Here is a quick summary of the current status for the DWScript JavaScript CodeGen, aka OP4JS.
JavaScript CodeGen for DWScript: what works, what doesn’t
Pretty much everything is working, including class methods (virtual or not), var & lazy parameters, records, arrays (dynamic and static), functions/method pointers/delegates, interfaces, operator overloading, etc. in short, the list of “what works” would amount to listing almost every DWS language feature, so I’ll just list what isn’t supported instead, or supported with quirks.
Of course, take this list with a grain of salt, it may be obsolete in the near future 😉
Destructors: JavaScript doesn’t exactly have the concept, you can just “delete” and null fields. Though DWS being garbage-collected, destructors are in DWS more for compatibility with Delphi and special tasks, and you can often go on just fine without them. In practice the limitation is that in a Delphi-side execution, when an object is GC’ed, it’s destructor code will be executed, while on the JS-side, a destructor will only get executed if manually invoked.
DateTime: Date/Time functions are currently not supported JS-side. Though contrarily to the rest of the RTL functions, these will involve a rather heavy framework, as JavaScript built-in features for manipulating or converting date & time are rather lacking.
Variants: they are supported only in limited cases, mostly revolving around passing them around, f.i. as parameters. The reason they aren’t in lies with figuring out what should happen when you do cross-type operations and comparisons (f.i. when comparing a string to a variant, a variant to a string, etc.). The JS resolutions for those are different than Delphi’s, and to be honest, I’m not really sure what the Delphi rules are in all the cross-type cases in the first place. Another option being considered is to not worry about Delphi-side behaviors, and just use the JS behaviors.
StackTrace: Exception’s StackTrace member isn’t supported yet. At the moment stack traces in JS involve browser-specific code, so this may require a significant framework.
ExceptObject: this is a special entity that in Delphi allows to access the current exception, there is AFAIK no direct equivalent in JS, and emulating it would involve a significant amount of extra JS code. Since its usage scenarios are infrequent, it is currently of low priority.