Error Handling

What you'll walk away with

Explain the core ideas behind Error Handling
Run the sample GraphQL query or code and verify its output
Apply the technique correctly to the Tutorial Platform and production scenarios

Build the mental model

Where REST expresses errors through HTTP status codes (404, 500), a GraphQL response returns HTTP 200 and carries a separate `errors` array alongside `data`. If one field fails but sibling fields can still resolve, GraphQL returns a partial response containing both data and errors. The error object's `extensions` can carry a custom `code` (such as `NOT_FOUND` or `UNAUTHENTICATED`) so clients can handle errors programmatically.

Connect it to a real scenario

Querying tutorial 999, which does not exist, causes the resolver to throw a `GraphQLError` with `extensions.code: 'NOT_FOUND'`. The response's `data.tutorial` is `null`, `errors[0].path` is `["tutorial"]`, and the frontend checks `error.extensions.code` to show a "Tutorial not found" UI.

GraphQL responses return HTTP 200 plus an errors array
Sibling fields can still resolve, giving a partial response
extensions.code enables programmatic client-side handling

Try the working example

json

{
  "data": {
    "tutorial": null
  },
  "errors": [
    {
      "message": "Tutorial 999 not found",
      "path": ["tutorial"],
      "extensions": {
        "code": "NOT_FOUND"
      }
    }
  ]
}

You should see

You can read a GraphQL error response and write handling logic based on `extensions.code`.

5-minute try-it

Write the response JSON shape you'd expect when the `author` field's resolver throws a `GraphQLError` with `extensions.code: 'NOT_FOUND'`.

One important caution

Client code that only checks HTTP status can miss GraphQL errors entirely — an HTTP 200 response can still carry an `errors` array, so always check it.

Apollo Server — Handling GraphQL Errors — GraphQL

Easy traps

Client code that only checks HTTP status can miss GraphQL errors entirely — an HTTP 200 response can still carry an `errors` array, so always check it.
Validate sample queries and mutations on a local or test server with recoverable data before applying them to production.

Exercise

Write the response JSON shape you'd expect when the `author` field's resolver throws a `GraphQLError` with `extensions.code: 'NOT_FOUND'`.

You'll know it worked when: You can read a GraphQL error response and write handling logic based on `extensions.code`.