Data type classification
basic type
- String
- Number
- Boolean
- undefined (indicating that the definition is assigned)
- null (indicating the definition and assignment, but the value is null; the initial assignment is null, indicating that the object will be assigned; in addition, re-assigning the object to null can also release the memory occupied by the object)
Reference type
- Object : any object
- Function : a special object (internally contains executable code)
- Array : a special object (with numerical subscripts, internal data is ordered)
Data type judgment
typeof
The return value is the string expression of the data type (lowercase words)
can be judged: undefined / numeric value / string / boolean / function
cannot be judged: null and object, array and object
instanceof
Determine the specific type of object
===
You can directly judge the two types of undefined and null, because both types have only one value.
Variable type
Type of value in variable memory
basic type
The basic type refers to the basic type of data stored in the memory
Reference type
The pointer address is stored in the reference type