JavaScript correct usage of a thunk function (with variables)

Question

I'm stuck in a thunk function usage and I need to better understand how it works.

Code that DOES NOT WORK:

const fields = {
    id: {
        type: new GraphQLNonNull(GraphQLID),
        resolve: (obj) => dbIdToNodeId(obj._id, "History")
    },
    timestamp: {
        type: GraphQLLong
    },
    objectId: {
        type: GraphQLString
    },
    user: {
        type: require('../User/connections').default,
        args: connectionArgs,
        resolve: (source, args) => {
            return UserModel.findOne({ id: source.id }).exec();
        }
    },
    action: {
        type: new GraphQLNonNull(GraphQLString)
    }
};

export const HistoryType = new GraphQLObjectType({
    name: 'History',
    description: 'History',
    interfaces: () => [NodeInterface],
    isTypeOf: (value) => value instanceof HistoryModel,
    fields: () => (fields)
});

Code that DOES WORK:

export const HistoryType = new GraphQLObjectType({
    name: 'History',
    description: 'History',
    interfaces: () => [NodeInterface],
    isTypeOf: (value) => value instanceof HistoryModel,
    fields: () => ({
    id: {
        type: new GraphQLNonNull(GraphQLID),
        resolve: (obj) => dbIdToNodeId(obj._id, "History")
    },
    timestamp: {
        type: GraphQLLong
    },
    objectId: {
        type: GraphQLString
    },
    user: {
        type: require('../User/connections').default,
        args: connectionArgs,
        resolve: (source, args) => {
            return UserModel.findOne({ id: source.id }).exec();
        }
    },
    action: {
        type: new GraphQLNonNull(GraphQLString)
    }
})
});

From my "not very skilled on JavaScript thunks" point of view, both codes are the same, but they are not as only the second one works, so:

a) Why the codes does not behave the same ? Why storing the thunk content in a function (fields) makes it behave differently ?

b) My fields variable is used in the sequence of the code, so is there a way to make the thunk work keeping its object in a separate variable, as code 1 (in fact a fix for code 1 that keeps the field variable) ?