Mutation response is undefined, but shouldn't be

Intended outcome: Using Apollo Client and hooks, the typings tell me that the response from an operation will always be defined, but that response.data might not be. As such, we always use response.data?.something when unwrapping the response.

We use the built-in options for onCompleted and onError, and as such, do not wrap our calls in try/catch assuming that any failures in the request will propagate to the onError callback.

Actual outcome: We get Sentry reports that says TypeError: undefined is not an object (evaluating 'n.data')

How to reproduce the issue:

const response = await ourMutation({
  variables: {
    input: {
      id: someId, 
      otherVar: theValue
    },
  },
});

if (response.data?.ourMutation) {
  setShowSuccess(true);
}

Versions

System:
    OS: macOS 10.15.7
  Binaries:
    Node: 14.15.4 - /usr/local/bin/node
    Yarn: 1.22.10 - ~/Documents/git/MMS-Ticket/node_modules/.bin/yarn
    npm: 6.14.10 - /usr/local/bin/npm
  Browsers:
    Chrome: 88.0.4324.192
    Safari: 14.0.3
  npmPackages:
    @apollo/client: 3.3.6 => 3.3.6 
    apollo: 2.31.0 => 2.31.0 
    react-apollo: 3.1.5 => 3.1.5

[extjbhlego]

@jcreighton - Eureka! 😄

Today, I accidentally stumbled across a way to reproduce the error. It turns out that it can be reproduced if you provide an onError callback to the useMutation config.

I’ve set up an updated sandbox that produces the issue when you press the button:

https://codesandbox.io/s/late-sound-ettg7?file=/src/App.js

We have a default error handling function that displays a nice default error message for the user in the few cases where we don’t handle errors explicitly, which we provide to the onError callback in the useMutation/useQuery configs. From the types and documentation, I would expect that in this case

1. the callback will be invoked
2. response will still be defined as the types indicate, but data will be null, and errors will instead be populated:

[extjbhlego]

@jcreighton Can you give any estimate as to when we can expect this to land in a patch? 😃 Thanks