Gracefully handle a Promise using async
/await
.
With the addition of async
/await
keywords in ECMAScript 2017 the handling of Promises became much easier. However, one must keep in mind that the await
keyword provides no standard error handling API. Consider this usage:
async function getUser(id) {
const data = await fetchUser(id)
// Work with "data"...
}
In case fetchUser()
throws an error, the entire getUser()
function's scope will terminate. Because of this, it's recommended to implement error handling using try
/catch
block wrapping await
expressions:
async function getUser(id) {
let data = null
try {
data = await asyncAction()
} catch (error) {
console.error(error)
}
// Work with "data"...
}
While this is a semantically valid approach, constructing try
/catch
around each awaited operation may be tedious and get overlooked at times. Such error handling also introduces separate closures for execution and error scenarios of an asynchronous operation.
This library encapsulates the try
/catch
error handling in a utility function that does not create a separate closure and exposes a NodeJS-friendly API to work with errors and resolved data.
npm install until-async
import { until } from 'until-async'
async function getUserById(id) {
const [error, data] = await until(() => fetchUser(id))
if (error) {
return handleError(error)
}
return data
}
import { until } from 'until-async'
interface User {
firstName: string
age: number
}
interface UserFetchError {
type: 'FORBIDDEN' | 'NOT_FOUND'
message?: string
}
async function getUserById(id: string) {
const [error, data] = await until<UserFetchError, User>(() => fetchUser(id))
if (error) {
return handleError(error.type, error.message)
}
return data.firstName
}
This has been intentionally introduced to await a single logical unit as opposed to a single Promise
.
// Notice how a single "until" invocation can handle
// a rather complex piece of logic. This way any rejections
// or exceptions happening within the given function
// can be handled via the same "error".
const [error, data] = until(async () => {
const user = await fetchUser()
const nextUser = normalizeUser(user)
const transaction = await saveModel('user', user)
invariant(transaction.status === 'OK', 'Saving user failed')
return transaction.result
})
if (error) {
// Handle any exceptions happened within the function.
}
- giuseppegurgone for the discussion about the original
until
API.