7、 JS data type

Why is there a concept of data type ？
In order to use memory space more reasonably

1、 Basic data type

1、string

Wrap in single or double quotation marks , Single quotation marks and double quotation marks can be nested with each other , But you can't nest yourself , If you really need to nest , You can use escape characters

2、number

stay JS in , Don't distinguish between decimals and integers , All are number type

Special values ：

• Maximum ：Number.MAX_VALUE

• minimum value ：Number.MIN_VALUE

• infinity ：Infinity 10 / 0

• An infinitesimal ：-Infinity -10 / 0

• Illegal number NaN
  JS When calculating a value, no result is returned , Then return to NaN, Any number and NaN Calculation , Are all NaN,NaN Compare with any number , Are all false, Determine whether a variable is NaN, adopt isNaN()

3、boolean

Values are true and false Two , Some data will have implicit conversion of data type at some time

4、und

It's a data type , There is only one value corresponding to this data type , yes und

// Later, it is generally used to judge whether a variable has a value 
var x;
console.log(x);     //undefined

5、null

• Null Represents an empty... Of a reference data type
• Represents an empty object , use typeof Check null Will return a object
• use ： Through the assignment null This special value , To make the system clear the reference of variables to objects

6、und And null The difference between

• und and null Both are empty
• und Represents the null of the basic data type
• null Represents an empty... Of a reference data type
• Both indicate that the stored content is empty , Determine whether the two values are equal , The result is true.

Equal sign ：==
Determine whether the operands on both sides are equal
In judgment , Do implicit type conversion first , Will be able to null and und All turned into false
The final judgment is false == false

Absolutely equal to ：===
At this point, it first judges the type , If the types are inconsistent , The result is false