The data type

• classification
• judge

classification

• Basic (value) types
• Object (reference) type

Base value type

• String Any String
• Number Indicates an arbitrary Number
• boolean true/flase
• undefined undefined
• null null

Object (reference) type

• Object Any Object
• Function a special object (that can be executed)
• Array A particular type of object (numeric subscript, internal data ordered)

judge

• typeof
  • Returns a string representation of the data type
  • Can judge undefined
  • You can determine the value
  • Can be used to determine the string
  • Boolean values can be determined
  • Function
  • Null and object cannot be determined
  • Object and array cannot be determined
• instanceof
  • Determine the specific type of the object
  • You cannot judge undefined
  • The other two will do
• = = =
  • Judge undefined
  • Determine the null

The problem

1. Undefined is null.

• Undefined means the definition has no value assigned
• Null is defined and assigned, but the value is null

2. When is a variable assigned to null?

• Initial assignment, indicating that an object will be assigned
• Before you finish, make the object a garbage object (to be collected by the garbage collector)

3. Strict distinction between variable types and data types?

• Type of data
  • Basic types of
  • Object type
• Type of variable (variable memory value type)
  • Basic type: Save data that is the basic type
  • Reference type: Holds the address value

The instance

• Instance: Instance object
• Type: Type object

data

Question: when a variable parameter is passed in a JS call, is it passed by value or by reference

How does the JS engine manage memory

• 1. Memory life cycle
  • Allocate a small memory space and get the use of it
  • Stores data and can be manipulated repeatedly
  • Free up small memory space
• 2. Release memory
  • Local variables: the function is automatically released after execution
  • Object: become garbage object ==> garbage collector collection