What's a convenient way to look up a `DispatchError` in polkadot-js?

Question

A DispatchError will refer to a pallet and a member of the pallet's Error enum by index:

DispatchError: {"module":{"index":123,"error":456}}

It's tiresome and error-prone to look these up manually. How can I automate the process using the polkadot-js API or the web interface?

Answer 1

For polkadot-js, you can retrieve more information about the error via the event and the findMetaError api:

events
// find/filter for failed events
.filter(({ event }) =>
  api.events.system.ExtrinsicFailed.is(event)
)
// we know that data for system.ExtrinsicFailed is
// (DispatchError, DispatchInfo)
.forEach(({ event: { data: [error] } }) => {
    if (error.isModule) {
    // for module errors, we have the section indexed, lookup
    const decoded = api.registry.findMetaError(error.asModule)
    const { docs, method, section } = decoded
    console.log(`${section}.${method}: ${docs.join(' ')}`)
    } else {
        // Other, CannotLookup, BadOrigin, no extra info
        console.log(error.toString())
    }
})

This was extracted from the polkadot-js cookbook recipe "How do I get the decoded enum for an ExtrinsicFailed event".

Please note that for SUDO you have to apply a different filtering process:

events
// We know this tx should result in `Sudid` event.
.filter(({ event }) =>
  api.events.sudo.Sudid.is(event)
)
// we know Sudid just returns a result
.forEach(({ event: { data: [result] } }) => {
    // check whether extrinsic was successful
    if (result.isErr) {
        const error = result.asErr
        if (error.isModule) {
            // for module errors, we have the section indexed, lookup
            const decoded = api.registry.findMetaError(error.asModule)
            const { docs, name, section } = decoded
            console.log(`Sudo Transaction Error: ${section}.${name}: ${docs.join(' ')}`)
        } else {
            // Other, CannotLookup, BadOrigin, no extra info
            console.log(`Sudo Transaction Error: ${error.toString()}`)
        }
    }
})