Checking if a provider supports subscriptions by checking for "on" property often breaks

[dmihal]

When a transaction is sent, web3 chooses to either poll for a receipt, or subscribe to new blocks. However, it makes this decision by checking if on is a property of the provider:

https://github.com/ethereum/web3.js/blob/af6472976c23cb61c05e925eebe5734d141384f4/packages/web3-core-method/src/index.js#L515-L521

This means if a project is using a provider that includes on, such as web3-provider-engine, it will incorrectly try to subscribe. This can cause errors when sending a transaction:

The workaround I've used is to remove on from a provider, however I imagine this may break some providers or apps.

const provider = new ProviderEngine();
provider.on = null;
const web3 = new Web3(provider)

I'm not sure what the correct way is to determine whether a provider supports subscriptions is, maybe this needs to be addressed in the provider spec/API?

[cgewecke]

@dmihal Thanks for reporting. Out of curiosity, which provider-engine version are you using?

[dmihal]

Thanks @cgewecke, I'm using web3-provider-engine@15.0.6

I also had the same problem while building burner-wallet/burner-connect-provider. I wanted to implement the EventEmitter interface, but had to remove on as well: