JS undefined, null, and Nan

Undefined: A variable is not declared or declared but not assigned, or the variable never appears.

Null: A variable has no value

NaN: Non-numeric

Text and Boolean types of data are theoretically considered Nan because they are not numbers, but in practice, they are not. Nan is designed to indicate that numeric data that is not what we expect, is an indicator of the wrong type of digital data, and that Nan is usually only seen when numbers are needed in the calculation, but when data is received that is non-numeric.

such as: a= (1+12) *?;

alert (a);//nan

B= "";//null

alert (c);//undefined

JS undefined, null, and Nan