JavaScript data type classification

1.  Basic types (6 types):

　　Number (String) String (String) Boolean (Boolean) 

　　Undefined Null ES6-Symbol (Unique)

1.1 Number 

the let NUMBER_1 = 15; // decimal notation 
the let NUMBER_2 = 017; // octal notation 
the let NUMBER_3 = 0xF; // hexadecimal notation 

// However, the output of the above procedures will be converted to decimal notation output 
console .log (number_1, number_2, number_3)

 

1.1.1 Decimal (float)

　　Numbers with a decimal point use values ​​of type float.

let height = 1.75;

 

1.1.2 Special values

　　NaN (not a number) calculation error, type conversion failed

　　Infinity (divisor is zero)

1.2 String

　　Use double quotation marks ("") or single quotation marks ('') to represent data of type string

let name = "Someone";

1.3 Boolean type (boolean)

　　There are only two values ​​of type boolena: true or false, representing true and false

1.4 undefined

　　The data type undefined has only one value undefined.

　　The difference between undefined and undeclared variables

　　Undefined variable: a variable has been declared but not assigned, the default value is undefined;

var address;
console.log(address);//undefined

　　Undeclared variable: use it without declaring the variable-the console will report an error

console.log (add); // error -add not defined, add this variable is not declared;

1.5 empty

　　The data type null has only one value null. You can clear the contents of a variable by assigning a null value to the variable

1.6 ES6 - Symbol

　　Did not learn here (omitted)

2. View the data type of the basic type value

　　Syntax: typeof variable name / value or typeof (variable name / value)

console.log( typeof 12 )              //number
console.log( typeof '曹操' )          //string
console.log( typeof true )           //boolean
console.log( typeof undefined)    //undefined
console.log( typeof null )            //object 这是个bug
console.log( typeof Symbol() )   //symbol

　　Why typeof null gets object instead of null --- It doesn't know too much about the underlying layer. I only know that it is a bug and because null starts with 000 in JavaScript binary, and if the first three bits of binary in JavaScript are all 0, it will be judged as object type