refer to : https://www.udemy.com/course/the-web-developer-bootcamp/
1. primitive types基本类型
- the basic building blocks
- Number
- String
- Boolean
- Null
- technically, there are two other primitive types, Symbol and BigInt
Undefiend. eg: youtube web page, 点赞数：number; 标题： string；是否订阅: boolean; 在console里面写js注释的时候，返回类型是undefined
2. Numbes IN JS
Js has one number type: positive numbers, negative numbers, whole numbers(integers), decimal numbers
3. basic operations：addtion, substraction, multiplication, division.
% modulo. remainder operator： check number is odd or even
** exponentiation 2**4 = 16. two to the fourth power
4. what is NAN?
It is NOT A NUMBER. eg: 0/0. 1 + NAN
5. Variables & let
basic syntax: let someName = value; keywork: let
6. clear(). to clear console
7. const. const works just like let, except you can not change the value.
why use const: to define some unchanged variables, const variables can not be reassigned.
eg: const pi = 3.14159. const daysInWeek = 7.
8. camel-cased, with the first letter lowercased.
9. In JS, the variables can be changed into another type, but in typescript, there are restrictions, you can not change the variables' type.