JS data types and mutual conversion

data type

In the computer , Different data requires different space , In order to facilitate the division of data into data of different sizes , Make the most of storage space , So different data types are defined

JS Is a weakly typed or dynamic language , This means that the type of the variable is not declared in advance , While the program is running , The type will be automatically determined

The data type of a variable is defined by JS engine according to = The data type of the variable value on the right is used to judge Of , After running , Variables determine the data type

JS Have dynamic types , It also means that the same variable can be used as a different type

Classification of data types

• Simple data type （Number、String、Boolean、Undefined、Null）
• Complex data type （object）

  Digital Number

JS Numeric types can be used to hold integer values , You can also save decimals （ Floating point numbers ）

1. Digital base system

The most common system is binary 、 octal 、 Decimal system 、 Hexadecimal

octal ： Numbers Add in front 0 For octal , Every eight enters one.

Hexadecimal ：0~9     a~f    Numbers Add in front 0x For hexadecimal

2. Digital range

js The maximum and minimum of the values in

 3. Three special values of digital type

•  infinity For infinity , Greater than any number
• -infinity For infinitesimal , Less than any number
• NaN,Not a number Represents a non numerical value

isNaN（） Used to judge non numbers , And return a value , If it is a number, it returns false, If it's not a number, return true

String type String

String type can be any text in quotation marks , Its grammar is “ Double quotes ” and ‘ Single quotation marks ’    Single quotation marks are recommended

1. String quotes nested

JS Can be used in the Single quotes nested double quotes , Or use Double quotes nested single quotes （ Double outside and single inside , Single outside and double inside ）

2. String escape character

similar HTML The special characters in it , There are also special characters in strings , We call it the escape character

All the escape characters are \ At the beginning , The common escape characters and their descriptions are as follows ：

 3. String length

Use length To detect the length of the string

4. String splicing

• Multiple strings can be used + Splicing , The splicing method is character string + Any kind of = The new string after splicing
• Any type added to the string will be converted into a string before splicing , And then splice it into a new string

  + To sum up the pithy formula ： Add the values , The characters are connected

 5. String concatenation strengthens

var str = ` I this year ${age} Year old `

Don't write variables into strings , It is realized by connecting with a string

A formula for connecting variables to strings ： Lead and add     Cut out the numbers , Variable plus middle

• We will splice strings and variables , Because variables can easily modify the values inside
• Variables cannot be quoted , Because quoted variables become strings

  Boolean type    Boolean

The boolean type has two values ：true and false, among true Said really ,false Said the false

Undefined and Null

A variable that has not been assigned a value after declaration will have a default trace undefined（ Data type not defined ）

undefined Add it to the number and the result is NaN

Null  （ Null value ）

Symbol（ES6）

var test = Symbol（‘123’）;

Get the data type of the test variable

typeof + Spaces can be used to detect the data type of variables

Literal

Is a representation of a fixed value , A literal quantity is how to express this value

• Number literal quantity ：8,9,10
• Literal of a string ：‘ Black horse programmer ’
• Boolean literal ：true,false

Conversion of data types

hold A variable of one data type is converted to another data type

Convert to string

Variable .toString（）      console.log（String（ Variable ））    console.log( Variable +")

• toString() and String() It's not the same way
• Plus sign splicing conversion is also called implicit conversion

  Convert to digital

• parseInt and parseFloat The case of words

Convert to Boolean

•   representative Empty , no The value of will be converted to false, Such as ：0 NaN、null、undefined
• The rest of the values are converted to true

Interpreted and compiled languages

A tool for translating programming languages into machine languages , It's called a translator

• There are two ways of translator translation ： One is compile , The other is explain , The difference lies in The time point of translation is different
• The compiler is in Compile before code execution , Generate intermediate code file
• The interpreter is in Compile at runtime , And execute immediately

identifier

Developers are variables 、 attribute 、 function 、 The name of the parameter

Identifier cannot be keyword and Reserved words

keyword

JS Words that have already been used , They can no longer be used as variable names , Method name

Reserved words

Reserved keywords