Detailed explanation of comparison examples between simple data types and complex data types in JavaScript

Data types in JavaScript

Simple (basic) data types: Number, String, Boolean, Undefined, Null

Complex (reference) data types: Object , Array, Date, function, etc.

The following is an introduction to the difference between simple (basic) data types and complex (reference) data types:

Simple data types: store values ​​directly in the stack            

Complex data types: storing references on the stack

After understanding the storage methods of the above two data types, you can distinguish the difference between the two. You can do the following exercises:

var a =10;
  var b = a;
  //问：当改变a的值后，b的值是否发生改变
  a=20;
  console.log(b);  // 10

var s1 = new Object();
var s2 = s1;
//问：改变s1的属性后，s2相同的属性是否改变
s1.name = "mh";
console.log(s2.name);    //mh

function f2(arr)
     {
       arr = [9,8,7,6,5];//产生新的对象
       arr[0]=-100;
     }
     var array = [1,2,4,7,5];
     f2(array);
     console.log(array[0]);//  1

Variable promotion, function declaration and variable scope in JavaSript

First look at the following interview question:

var num = 10;
    fun();
    function fun()
    {
      console.log(num);
      var num =20;
    }

Before learning, I would have directly answered 10, but now I know the concept of precompilation in JavaScript. The promotion concept of var keyword and the declaration concept of function know that the above code is equivalent to the following code:

var num;//全局作用域  遇到var和function 提升
     function fun() {
       var num ;  //局部做用户 遇到var 提升
       console.log(num);
       num =20;
     }
    num = 10;
    fun();

Let’s look at the following question again:

//问题：为什么会出现下面的错误？
      //Uncaught TypeError:fnName is not a function

      console.log(fnName(1,2));
      var fnName = function (a,b) {
        return a + b;
      }

The above code is equivalent to the following code:

var fnName;
      console.log(fnName(1,2));
      fnName = function (a,b) {
        return a + b;
      }

You can see that because function is on the right side of the equal sign, only var will be promoted. As a result, the "Uncaught TypeError: fnName is not a function" error will be reported.

To understand the scope of variables, you can look at the following code:

f1();
        console.log(c);
        console.log(b);
        console.log(a);
        function f1() {
          var a = b = c = 20;
          console.log(c);
          console.log(b);
          console.log(a);
        }

For continuous assignments such as var a=b=c=20;, they work locally Only a will be declared again within the scope, while the scopes of b and c are global scopes. Therefore, only a in the global scope reports the error "Uncaught ReferenceError: a is not defined".

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