Error handling

In this section we'll see how to handle the failure case from the previous example. Let's suppose that our API function Api.fetch returns a Promise which gets rejected when the remote fetch fails for some reason.

We want to handle those errors inside our Saga by dispatching a PRODUCTS_REQUEST_FAILED action to the Store.

We can catch errors inside the Saga using the familiar try/catch syntax.

import Api from './path/to/api'
import { call, put } from 'redux-saga/effects'

// ...

function* fetchProducts() {
  try {
    const products = yield call(Api.fetch, '/products')
    yield put({ type: 'PRODUCTS_RECEIVED', products })
  }
  catch(error) {
    yield put({ type: 'PRODUCTS_REQUEST_FAILED', error })
  }
}

In order to test the failure case, we'll use the throw method of the Generator

import { call, put } from 'redux-saga/effects'
import Api from '...'

const iterator = fetchProducts()

// expects a call instruction
assert.deepEqual(
  iterator.next().value,
  call(Api.fetch, '/products'),
  "fetchProducts should yield an Effect call(Api.fetch, './products')"
)

// create a fake error
const error = {}

// expects a dispatch instruction
assert.deepEqual(
  iterator.throw(error).value,
  put({ type: 'PRODUCTS_REQUEST_FAILED', error }),
  "fetchProducts should yield an Effect put({ type: 'PRODUCTS_REQUEST_FAILED', error })"
)

In this case, we're passing the throw method a fake error. This will cause the Generator to break the current flow and execute the catch block.

Of course, you're not forced to handle your API errors inside try/catch blocks. You can also make your API service return a normal value with some error flag on it. For example, you can catch Promise rejections and map them to an object with an error field.

import Api from './path/to/api'
import { call, put } from 'redux-saga/effects'

function fetchProductsApi() {
  return Api.fetch('/products')
    .then(response => ({ response }))
    .catch(error => ({ error }))
}

function* fetchProducts() {
  const { response, error } = yield call(fetchProductsApi)
  if (response)
    yield put({ type: 'PRODUCTS_RECEIVED', products: response })
  else
    yield put({ type: 'PRODUCTS_REQUEST_FAILED', error })
}

onError hook

Errors in forked tasks bubble up to their parents until it is caught or reaches the root saga. If an error propagates to the root saga the whole saga tree is already terminated. The preferred approach, in this case, to use onError hook to report an exception, inform a user about the problem and gracefully terminate your app.

Why I cannot use onError hook as a global error handler? Usually, there is no one-size-fits-all solution, as exceptions are context dependent. Consider onError hook as the last resort that helps you to handle unexpected errors.

What if I don't want an error to bubble? Consider to use safe wrapper. You can find examples here