Problem

Continuing our convo

• Feature Request: local eval for SES (Secure EcmaScript) support #957

We have no plans to support non-strict local eval or with - unfortunately they are incompatible with our code generation strategy and bytecode instructions. It is theoretically possible to implement them on top of JS objects with the existing bytecode, but the effort would be significant and the performance truly appalling...

...FWIW, I think we might be able to add support for with. It feels wrong to completely ignore a language feature that has been with us for many years. It also feels good to pass most of test262. Plus, the implementation is interesting and will not slow everything down. But I can't promise it is high priority.

in order to introduce SES to our RN ecosystem (starting with metamask-mobile)

we can get by without local eval, but with statements are required

Solution

with statements support 🙏

Alternatives considered

• facebook/hermes community contribution: with statements
• more detailed in Feature Request: local eval for SES (Secure EcmaScript) support #957
• Idea: Compartment Sandbox without with-statement or proxy endojs/endo#1561

Additional Context

https://github.com/facebook/hermes/blob/main/doc/Features.md#excluded-from-support

Excluded From Support

• Symbol.unscopables (Hermes does not support with)
• with statements

hermesengine.dev/playground

# with ([1, 2, 3]) {
#   console.log(toString()); // 1,2,3
# }

/tmp/hermes-input.js:1:1: error: invalid statement encountered.
with ([1, 2, 3]) {
^~~~~~~~~~~~~~~~~~
Emitted 1 errors. exiting.