How to decode error that occurred while calling a write method on ink contract, on the client side?

Question

While calling a read only method on my ink contract, I get an error that looks like following

On my contract, I have a check to throw an error like:

if paid <= MIN_VESTING_AMOUNT{
    return  Err(Error::VestingAmountTooLow);
}

However the error message that I have received on the client side doesn't exactly tell me what went wrong in the call. Instead it just gives a generic message ContractTrapped on every error on the contract.

Is there a way to decode the actual error message?

Answer 1

You can query the contract method before creating a transaction as it will return any possible error that can occur during the contract call. This will also save the gas fee of the callee since the query will not deduce any gas fee

        const res = await contractPromise.query[queryFunc](accountId, { gasLimit: -1, storageDepositLimit: null}, ...args);
        const { output, result} = res;

        if (output?.isErr || result?.isErr) {
            const errorMsg = output?.toJSON()?.err || result?.toJSON()?.err;
            throw new Error(errorMsg || 'occurred due to unknown reason');
        }

Note1: Please replace "queryFunc" with contract method name.

Note2: you can get information about contract query from this link.

Answer 2

Your error is coming from one of the pallets. You can tell that since it has Module which is an enum from DispatchError.

Module {
    index: 20,
    error: 0x0b
}

• Index 20 is pointing to the concerned pallet on the node you are using. Go to runtime/src/lib.rs of the node you are using to find construct_runtime! macro and check which pallet has index 20. It can be assigned number or just count pallets until you come to pallet on position 20. Start counting with 0.

• To find error 0x0b, you need to go to the lib.rs of the pallet (with index 20) and again find the error on position 11 (0xb). Start with index=0.

However if this error is from pallet-contracts the easiest way to find the error is from your unit test like this:

let r = contract.my_function();
println!("result {:?}", r);

or wrap it in assert!

assert_eq!(contract.my_function(), expected_result);

Using polkadot.js to debug your contract is not very helpful.