How to write unit test for ApolloClient cache update using readQuery/writeQuery?

Question

Given this contrived example React component using apollo-client useMutation hook with an update method to customize the cache update, how can I write a unit test to test the update method?

Because readQuery will return null if anything is query is not in the cache, I want a unit test to verify the read/writeQuery work as expected so we can prevent possible regressions.

Ideally I would like to test the customUpdateFunction , but if that is not possible we can test the ExampleComponent in a way to verify the cache update works as expected. As long as the result is adding some safety to the custom cache update in the useMutation update method.

function ExampleComponent() {
    const {data} = useMutation(REMOVE_ITEM_MUTATION, {
        variables: VARIABLES,
        update: customUpdateFunction,
    });
    
    return ...
}

function customUpdateFunction(cache, data) {
    const cachedData = readQuery({
        query: LIST_QUERY,
    });

    if (!cachedData) {
        return;
    }

    // Some immutable update to remove an item from a list
    const updatedData = immutableUpdate(cachedData, data)

    writeQuery({
        query: LIST_QUERY,
        data: updatedData,
    });
}

Answer 1

I don't understand what is readQuery and writeQuery in your code.

If instead of readQuery and writeQuery you can use cache.readQuery and cache.writeQuery , then you can use import { InMemoryCache } from '@apollo/client' to create cache and test it like this:

describe("customUpdateFunction", () => {
    it("should remove data from LIST_QUERY in the cache", async () => {
        //easy way to recive an initialCacheState is cache.extract() 
        const initialCacheState = ....
        const cache = new InMemoryCache().restore(initialCacheState);
        const data = .... // mutation result

        customUpdateFunction(cache, data)
        const cachedData = cache.readQuery({
            query: LIST_QUERY,
        });
        expect(cachedData.something).not.toContain(data.something);
        //or map array  
        //expect(cachedData.something.map(i=>i.id)).not.toContain(data.something.id);
    });
});