Basic data type and complex data type

let Variables are declared , And variable quantity , If the assignment is repeated , And the latter will cover the former
const Declared constant , And will not change , You can't repeat the assignment , And the value must be assigned when declaring

0.1 + 0.2 == 0.30000000000000004 Why? ?

Because machine language is converted to binary , There is a lack of precision in binary calculation ,

Seven basic data types :

1、Number: Numerical type The range of data calculation is 22013 ~ -22013
2、String: character string ,js Can identify + Both sides of No , But where there is one side + Number That is splicing
3、Boolean: Only eight will turn into false, Everything else true, Namely Boolean(0),Boolean(0n),Boolean(undefined),
Boolean(null),Boolean(false),Boolean(" "),Boolean(NaN),Boolean( ), It is mainly used for judging
4、Null: Null value give an example : There is a piece of land that is nominally mine , But there is nothing on the ground , Not taking up space is null,
5、undefined: give an example : There is a piece of land that is nominally mine , But there are my belongings on the ground , Occupied space is undefined,
6、Symbol: Unique value
7、Bigint: Super integer , When using, you must add n

Complex data type :

arr[] ,object{}, function function, Built-in objects :window ,document(Dom), Math Mathematical objects ,Date Date object , Except for the foundation type , Others are complex data types

An array can hold multiple pieces of data , The serial number from 0 Starting number , and lengrh It's from 1 Start counting , The value taking method is similar to arr[]
Common object / Instance object { } Is stored in the form of key value pairs , Key name : Key value , Value method 1：obj spot , And the dot cannot be followed by a string or a number , Strings and numbers must be wrapped in brackets , Value method 2：obj[" "] , If the value is taken without quotation marks in the brackets , Will be the first to identify whether it is a variable

What is stack memory and heap memory ?

The data stored in the basic data type will be stored in the stack memory , Data stored in complex data types will be stored in heap memory ,
So why are they not equal ? The reason is because : For example arr The saved data will be saved to 13 building 1307 between , and arr1 The saved data will be saved to 14 building 1307 between , All say their memory addresses are different , It won't be equal

        let arr = [1,2," Finch ",true]
        let arr1 = [1,2," Finch ",true]
        console.log(arr == arr1);  //false

How to modify and add values in common objects

        let age = "age"

        let obj = {
    
            name:" Jay Chou ",
            age:18,
            "abc" :"123",
            10:123,
            "10":" Hello ",
        }

        //  modify 
        obj.name = " Jerungo "
        //  newly added 
        obj.hobby = " Love playing basketball "

        //  Modified value 
        obj[10] = "ABC"
        console.log(obj);
        console.log(obj.length);