How to understand types, parameter passing and execution environment in JavaScript

The basic type values ​​are: 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: numerical value, Boolean value, null, undefined.
(2) Reference type: object, array, function.

If the value assigned is a reference type value, 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 references Types are also different. Primitive types copy the value itself, while reference types copy the address.

Pass parameters

In ECMAScript, the parameters of all functions 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 is not passed by reference. If it exists If passed by reference, the variables within the function will be global variables and can also be accessed externally. But this is obviously impossible.

Execution environment

The 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, all programs need to It will be destroyed after the execution is completed or the web page is 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("trigkit4");//弹出trigkit4
   alert(name);//弹出Jack
</script>

The function body also contains a function, and only this function can be accessed The functions of the inner layer

<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 through 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>

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