js basics

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

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2. Numbes IN JS

Js has one number type: positive numbers, negative numbers, whole numbers(integers), decimal numbers

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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

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 4. what is NAN?

It is NOT A NUMBER.  eg: 0/0.   1 + NAN

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5. Variables & let

basic syntax: let someName = value;   keywork: let

 

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6. clear().  to clear console

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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.  

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 8.  camel-cased, with the first letter lowercased.

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9. In JS, the variables can be changed into another type, but in typescript, there are restrictions, you can not change the variables' type.

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