Make keyFields a readonly array

[julienfouilhe]

Just a small typing change.

The current behaviour prevents doing things like this:

const apolloMainTypePolicies = {
  User: {
    keyFields: ['id'],
  },
} as const;

export type ApolloMainTypePolicies = typeof apolloMainTypePolicies;

export const getCacheForApollo = () => {
  return new InMemoryCache({
    possibleTypes: introspectionQueryResultData.possibleTypes,
    typePolicies: apolloMainTypePolicies,
    // Type 'readonly ["id"]' is not assignable to type 'false | KeySpecifier | KeyFieldsFunction | undefined'.
    //    The type 'readonly ["id"]' is 'readonly' and cannot be assigned to the mutable type 'KeySpecifier'.
  });
};

[apollo-cla]

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[benjamn]

Seems reasonable, especially since mutating these arrays is unlikely to produce desirable results (whatever the goal might be). Thanks @julienfouilhe!

[julienfouilhe]

@benjamn Typescript seems to be having trouble discriminating KeySpecifier in some cases now though, I'll come up with something to fix this!

[julienfouilhe]

@benjamn I added a isReadonlyArray helper, it was the easiest/most simple solution I could find. Tell me if you think of something smarter!

[sztadii]

Thanks for catching the issue, but the name of the function is misleading.
It is not really filtering the read-only types but fixing the issue with the isArray method.

[sztadii]

Can you rename the helper to isArray?

[sztadii]

export const isArray = (a: any): a is ReadonlyArray<any> | Array<any> => Array.isArray(a)

[julienfouilhe]

@sztadii you're right, done!

[sztadii]

@julienfouilhe I am only worried that someday TS will change the behavior of read-only arrays.
Sometimes maybe we will prefer to use a mutable array.
Can we change isArray to something like that

export const isArray = (a: any): a is any[] | readonly any[] => Array.isArray(a)

?

[julienfouilhe]

@sztadii done! 👍

[benjamn]

LGTM, thanks again @julienfouilhe!

[benjamn]

These changes should now be available from npm in @apollo/client@3.5.9 (just published).