How to decode an error that happened in a proxy.proxy call

Question

I'd like to show my users if an error occurred while doing a proxy.proxy call. While I know how to decode a system.ExtrinsicFailed, as this is well documented, I have difficulties finding the correct way to decode an error from proxy.ProxyExecuted when printed to string, it shows [{"err":{"module":{"index":4,"error":"0x02000000"}}}] when there's an error or [{"ok": null}] in case it succeeded.

What would be the best way to check the status and decode the error?

I am ideally looking at something similar than

const [dispatchError] = data
const decoded = api.registry.findMetaError((dispatchError as any).asModule);
const errorInfo = `${decoded.docs} - ${decoded.section}.${decoded.name}`;

But checking if dispatchError.err exists or forcing it with asModule doesn't end well, although it looks like a module. It needs a step in between to be decoded it seems, that I'm missing.

Answer 1

Thanks a lot forgetso for your help. this definitely got me on the right track. For some reason, the asModule didn't work. I had subsequent TS errors such as toNumber is not a function.

I had to use toJSON to what came back from the event to work my way through. That's the working code:

const [dispatchError]: any = data.toJSON();

// if proxy has an error
if (dispatchError?.err?.module) {
  const mod = dispatchError.err.module
  const error = api.registry.findMetaError(
    new Uint8Array([Number(mod.index), Number(mod.error.slice(0, 4))])
  )

  errorInfo = Array.isArray(error.docs) ? error.docs.join('') : error.docs || ''
}
  

Answer 2

I think you need to strip the trailing zeroes from the error code and decode the error as follows:

            const mod = dispatchError.asModule
            const error = api.registry.findMetaError(
                new Uint8Array([mod.index.toNumber(), bnFromHex(mod.error.toHex().slice(0, 4)).toNumber()])
            )
            message = `${error.section}.${error.name}${
                Array.isArray(error.docs) ? `(${error.docs.join('')})` : error.docs || ''
            }`

This will give you the module at index 4, error with index 2. e.g. in substrate

node_modules/@polkadot/types-support/metadata/v14/substrate-json.json

        {
          "name": "Authorship",
    
          ...

          "errors": {
            "type": 424 // use this value to look for the errors
          },
          "index": 4
        },

node_modules/@polkadot/types-support/metadata/v14/substrate-types.json
  {
    "id": 424,
    "type": {
      "path": [
        "pallet_authorship",
        "pallet",
        "Error"
      ],
      ...
      "def": {
        "variant": {
          "variants": [
      ...
            {
              "name": "TooManyUncles", // this might be your error
              "fields": [],
              "index": 2,
              "docs": [
                "Too many uncles."
              ]
            },