QuickJS supports (almost) the full ES2020 spec, and boasts close to a 100% pass rate of the ECMAScript Test Suite. For more information on QuickJS itself you can visit https://bellard.org/quickjs/
While care has been taken to make sure this update is backward compatible with existing Umajin projects, there are some things to take note of.
The const keyword
Probably the most common thing to note is that the const keyword was not enforced in the old engine, but now it is. This means any code that redefines or modifies the value of a const variable would have been working before, but now will throw an error.
QuickJS supports the ES2020 spec, but ES modules are probably the most notable missing feature. However, Umajin still support loading CommonJS module files using require, just like it always has.
ES modules are still planned for a future update though, so keep an eye out for that.
QuickJS has mathematical extensions that add support for bigint, as well as BigDecimal and BigFloat, but they have not been enabled in the current release due to compatibility.
Component reference types have changes
New language features