JS -- first understand the naming rules and data types of JS and variables

List of articles

• Preface
• One 、 What is? JS
• Two 、 Variable
• summary

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Preface

        From today on, I will open a new JS series , And will continue to update , If it can help you a little bit in front of the screen , I will be very happy ~（ In this paper, Js Variables section in ）

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One 、 What is? JS？

• JS（JavaScript）

A scripting language originally developed by Netscape ,

• JS  Component part

1. ECMAScript

It defines the basic syntax and basic objects of the script language , Now every browser has low ECMAScript The implementation of the

    2.DOM（Document Object Model）

Document object model , In the browser DOM Solidify the whole web page into a tree structured document composed of node levels ,3

    3.BOM（Broswer Object MOdel）

Browser object model , Describes the method and interface of accessing and operating the browser window .

• JS Basic usage

   1. Inline ：    take js The code is written directly inside the label

<a href="javascript:alert(' Click on me ');">  You order me too </a>

   2. embedded ：  take JS The code is written in script In the label   , Written in head Medium or body in

    3. Outer chain ：  take script The label is written in an external file ,script Once the label is introduced outside js file , You can't write code anymore , You need to recreate a script Tags are used to write internal code  , for example ：

,<script src="./01.js"></script>

Two 、 Variable

1. Declaration of variables

Use    var  Keyword to declare a variable    

• Use var  Keyword declared variables , Will be declared before all code is executed （ But no assignment ）
• But if you don't use var  keyword , Then the variable will not be declared in advance ,

for example ：

var a=123;            

2. Identifier naming rules

1. It can contain letters , Numbers , Underline ,$
2. Cannot start with a number
3. It can't be ES Keywords or reserved words in
4. Generally, hump is named （ Initial lowercase , Capitalize the beginning of each word , The rest of the letters are lowercase    xxxZzzYyy）

3. data type

Basic data type ： string   Number     Null        Undefined  Boolean

JS All variables of are saved to Stack memory Medium , Values are independent of each other , Modifying one variable does not affect the other variables

Reference data type ： object

Object is saved to Heap memory Medium , Every time a new object is created , It will open up a new space in the heap memory ,

The variable stores the memory address of the object （ References to objects ）, If the object points to the same object , When one of the variables modifies the attribute , The other will also be affected

But if one of them is set to   null  The other one will not affect （ This is equivalent to the disconnection of this , The other is still connected , Because two points to the same object .

var  obj3 = new Object( );

var obj4  = new  Object( );

When comparing two basic data types , That's the comparison value

When comparing multiple reference data types , It's the memory address of the object being compared , If two objects are as like as two peas , But the address is different , And will return to false

3.1 character string   string

1. Use quotation marks to express as a string
2. Single quotation marks , And double quotation marks cannot be mixed
3. Quotation marks cannot be nested , Double quotation marks can't be put in double quotation marks , You can't put a single quote
4. have access to     \      Character escape

• \''  Express   ''
• \' Express    '
• \n  Means line break
• \t    Generate tabs , It's equivalent to pressing once tab key
• \\  Express   \

3.2 The number    number

stay Js All the values in are number  type , Including integers and floating-point numbers ( decimal ) 

stay JS The maximum number of digits that can be represented in    Number.MAX_VALUE  , Greater than 0 Of minimum value Number.MIN_VALUE  If the maximum value is exceeded, return    Infinity  Positive infinity ,-Infinity  Negative infinity ,

typeof      You can check the type of a variable

typeof  Variable

When checking strings , return string

When checking the values , return number

NaN It's a special number ,  Express Not a Number  Use typeof  Check , And will return to number

Not in JS  Precision in requires precise calculation

3.3 Boolean value

Make logical judgment

   true  Said really   false  Said the false

Use   typeof    When checking a Boolean value , Returns the boolean;

Null    And    Undefined

Null（ Null value ）  It's worth only one , Namely null,  Used to represent an empty object

Use typeof  Check one null When the value of , Returns the object

Undefined（ Undefined ） It's worth only one , Namely undefind

When a variable is declared but not assigned , The value is undefined

Use typeof  Check one undefind When the value of , Returns the undefined

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summary

    That's what I want to share today , Beginners JS Are some basic grammatical parts , But it is the foundation of future learning , Beginners of front-end students must take it seriously . Finally, I still see your peace and happiness in front of this screen , Health and happiness , Wish you and me ~