JavaScript data type (a)

JavaScript data type of the undefined and null

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typeof

Features

Detection variable type

grammar

typeof variable or typeof (variable)

return value

string type, there may be a string, number, boolean, object, undefined, function

undefined

Only one type undefined value, i.e. special undefined

Explanation

In general, there is no need to explicitly set the variable to a worthy case undefined

null

1. A null value indicates a null object pointer
2. If you define a variable to hold the object to prepare in the future, it is best to change the amount is initialized to null value and not the other
   
   

Explanation

undefined values ​​are derived from the null value, it returns undefined == null result is true