Read the documentation: MDC String.indexOf 🙂
indexOf returns the index the match was found. This may be 0 (which means “found at the beginning of string”) and 0 is a falsy value.
indexOf will return -1 if the needle was not found (and -1 is a truthy value). Thus the logic on the test needs to be adjusted to work using these return codes. String found (at beginning or elsewhere): index >= 0 or index > -1 or index != -1; String not found: index < 0 or index == -1.
Happy coding.