Overview
Handles the Err case of a ResultAsync by applying a recovery function. Useful for error recovery, fallbacks, and error transformation.
Signature
class ResultAsync<T, E> {
orElse<U, A>(
f: (e: E) => Result<U, A> | ResultAsync<U, A>
): ResultAsync<U | T, A>
}
f
(e: E) => Result<U, A> | ResultAsync<U, A>
required
Function that takes the Err value and returns a new Result or ResultAsync
ResultAsync<U | T, A> - A ResultAsync with potentially recovered value
Usage
Simple fallback
const result = await fetchFromCache(key)
.orElse(() => fetchFromDatabase(key))
.orElse(() => okAsync(defaultValue))
// Tries cache, then database, then default
Conditional recovery
type ApiError = { code: number, message: string }
const result = await callApi()
.orElse((error: ApiError) => {
if (error.code === 404) {
return okAsync(null) // Treat 404 as empty result
}
if (error.code === 503) {
return retryAfterDelay() // Retry on service unavailable
}
return errAsync(error) // Propagate other errors
})
Multi-tier fallback
const loadConfig = () => {
return readFromFile('./config.json')
.orElse(() => readFromFile('./config.default.json'))
.orElse(() => fetchFromRemote())
.orElse(() => okAsync(HARDCODED_DEFAULTS))
}
Key characteristics
- Only runs on Err: If the ResultAsync is Ok, orElse is not executed
- Can change type: The recovery function can return a different value type
- Error type changes: The new error type replaces the original
Use orElse to implement graceful degradation in your async operations.
andThen
Chain successful operations
unwrapOr
Provide a simple default value