Hoist await and yield

[novemberborn]

General code cleanup and dependency upgrades, as well as a fix for avajs/ava#987 (see last commit).

The await and yield expressions may still throw of course. I'm wondering if we want to capture those exceptions with this helper? They wouldn't be due to misuse of t.throws() however.

[sindresorhus]

Looks good :)

@jfmengels Could you review 0f850a2 ?

[jfmengels]

This looks great. And pretty nice to read too (haven't looked at Babel code in a while).

I just have one concern for which I'd like to see some tests, but otherwise, LGTM :)

[jfmengels]

You are replacing the Callxpression by an ExpressionStatement here. Would the following test code stll work, or would Babel crash or something because you are building an odd AST?

test('should foo...', async t => {
  t.is(
      t.throws(foo(await quux())).message,
     'Could not find X'
   );
});

My worry is that you can't AFAIK pass a statement as the argument of a function (here, the `t.is(){ call). But I don't know how Babel would react to that. Would you mind adding a test case and see how that pans out?
I would be fine with not entirely supporting this edge case, but it should at the very least not crash.

[novemberborn]

Yea I doubt that'll work… in your example t.throws is inside an ArgumentExpression so we'd have to walk the path back to the test implementation and detect any ArgumentExpression nodes, then prepend the hosted statements to node's parent.

But then what about:

test(async t => {
  (function* () {
    const err = await t.throws(foo(yield))
    t.true(err.message === 'hahaha')
  })()
})

That scenario is further complicated by this plugin running after async/await has been transpiled into a generator…

[sindresorhus]

So, what should we do about it? Just ignore it as an edge-case?

[novemberborn]

I haven't had the time to get back to this. We could explore whether we can run this plugin before any other presets and plugins. Alternatively the generator compilation (for Node.js v4 and up) should reduce to yield expressions.

[sindresorhus]

Relevant: babel/babel#3281

---

Alternatively the generator compilation (for Node.js v4 and up) should reduce to yield expressions.

I would be ok with only supporting Node.js 4 for this.

[novemberborn]

Another approach may be to rewrite t.throws to something like:

t.throws(
  (
    t._throwsArgStart(),
    t._throwsArgEnd(fn(await foo))
  ),
  TypeError
)

_throwsArgStart() could set a flag, which is unset by _throwsArgEnd(). This function returns its first argument. If an exception occurs while the flag is set (caused by the fn(await foo) invocation) then that's because of improper use. Otherwise the promise or callback is passed to t.throws() and the assertion runs as normal.

[sindresorhus]

@novemberborn I like that approach. I can't really think of any immediate downsides of doing that.

@jfmengels Thoughts? ⬆️

[novemberborn]

Closing in favor of #8.