Hermes and Jest integration

[joe-sam]

Problem

When jest unit tests run they are typically run in a virtual environment vmContext typically emulating nodejs or browser (jsdom).

To make a clean and safe transition from plain JSC to Hermes it would be considerably quicker to troubleshoot by running various fundamental unit jest tests under hermes VM engine, before any e2e tests are required.

Solution

Should be an easier way to configure the setup so as to run the unit tests under a specially built Hermes VM context module, and then compare the unit test results to a JSC context.

Additional Context

For more info see

[nandorojo]

Did you figure out a way to do this? I’m migrating to Hermes for iOS and am very concerned about the lack of Intl support. I’m thinking of using a dates polyfill, but I want to run tests to ensure their accuracy.

[joe-sam]

No, unfortunately I haven't figured out a way to do this exactly as there were still some critical missing features in Hermes implementation at that time notably the implementation of a runtime eval which makes awkward sense as hermes is a bytecode compiler. If this changes it would be entirely possible to write your own testbench framework in a few hundred lines as explained by Christoph Nakazawa building a javascript testing framework such a gem of wonderful tutorial on understanding and expanding the usage of the Jest f/w by an original implementer.

The status for Hermes iOS support seems to be open ended with some users claiming that Intl is fully (???) supported but RN framework apparently hasn't yet re-wired support to the new libs (projected ETA @ RNv0.71) (statistically that would be about 1.2 years as each major release seems to take a more than a quarter of a year). Hermes itself undergoes a battery of Intl tests but it is hard to see which ones have been skipped to pass the CI.

Follow this other thread for further updates on Intl , which in my opinion is a huge project having thousands of edge cases and my estimate is it would take many man months of effort.

[steveluscher]

I would love to have this to test https://github.com/solana-labs/solana-web3.js. I want unit tests that protect against the React Native bundle breaking because someone introduced a primitive that's not available in Hermes. Things like this have happened countless times in the past (SubtleCrypto not being available, JSC not supporting BigInt literals, Intl differences, et cetera).

[tmikov]

As far as I can tell, there is no concrete ask for the Hermes team here. We are not experts on Jest and unfortunately we don't know why Jest doesn't work with Hermes or what would be required to change that either in Hermes or in Jest.

Moving this to Discussions. If concrete Hermes features get request as a result, please file them as issues.

[ChALkeR]

Made https://github.com/ExodusMovement/test which kinda works on hermes (with --engine=hermes:bundle) and is (mostly) jest-compatible as a drop-in replacement

Found some Hermes bugs with it :-/