JavaScript in-depth understanding of the values of basic types and reference types

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A variable can hold two types of values, the values of the base type (primitive values) and the value of the reference type (reference values).

ES6 introduces a new primitive data type, Symbol, that represents a unique value. It is the seventh data type of the JavaScript language, the first six kinds are: Undefined, Null, Boolean (Boolean), String (string), number, (object).

Basic Type

JavaScript has 6 basic data types:,,,,, Undefined Null Boolean Number String Symbol (new in ES 6) !

conventions: 基本数据类型 and 原始数据类型 other meanings.

The value of the base data type is accessed by value.

• The value of the base type is immutable

  var str = "123hello321";str.toUpperCase();     // 123HELLO321console.log(str); // 123hello321

• Comparisons of

• Base types are comparisons of their values

   var a = 1;< Span class= "Hljs-keyword" >var B = true; Console.log (A = = B); //trueconsole.log (a = = B); //false          

  The data types above A and B are different, but you can also compare values because the   of the data type is automatically performed before the comparison, and implicitly conversions .

  • = =  : Comparison of values only

  • = = =  : Not only is it worth comparing, but also the comparison of data types

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• The basic type of variable is stored in stack memory (stack).

  var a,b;a = "zyj";b = a;console.log(a);   // zyjconsole.log(b); // zyja = "呵呵"; // 改变 a 的值，并不影响 b 的值console.log(a); // 呵呵console.log(b); // zyj

The diagram is as follows: The stack memory includes the identifier of the variable and the value of the variable

Reference type

In addition to the above 6 basic data types, the remaining is the reference type, collectively referred to as Object 类型 . Subdivision words, there are:,,, Object 类型 Array 类型 , and Date 类型 RegExp 类型 Function 类型 so on.

The value of a reference type is accessed by reference.

•  <   Span class= "Hljs-keyword" >var obj = {name: "Zyj"}; //Create an object obj.name =  "Percy"; //Change the value of the Name property Obj.age = 21; //add age Property Obj.givemeall =  function (return this.name + " Span class= "hljs-string" > ":" + this.age;}; //Add Givemeall method Obj.givemeall ();         

• Comparison of reference types is the comparison of references

       var obj1 = {};    //Create a new empty object Obj1var obj2 = {}; //Create a new empty object Obj2console.log (obj1 = = obj2); //falseconsole.log (obj1 = = = Obj2); //false           

  because Obj1 and Obj2 refer to 2 different objects stored in heap memory respectively, the value of the variable obj1 and OBJ2 (reference address) is not the same!

• The value of a reference type is the object that is stored in the heap memory (heap), unlike other programming languages, where JavaScript cannot directly manipulate the object's memory space (heap memory).

  var a = {name:"percy"};var b;b = a;a.name = "zyj";console.log(b.name); // zyjb.age = 22;console.log(a.age); // 22var c = { name: "zyj", age: 22};

The illustrations are as follows:

• The variable identifier is saved in the stack memory and a pointer to the object in the heap memory

• The contents of the object are saved in the heap memory

Detection Type

• typeof: Often used to detect whether a variable is the most basic data type

  var A;typeof A;Undefineda =Nulltypeof A;Objecta =Truetypeof A;//Booleana = 666; typeof A; //number a =  "hello"; //Stringa = symbol (); //Symbola = function (typeof A; //Functiona = []; typeof A; //objecta = {}; typeof A; //objecta = /aaa/g; typeof A; //object             

• Instanceof: Used to determine whether the object pointed to by the prototype property of a constructor exists on another prototype chain to detect the object

  • The simple thing is to judge whether a variable of a reference type is specific to a certain type of object

    instanceof Object              // true([]) instanceof Array // true(/aa/g) instanceof RegExp // true(function(){}) instanceof Function // true

For more detailed type testing, see Resources!

Where there is a mistake, you are welcome to point it out!

References

• "article" [JS advanced] Basic type reference type simple Assignment object reference (recommended)

• "Stack Overflow" Javascript by reference vs. by value

• "article" [[JS] Let the person faint JavaScript variable assignment] (Http://hellobug.github.io/blo ...

• How does the article check the JavaScript variable type? Recommended

• Three meanings of the "article" stack

JavaScript in-depth understanding of the values of basic types and reference types