Alternate "slim" version of empower.

[jamestalmage]

@twada

Poking through the code, I think there is room for a "slim" version of empower.

If my understanding is correct, then the following:

saveContextOnRethrow: true,
modifyMessageOnRethrow: false

Does not require a formatter at all. It just attaches the context and re-throws a copy.

In the "slim" version I am picturing, it would only take a patterns argument.

Also, instead of re-throwing an error (which messes with the existing stack trace needlessly). You could just use serialize-error attach the context, and throw.

We would format and print the message on the main thread using powerAssertContext.

Is what I am saying making sense?

[twada]

@jamestalmage Yeah, that's exactly what I'm thinking of.

[twada]

Let me explain a history behind the code.

Also, instead of re-throwing an error (which messes with the existing stack trace needlessly). You could just use serialize-error attach the context, and throw.

I've implemented the same strategy, but some of old browsers (including PhantomJS) have strange behavior. When I rethrow Errors after attaching some props, the props disappear.

try {
    try {
        ...
    } catch (e) {
        e.foo = 'bar';
        throw e;
    }
} catch (e) {
    assert(e.foo === 'bar');  // Error on old PhantomJS
}

refactor(empower): always re-instantiate AssertionError on rethrow · power-assert-js/empower@997da24

However, I found that the attached props are remaining on current PhantomJS.
So I can just attach and rethrow the original Error now.

It's a delightful discovery. Thanks!

[jamestalmage]

@twada - serialize-error actually does not create an error. It creates a plain old javascript object with deep copies of the Errors properties. An Error object does not survive JSON.stringify() very well. Especially assertion errors that have circular references.

Check the unit tests for all it does.

For use outside AVA, throwing non-errors is probably a bad idea. Could you maybe split most of the functionality into another module? empower-core or something. It should take a copyAssertion callback. empower can continue to copy the Assertion the way you've been doing it here, and we can make a custom version that uses serialize-error to make the copy.

[twada]

@jamestalmage

Could you maybe split most of the functionality into another module? empower-core or something. It should take a copyAssertion callback.

Yeah I'm working on that !

[twada]

Update: Now in a beta stage

[twada]

Close this issue since twada/empower-core is going on.