JS variable types and common type conversions

JS Variable type

The original type ：Number、String、Boolern、undefined、NULL– Store in the stack
Reference type ：Object -- Store in the pile
1. Numerical type –Number： Numbers
（1） integer ： Positive and negative integers and zero
（2） floating-point ： decimal （1.11e+1 = 11.1）
2. String type –String
single quote (‘abc’)、 Double quotes (“abc”)、 Template string (``) Wrapped character set .
3. Boolean type –Boolern
true and false
4. Undefined type –undefined
5. Empty type –NULL
Its type is actually an object
6. object type –Object
Objects store properties and methods
var obj = {
a:a,
b:func1
}
classification ：
Host object The host is the environment
There are two kinds of environments that the front end contacts ：
1. Browser environment -- Provided window object ( Global object ) And its children
2. Server environment – nodejs Environment provided – global object ( Global object ) And its children
Both environments can run js Code .
The same thing is that they all use ChromeV8 engine – Because it is the highest performance engine at present
The difference is The two environments provide API Different
Built-in objects
JavaScript Provides some built-in objects and functions ：
String、Math、Date、Array…
Custom object
Class instantiation object defined by ourselves
7.Symbol type
Please check the details
https://es6.ruanyifeng.com/#docs/symbol
7.BigInt type
var bg = 121212n; // Add... After the number ‘n’
Be careful ：
1.BigInt Type cannot be used Math Object method .
2. Unable to join Number Example hybrid operation .

Type conversion

It is divided into ： Implicit conversion and cast

Other types are converted to numbers ：
Implicit conversion – Automatic conversion during program operation

The plus sign is compared with other operators first except Arithmetic operation also String concatenation The role of
As shown in the figure above ： Spliced strings have priority over arithmetic operations （ Will first string splicing If there is no string splicing, the arithmetic operation will be performed ）

true Implicitly convert to 1
false and null Implicitly convert to 0
undefined Into the NaN =>Not a Number

reduce 、 ride 、 except 、 modulus Can convert a string into a number Convert a number to a string (1+’’) that will do
~~ Operators can be used properly to easily convert strings and numbers into each other in work

NaN operation

NaN The results of mathematical operations are NaN Even if it's NaN It is also NaN
The function used for implicit conversion is Number

Coercive transformation – Manual switching

Forced conversion to numbers
Number(‘1’) //1
Number(‘1a’) //NaN
Number(true) //1
Number(false) //0
Number(undefined) //NaN
Number(null) //0

Forced to integer
You can only turn numbers Encounter letters or decimal points or other characters Stop intercepting The result of conversion failure is NaN

parseInt Back to decimal perhaps NaN, It has two parameters parseInt( Values to convert , Converted base ) The first white space character of the string is ignored

Force conversion to floating point

Encounter letters, etc Stop intercepting The result of conversion failure is NaN The first white space character of the string is ignored

Force to string

notes ： If directly in numbers 、true And so on API You can use Numbers + Space +.API

Strong to Boolean

Turn Boolean to false Of situation ：’’ NaN 0 null undefined
Implicitly convert to Boolean
!!num // Double exclamation + Variable name
~~// I often write this in the program First, verify the errors we can expect at the beginning of the method If something goes wrong be return;
if(!num){
// Error prompt code
return;
}