How do I call the FA2 balance_of endpoint through Truffle?

Question

I have written a contract to implement FA2 and I have a Truffle/JavaScript setup to test this contract while running against a sandboxed node. I can test the transfer endpoint and the update_operators endpoint, but I don't know how to construct the right JavaScript object to test the balance_of or the token_metadata_registry endpoint.

The input value as defined in LIGO (Pascal dialect) to token_metadata_registry is:

type token_metadata_registry_parameter is contract(address);

The input type to the balance_of endpoint is balance_of_parameter_michelson defined as

type balance_of_request is record
    owner: token_owner;
    token_id: token_id;
end;

type balance_of_response is record
    request: balance_of_request;
    balance: token_balance;
end;

type balance_of_callback is contract(list(balance_of_response));

type balance_of_parameter is record
    requests: list(balance_of_request);
    callback: balance_of_callback;
end;

type balance_of_request_michelson is michelson_pair_right_comb(balance_of_request);

type balance_of_response_auxiliary is record [
    balance: token_balance;
    request: balance_of_request_michelson;
]

type balance_of_response_michelson is michelson_pair_right_comb(balance_of_response_auxiliary);

type balance_of_callback_michelson is contract(list(balance_of_response_michelson));

type balance_of_parameter_auxiliary is record [
    requests: list(balance_of_request_michelson);
    callback: balance_of_callback_michelson;
]

type balance_of_parameter_michelson is michelson_pair_right_comb(balance_of_parameter_auxiliary);

Answer 1

As far as I know, you cannot check if the entrypoint returns the balance with JavaScript, because the entrypoint doesn't return a value but emits a transaction. I didn't try it for a FA2 token contract but to test a FA1.2 token contract with Truffle, I wrote a simple contract that would call the FA1.2 balance_of entrypoint and receive the balance on a different entrypoint before saving it in its storage. I would then fetch the storage of this contract to verify that the balance is what was expected.