JavaScriptUndefined, difference between the Null type and the NaN value _ basic knowledge

Differences between null values and undefined values in Javascript Undefined type
Only one undefined Value
The following three types of typeof return types are undefined:
1. When the variable is not initialized
2. When the variable is undefined
3. When the function does not return a specific value (undefined is returned when the function does not return a value)
Undefined is not equivalent to an undefined value.
Typeof does not really distinguish whether it is undefined
See the following sample code:
Var oTemp;
Alert (typeof oTemp); // outputs "undefined"
Alert (typeof oTemp2); // outputs "undefined"
Alert (oTemp2 = undefined); // error oTemp2 undefined cannot use operations other than typeof but not oTemp2
When the variable
Binary Null type
There is only one null. undefined is derived from null, so undefined = null
Undefined is a variable declared but not initialized,
Null indicates that no existing object exists.
Three NaN values
Is a special value, indicating a non-Number (Not a Number)
If the type conversion fails, NaN is returned.
E.g. to convert a word blue to a value, it will fail because there is no equivalent value.
NaN is not equal to yourself
That is, NaN = NaN is false.
Determine whether NaN uses isNaN ();