How can we get a Pallet's Sub-Account Address using PolkadotJS API

Question

We have a pallet which has their own account e.g *b"replace_" and we are able to generate sub accounts addresses like so:

let sub_account: T::AccountId = T::PalletId::get().into_sub_account((0, 1))

In our react frontend, how can we fetch the address of a pallet's sub-account address using PolkadotJS API? (in this example the id is tuple of (0,1))

We set up the pallet account as detailed in Shawn's accepted answer: https://stackoverflow.com/questions/67082488/transferring-assets-between-accounts-and-pallet

and i found from this site https://www.shawntabrizi.com/substrate-js-utilities/ we can get the Pallet's account address using:

let address = stringToU8a(("modl" + moduleId).padEnd(32, '\0'));
encodeAddress(address);

Our issue is getting the sub account address for a given id.

Answer 1

You just need to look at the Rust source code:

fn into_sub_account<S: Encode>(&self, sub: S) -> T {
    (Id::TYPE_ID, self, sub)
        .using_encoded(|b| T::decode(&mut TrailingZeroInput(b)))
        .expect("`AccountId` type is never greater than 32 bytes; qed")
}

So basically:

1. Take the bytes of the TYPE_ID.

   For a PalletId that is:

   impl TypeId for PalletId {
       const TYPE_ID: [u8; 4] = *b"modl";
   }
   

   As hex bytes, "modl" is: 0x6d6f646c

2. Take the PalletId itself as bytes, for example:

   pub const SocietyPalletId: PalletId = PalletId(*b"py/socie");
   

   As hex bytes, "py/socie" is: 0x70792f736f636965

3. Take your sub account seed as bytes:

   /// The account ID of the payouts pot. This is where payouts are made from.
   ///
   /// This actually does computation. If you need to keep using it, then make sure you cache the
   /// value and only call this once.
   pub fn payouts() -> T::AccountId {
       T::PalletId::get().into_sub_account(b"payouts")
   }
   

   As bytes, "payouts" is 0x7061796f757473

4. Then fill the rest of the account address with 0 to fill the expected account encoded length.

So putting this all together, you can find the account:

0x6d6f646c70792f736f6369657061796f75747300000000000000000000000000
--1-------2---------------3-------------4-------------------------

Which leads to SS58 Address:

5EYCAe5ijiYfqiTEFBxuWTX5AH6oQUT7Fe1Pz5wNvsNWXux7

And there you go! Check out the Society Pallet Payouts sub-account here:

https://kusama.subscan.io/account/5EYCAe5ijiYfqiTEFBxuWTX5AH6oQUT7Fe1Pz5wNvsNWXux7