Almost a year after first working on this, it's finally done: an
implementation of Promises for LibJS! :^)
The core functionality is working and closely following the spec [1].
I mostly took the pseudo code and transformed it into C++ - if you read
and understand it, you will know how the spec implements Promises; and
if you read the spec first, the code will look very familiar.
Implemented functions are:
- Promise() constructor
- Promise.prototype.then()
- Promise.prototype.catch()
- Promise.prototype.finally()
- Promise.resolve()
- Promise.reject()
For the tests I added a new function to test-js's global object,
runQueuedPromiseJobs(), which calls vm.run_queued_promise_jobs().
By design, queued jobs normally only run after the script was fully
executed, making it improssible to test handlers in individual test()
calls by default [2].
Subsequent commits include integrations into LibWeb and js(1) -
pretty-printing, running queued promise jobs when necessary.
This has an unusual amount of dbgln() statements, all hidden behind the
PROMISE_DEBUG flag - I'm leaving them in for now as they've been very
useful while debugging this, things can get quite complex with so many
asynchronously executed functions.
I've not extensively explored use of these APIs for promise-based
functionality in LibWeb (fetch(), Notification.requestPermission()
etc.), but we'll get there in due time.
[1]: https://tc39.es/ecma262/#sec-promise-objects
[2]: https://tc39.es/ecma262/#sec-jobs-and-job-queues
(...and ASSERT_NOT_REACHED => VERIFY_NOT_REACHED)
Since all of these checks are done in release builds as well,
let's rename them to VERIFY to prevent confusion, as everyone is
used to assertions being compiled out in release.
We can introduce a new ASSERT macro that is specifically for debug
checks, but I'm doing this wholesale conversion first since we've
accumulated thousands of these already, and it's not immediately
obvious which ones are suitable for ASSERT.
