Define message for non-Error errors

[aryanjassal]

Description

If a non-Error error is thrown from within a RPC handler, the message field would be undefined. This would result in the message field (which is expected to be present) be omitted, breaking the parsing step.

This PR addresses that by wrapping that in a nicer message. For example, if we do throw 123, the message becomes 'Non-error class Number was thrown'. If we do throw new ReadableStream(), then the message becomes 'Non-error class ReadableStream was thrown'. You get the idea.

Issues Fixed

• Relates to PR Wrapping VaultOps errors in ErrorPolykey errors and updating tests Polykey#838

Tasks

• 1. If the message does not exist, substitute it with something else
• 2. Run tests to ensure this behaviour is consistent

Final checklist

• Domain specific tests
• Full tests
• Updated inline-comment documentation
• Lint fixed
• Squash and rebased
• Sanity check the final build

[aryanjassal]

I was able to make the message guessing function more complex, so now it should return something like this:

// For a number 123
> console.log(composeMessage(e));
Non-error literal 123 was thrown

// For regular messages
> console.log(composeMessage(e));
this was the error message

// For objects, like ReadableStream or Array
> console.log(composeMessage(e));
Non-error object ReadableStream was thrown

I'm not too happy about needing to make a separate function to handle this logic as it won't fit in one line, and is required to be repeated, so this can't be defined where needed. Rather, I had to define composeMessage inside the RPCServer.ts itself. I didn't export it, so it becomes like a private utility function.

Thoughts, @tegefaulkes?

[aryanjassal]

All the tasks have been completed, and this PR has been approved. Merging.

[CMCDragonkai]

Generally I prefer not to make application concerns a library functionality. The exact reporting of a message - specifically it's template or toString impl should be generic and allowed to be overridden by the downstream user. This is how you ensure reusable modules/libraries.

[aryanjassal]

Generally I prefer not to make application concerns a library functionality. The exact reporting of a message - specifically it's template or toString impl should be generic and allowed to be overridden by the downstream user. This is how you ensure reusable modules/libraries.

Should I make an issue to track this?

[CMCDragonkai]

If you think about the levels of genericity:

1. js-errors is generic to any error usage
2. js-rpc is generic to any rpc usage - but now we know the error is specific to RPC
3. Polykey is generic to any Polykey usage - but now we know the error is specific to Polykey
4. ... ladder of consumption