JS TypeError: Cannot Read Property “…” of Undefined, Despite DATA being returned?

问题

I am following this Apollo Pagination tutorial:

Apollo Pagination Examples

Summary of my issue:

I have a known working GraphQL query that works in the playground. When I try to fetch data and use it in a React component, as outlined in that Apollo link above, I get the following error:

"TypeError: Cannot read property 'errorlogsConnection' of undefined"

However, when I check the response from the graphQL api in the web console, the query does in fact return data. Picture attached below.

I believe I'm probably trying to reference the object incorrectly but I can't spot what my mistake is.

Note: I have been able to query and use this same API endpoint in other React components for this very same project without issue.

Here is the code involved:

I am using this query, which works in my GraphiQL playground:

query errorlogsConnection($cursor: String) {
  errorlogsConnection(orderBy: errorid_DESC, first: 4, after: $cursor) {
    edges {
      node {
        errorid
        errorlog
        entrydate
      }
    }
    pageInfo {
      hasPreviousPage
      hasNextPage
      endCursor
      startCursor
    }
  }
}

Here is the ReactJS code that I've adapted from their tutorial:

function ErrorLogsPagination() {
    const {data: {errorlogsConnection: errorLogs}, loading, fetchMore, error} = useQuery(
        ERROR_LOG_PAGINATION
    );

    if (loading) return <p>Loading...</p>;
    if (error) return <p>Error :(</p>;


    return (
        <ErrorLogs
            entries={errorLogs || []}
            onLoadMore={() =>
                fetchMore({
                    variables: {
                        cursor: errorLogs.pageInfo.endCursor
                    },
                    updateQuery: (previousResult, { fetchMoreResult }) => {
                        const newEdges = fetchMoreResult.errorLogs.edges;
                        const pageInfo = fetchMoreResult.errorLogs.pageInfo;

                        return newEdges.length
                            ? {
                                // Put the new comments at the end of the list and update `pageInfo`
                                // so we have the new `endCursor` and `hasNextPage` values
                                comments: {
                                    __typename: previousResult.errorLogs.__typename,
                                    edges: [...previousResult.errorLogs.edges, ...newEdges],
                                    pageInfo
                                }
                            }
                            : previousResult;
                    }
                })
            }
        />
    );
}

回答1:

The data property will be undefined until the data is fetched from the server or the cache. To prevent the error, either avoid destructuring data until after loading is complete, or else provide default values where appropriate:

const {
  data: {
    errorlogsConnection: errorLogs
  } = {},
  loading,
  fetchMore,
  error,
} = useQuery(ERROR_LOG_PAGINATION)

来源：https://stackoverflow.com/questions/58601633/js-typeerror-cannot-read-property-of-undefined-despite-data-being-return

标签

reactjs

graphql

react-apollo