添加属性很怪异，因为只有对象才可以添加属性。
alert(box.age); //不是引用类型，无法输出；
</script>

In short, the heap memory stores reference values, and the stack memory stores fixed type values.

<script type="text/javascript">
  var man = new Object();//man指向了栈内存的空间地址
  man.name = "Jack";
  var man2 = man;//man2获得了man的指向地址

  alert(man2.name);//两个都弹出Jack
  alert(man.name);
</script>

Copy variable value

Look at the following example:

<script type="text/javascript">
  var man = new Object();//man指向了栈内存的空间地址
  man.name = "Jack";
  var man2 = man;//man2获得了man的指向地址

  man2.name = "ming";//因为他们都指向同一个object，同一个name，不管修改谁，大家都修改了
  alert(man2.name);//两个都弹出ming
  alert(man.name);
</script>

It can be concluded from the above: In terms of variable copying, basic types and reference types are also different. Basic types copy the value itself, while reference types copy the address.

Pass parameters

In ECMAScript, all function parameters are passed by value,

<script type="text/javascript">
   function box(num){   //按值传递
     num+=10;
     return num;
   }

   var num = 10;
   var result = box(num);
   alert(result); //如果是按引用传递，那么函数里的num会成为类似全局变量，把外面的number替换掉
   alert(num);  //也就是说，最后应该输出20（这里输出10）
</script>

Javascript does not pass by reference. If there is pass by reference, then the variables within the function will be global variables and can also be accessed externally. But this is obviously impossible.

Execution environment and scope

Execution environment is one of the most important concepts in JavaScript. The execution environment defines the permissions of variables or functions to access other data.

The global execution environment is the most peripheral execution environment. In a web browser, the global execution environment is the window object. Therefore, all functions of global variables are created as properties and methods of window.

<script type="text/javascript">
   var name = "Jack";      //定义全局变量
   function setName(){
     return "trigkit4";
   }

   alert(window.name);    //全局变量，最外围，属于window属性
   alert(window.setName()); //全局函数，最外围，属于window方法
</script>

When the code in the execution environment is executed, the environment is destroyed, and the variables and functions saved in it are also destroyed. If it is a global environment, it will not be destroyed until all programs have been executed or the web page has been completed.

Remove the local variable of var

<script type="text/javascript">
   var name = "Jack";
   function setName(){
     name = "trigkit4";  //去掉var变成了全局变量
   }

   setName();
   alert(name);//弹出trigkit4
</script>

By passing parameters, it is also a local variable

<script type="text/javascript">
   var name = "Jack";
   function setName(name){  //通过传参，也是局部变量
     alert(name);
   }

   setName("&#65532;trigkit4");//弹出trigkit4
   alert(name);//弹出Jack
</script>

The function body also contains a function. Only this function can access the inner layer of functions

<script type="text/javascript">
   var name = "Jack";
   function setName(){
     function setYear(){  //setYear()方法的作用域在setName()内
       return 21;
     }
   }
   alert(setYear());//无法访问，出错 
</script>

Can be accessed via the following methods:

<script type="text/javascript">
   var name = "Jack";
   function setName(){
     function setYear(){  //setYear()方法的作用域在setName()内
       return 21;
     }
     return setYear();
   }
   alert(setName()); //弹出21
</script>

Another scope example:

<script type="text/javascript">
   var name = "Jack";
   function setName(){
     function setYear(){  //setYear()方法的作用域在setName()内
       var b = "hi";   //变量b的作用域在setYear()内
       return 21;
     }
     alert(b);//无法访问 
   }
</script>

When code is executed in an environment, something called a scope chain will be formed. Its purpose is to ensure orderly access to variables and functions that have access rights in the execution environment (referring to the rule hierarchy). to access), the front end of the scope chain is the variable object of the execution environment.

Scope

A variable that is not declared within a function or declared without var is a global variable. It has a global scope. All properties of the window object have a global scope; it can be accessed anywhere in the code. It is declared inside the function and decorated with var. The variables are local variables and can only be used within the function body. Although the parameters of the function do not use var, they are still local variables.

No block scope

No block scope

// if语句：

<script type="text/javascript">
if(true){            //if语句的花括号没有作用域的功能。

var box = "trigkit4";
}
alert(box);//弹出 trigkit4
</script>

The same goes for for loop statements.

Query of variables

In variable queries, accessing local variables is faster than global variables, so there is no need to search up the scope chain.
Example below:

<script type="text/javascript">
   var name = "Jack";
   function setName(){
      var name = "trigkit4";
      return name; //从底层向上搜索变量
  }
  alert(setName());   
</script>

Memory problem

Javascript has an automatic garbage collection mechanism. Once the data is no longer used, it can be set to "null" to release the reference

Circular Reference

A very simple example: a DOM object is referenced by a Javascript object, and at the same time it references the same or other Javascript object. This DOM object may cause a memory leak. The reference to this DOM object will not be reclaimed by the garbage collector when the script stops. To break a reference cycle, the object referencing the DOM element or a reference to the DOM object needs to be assigned null.

Closure

When a variable outside the closure is introduced in the closure, the object cannot be garbage collected (GC) when the closure ends.

var a = function() {
 var largeStr = new Array(1000000).join('x');
 return function() {
  return largeStr;
 }
}();

DOM leak

When the original COM is removed, the child node reference cannot be recycled unless it is removed.

var select = document.querySelector;
var treeRef = select('#tree');

//在COM树中leafRef是treeFre的一个子结点
var leafRef = select('#leaf'); 
var body = select('body');

body.removeChild(treeRef);

//#tree不能被回收入，因为treeRef还在
//解决方法:
treeRef = null;

//tree还不能被回收，因为叶子结果leafRef还在
leafRef = null;

//现在#tree可以被释放了。

Timers leaked

Timers are also a common place where memory leaks occur:

for (var i = 0; i < 90000; i++) {
 var buggyObject = {
  callAgain: function() {
   var ref = this;
   var val = setTimeout(function() {
    ref.callAgain();
   }, 90000);
  }
 }

 buggyObject.callAgain();
 //虽然你想回收但是timer还在
 buggyObject = null;
}

Debug Memory

Chrome’s built-in memory debugging tool can easily check memory usage and memory leaks:
Click record in Timeline -> Memory: