Returns TRUE if value refers to or evaluates to any error value, including #N/A, and FALSE otherwise.
Use the ISERR function to test for any errors except#N/A.
returns TRUE, because taking the squareroot of -1 is an error.
where C5 contains 123, returns FALSE, because 123 is not an error.
returns TRUE, because NA() returns the #N/A error.
CHOOSE(0;"A";"B";"C";"D") returns Err:502 (correctly - the index is out of range) but ISERROR(CHOOSE(0;"A";"B";"C";"D")) also returns Err:502, rather than TRUE. The error is propagated rather than evaluated.