Building your API (server)
Error handling

Error handling

Handling errors can be a nuisance, we have opted to encourage a code based approach. Before we dig into that let's look at how errors work in GraphQL.

In GraphQL it's possible to both return data as well as errors in the same response, let's look at an example:

query {
  me {
  products(limit: 10, offset: 0) {
    nodes {

In here we query both me and products, me is a field that returns a User type and products returns a list of Product types. Imagine in the me field we have a check like the following

import { addQueryFields, AuthenticationError } from 'fuse';
addQueryFields(t => ({
  me: t.field({
    type: UserNode,
    resolve: async (parent, args, { user }) => {
      if (!user) {
        throw new AuthenticationError('You must be logged in to view this')
      return user

When we aren't logged in the json response coming through here will look like the following

  "data": {
    "me": null,
    "products": {
      "nodes": [{ "id": ".."}],
      "totalCount": 1
  "errors": [
      "message": "You must be logged in to view this",
      "locations": [{ "line": 3, "column": 3 }],
      "path": ["me"],
      "extensions": {
        "code": "UNAUTHENTICATED"

This is referred to as a GraphQL error, we get the path to what has errored as well as the message. At fuse we add in the extensions.code part, this makes it easier to reason about on the client as one can analyze the extensions.code and derive what to show the user from there. This while the product list can be rendered as usual.

The errors we export by default from fuse are the following:

  • AuthenticationError which has code UNAUTHENTICATED
  • ForbiddenError which has code FORBIDDEN
  • NotFoundError which has code NOT_FOUND
  • BadRequestError which has code BAD_REQUEST

Extending errors

The base list does not contain all problems, you could go more specific... Let's say we work with a lot of external providers, then we can import the base FuseError and create our own ExternalProviderError.

import { FuseError } from 'fuse';
export class ExternalProviderError extends FuseError {
  name = 'ExternalProviderError'
  constructor(message = 'External provider is offline') {
    super(message, { code: 'EXTERNAL_PROVIDER' })