Errors

Pallet errors are thrown out when things go wrong in the runtime, those are defined in the metadata. Available errors for each pallet are also exposed in ChainApi interface, so we can get information an error through this syntax: client.errors.<pallet>.<errorName>. E.g: Available errors for Polkadot network can be found here.

Similar to Event API, this API is helpful when we want to check if an error maches with an error that we're expecting.

Example to check if an error is AlreadyExists from Assets pallet:

// From system events
await client.query.system.events(async (records) => {
  for (const tx of records) {
    if (client.events.system.ExtrinsicFailed.is(tx.event)) {
      const { dispatchError } = tx.event.palletEvent.data;
      if (client.errors.assets.AlreadyExists.is(dispatchError)) {
        console.log('Assets.AlreadyExists error occurred!');
      } else {
        console.log('Other error occurred', dispatchError);
      }
    }
  }
});

Example checking for error details in transaction callback:

// Or from events in transaction callback
client.tx.balances.transferKeepAlive('<BOB_ADDRESS>', 1_000n)
  .signAndSend(aliceAddressOrKeyPair, ({ dispatchError, events }) => {
    if (dispatchError) { // DispatchError extracted from ExtrinsicFailed event if any
      // Check for specific error
      if (client.errors.balances.InsufficientBalance.is(dispatchError)) {
        console.log('Balance too low to send value!');
      } else {
        console.log('Other error occurred', dispatchError);
      }
      
      // Check by finding error metadata
      const errorMeta = client.registry.findErrorMeta(dispatchError);
      if (errorMeta) {
        const { pallet, name, docs } = errorMeta;
        console.log(`Error: [${pallet}][${name}] - ${docs.join(',')}`);
      }
    }
  });