Multiple catch patterns
Learn to handle different error types inside a single catch block.
Single catch, multiple patterns
JavaScript does not allow multiple catch blocks; instead you branch by error type inside one catch using instanceof. You write a separate handler for each type.
try {
parseInput(raw);
} catch (err) {
if (err instanceof SyntaxError) {
showFormatError(err);
} else if (err instanceof RangeError) {
showRangeError(err);
} else {
throw err;
}
}Leaving unknown errors
Inside a catch block the safest path for unknown error types is to let them bubble. In the else branch rethrow the error or send it to a central handler. Silently swallowing an unknown error makes debugging hard.
Handle the types you know and pass the rest to the caller. This prevents unexpected cases from vanishing.
Exercise Multiple-pattern error handler
Write a function handleError that takes an error parameter. If error instanceof ValidationError call showFormError(error); if error instanceof NetworkError call showRetry(error); otherwise rethrow with throw error.
Show example solution
function handleError(error) {
if (error instanceof ValidationError) {
showFormError(error);
} else if (error instanceof NetworkError) {
showRetry(error);
} else {
throw error;
}
}Test yourself
-
How do you tell a ValidationError apart in a catch block?
- if (err.name = 'ValidationError')
- if (err.type === ValidationError)
- if (err instanceof ValidationError)
- if (err.is(ValidationError))
Show answer
if (err instanceof ValidationError) — Correct! instanceof safely checks the error type.
-
Type check with instanceof
if (err ____ ValidationError) { showFormError(err); }Show answer
if (err instanceof ValidationError) { showFormError(err); } -
Match built-in error types
- SyntaxError
- Syntax error
- RangeError
- Out of range value
- TypeError
- Wrong type
- ReferenceError
- Undefined variable