Add to array in Apollo Client cache after create mutation

Question

In attempting to automate adding an item to a cached array in Apollo Client after it's created via a mutation, there's a TypeScript error on line 32 (data: {):

Type '{ [x: string]: TFetchResultData[]; }' is not assignable to type 'TQuery'.
  '{ [x: string]: TFetchResultData[]; }' is assignable to the constraint of type 'TQuery', but 'TQuery' could be instantiated with a different subtype of constraint 'Record<string, TFetchResultData[]>'.ts(2322)
DataProxy.d.ts(17, 9): The expected type comes from property 'data' which is declared here on type 'WriteQueryOptions<TQuery, TVariables>'

Here's the full code

import { ApolloCache, DataProxy } from '@apollo/client'

export const addToArray = <
  TVariables,
  TData,
  TFetchResultData,
  TQuery extends Record<string, TFetchResultData[]>,
  K
>(
  cache: ApolloCache<K>,
  {
    query,
    variables,
    fieldName,
    newItem,
  }: {
    query: DataProxy.Query<TVariables, TData>['query']
    variables: DataProxy.Query<TVariables, TData>['variables']
    newItem: TFetchResultData
    fieldName: keyof TQuery
  }
): void => {
  // Add to the documentFiles field, if it exists
  const queryResult = cache.readQuery<TQuery, TVariables>({
    query,
    variables,
  })
  if (queryResult) {
    cache.writeQuery<TQuery, TVariables>({
      query,
      variables,
      data: {
        [fieldName]: [...queryResult[fieldName], newItem],
      },
    })
  }
}

Answer 1

Typescript is complaining that TQuery might have additional fields that will be stripped by your code.

Add ...queryResult to your writeQuery code:

cache.writeQuery<TQuery, TVariables>({
  query,
  variables,
  data: {
     ...queryResult,
    [fieldName]: [...queryResult[fieldName], newItem],
  },
})