@mrousavy adding BigInt would touch a lot of components:

Parser support for BigInt literals.
BigInt literals and instructions have to be added to the IR and the type system (for type inference).
The bytecode file format has to be extended with a bigint table and instructions to load from it.
HermesValue has to be extended with a new type tag and new "first-class" VM object has to be added. This is assuming that BigInt-s will always be kept on the heap. If small ones will be kept inline, it gets even more complicated.
The interpreter has to support the new instructions.
All internal operations have to support the new type (like add, etc).
It is not totally clear whether interpreter fast paths will be affected. Presumably if BigInt-s are kept only on the heap, they will be relatively slow so probably should be kept out of the fast paths.
Lastly, JSLib (the standard JS library) has to extended.

Overall, I would say that it is a pretty involved project, but is certainly doable.

I am not actively working on Hermes anymore (I am currently focused on Juno), so I will let members of the Hermes team chime in here. cc: @neildhar

BigInt support #510

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