JavaScript Data types

In JavaScript, data types fall into two categories: the base data type and the reference data type. In JavaScript, declare variables using the keyword var.

I. Basic data types

Javascrip contains five basic data types:undefined,null,boolean, Number , and string.

• Boolean: Boolean, value is true  orfalse
• Number: Numeric value for any integral type floating point value
• string: A single character or consecutive character that is enclosed by single or double quotation marks (JavaScript is not distinguished by character type)
• null: null type, with only one value:null
• undefined: Not defined, it has only one value:undefined

typeof keyword : Because the variables in JavaScript are loosely typed, it provides a way to detect the data type of the current variable, that is, the typeof keyword, Of the 5 simple data types mentioned in ECMAScript (these 5 are just data types , representing a data type, like the int,string type in C #), by typeof keyword, The following values (shown as strings) are returned for these 5 types of data

• "boolean"--if the value of the variable is a boolean type;
• "Number"-The value is a numeric type;
• "string"-the value is a string type;
• "Object"-- object or value is null;
• "undefined"-- not stated , or the value of the variable is undefined or uninitialized;

The value of the primitive type is stored directly in the variable and can be detected with typeof . However, the typeof null detection is to return an object instead of returning null . Therefore, when NULL is detected, it is best to use all equals (= = =), which also avoids coercion of type conversions.

string number   and   boolean The original package type is automatically created. For example:

1 var name = "Tom"; 2 var char = Name.charat (0); 3 console.log (char);  // "T"

Things that happen in the JavaScript engine:

1 var person = "Tom"; 2 var New String (person); 3 var char = Temp.charat (0); 4 NULL ; 5 console.log (char);  // "T"

A reference to a string object is destroyed immediately after it is exhausted, so a property cannot be added to the string, and falseis returned when instanceof detects the corresponding type:

1 var person = "Tom"; 2 person.age = +; 3 Console.log (person.age);   // undefined 4 instanceof String);  // false

II. Reference data types

reference type : Saved as an object, essentially a reference to a memory location, so the object is not saved in a variable. In addition to custom objects, JavaScript provides a variety of built-in types.

Object Type: Theobject type is the originator of the JavaScript reference type, which is the same as in C # and Java, where you can add properties and methods after creating an instance of type object.

Based on Object , JavaScript provides built-in types:

• Array: An ordered list of arrays of type, indexed by number, of a set of values
• Date: Day and Time type
• error: Run-time error type
• function: Types of functions
• RegExp: Regular expression type

You can use new to instantiate each object, or to create an object in literal form:

1 varobj =NewObject ();2 varperson = {3Name: "Tom",4Sex: "Man",5"My Age": 226  };7Console.log (Person.sex);//Accessing Properties8Console.log (person["My Age"]); 9obj =NULL;//de-referencing

Obj does not contain an object instance, but rather a pointer (or reference) to the location of the actual object in memory. Because typeof returns object for all reference types of non-functions , it is necessary to use instanceof to detect reference types.

Object objects

An object is a reference type that is commonly used to create objects in two ways: the object constructor and the object literal form:

1 var p1 = {2     name: "Tom",3     age:224}; 5 var New Object (); 6 p2.name = "Jack";

In JavaScript, you can add properties to an object at any time:

1 p1.age = 0; 2 function () {3     alert (this. Name);   4 }5//"Tom"

There are two ways to detect if a property exists: in and hasOwnProperty (), which detects both the prototype attribute and its own (instance) attribute, which detects only its own (instance) attributes:

1  in p1);  // true 2 Console.log (P1.hasownproperty ("Age"));  // true 3  in p1);  // true 4 Console.log (P1.hasownproperty ("toString"));  // false

Object P1 does not define toString, which inherits from object, so in and hasownproperty () differences occur when the property is detected.

To delete a property, use the delete operator to delete the own property, and you cannot delete the prototype property:

1P1.tostring =function(){2Console.log ("P1 object");3 };4Console.log (P1.hasownproperty ("toString"));//true5P1.tostring ();//"Per1 Object"6 Deletep1.tostring;7Console.log (P1.hasownproperty ("toString"));//false8Console.log (P1.tostring ());//[Object Object]

JavaScript Data types