JavaScript undefined,null type and Nan value difference _ basics

A Undefined type
Only one value undefined
In the following three cases typeof return type is undefined
1. When the variable is not initialized
2. When the variable is undefined
3. When the function does not have an explicit return value (the function does not return a value is undefined)
Undefined is not equal to undefined value
typeof does not really distinguish whether it is undefined
Look at the following sample code:
var otemp;
Alert (typeof otemp); Outputs "undefined"
Alert (typeof OTEMP2);//outputs "undefined"
Alert (OTEMP2 = = undefined); Error OTEMP2 undefined cannot use an operation other than typeof that cannot be used OTEMP2
When the variable
Two-Null type
Only one null,undefined is handled by a null derivation, so undefined = = NULL
Undefined is the variable declared but not initialized.
Null indicates an object that does not exist
Three NaN values
is a special value that represents the non-number (not a numbers)
Type conversion fails to return Nan
e.g. to convert a word blue to a numeric value will fail because there is no equivalent value
NaN does not equal himself
That is, Nan = = Nan is False
Judge Nan using isNaN ();