Create hex encoded call (for XCM transact call) in javascript?

Question

I'm trying to create the encoded element for an XCM V0 transact call such as:

const message = {
    V0: {
        Transact: {
            originType: "Native",
            requireWeightAtMost: 100000,
            call: {
                encoded: "0x280016000000" //ex: templatePallet.doSomething(22)
            }
        }
    }
}

But how to create the encoded call ?

I've tried this without success:

let templatePallet_hex_call = await parachainApiInstInsurance.tx.templatePallet.doSomething(22).signAsync(account);
...
const message = {
    V0: {
        Transact: {
            originType: "Native",
            requireWeightAtMost: 100000,
            call: {
                encoded: compactAddLength(hexToU8a(templatePallet_hex_call.toHex()))
            }
        }
    }
}

Any clue or documentation for this ? Thanks in advance.

PS: I've seen this thread but it's not using javascript.

Answer 1

When you construct via api.tx.* is creates an unsigned extrinsic. So basically you want the call-only part of that.

// output as hex
const encoded = u8aToHex(
  // add length to the U8a (double-encoding)
  compactAddLength(
    // convert the method-only part to Uint8Array
    templatePallet_hex_call
      .method
      .toU8a()
  )
);

This part is not signed, so you can forgo the signAsync in your code. The requirement here is for the length + call indexes + call data. The signing is only for extrinsics, not call data.