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JavaScript — the reference

Target: JavaScript (browser & Node) · Lineage: compiler-integrated (this is purs) · By: the PureScript core team · Status: reference, definitional · Repo: purescript/purescript

The JavaScript backend is built into the compiler: purs lowers CoreFn to CoreImp and emits optimized ES modules. Its virtue is being everywhere — and being the semantic reference every other backend measures against. The browser and Node are the same code generator in two habitats; what differs is the runtime and FFI ecosystem, and therefore which programs make sense.

At a glance

	Browser	Node
ADT values	constructor functions + `instanceof`, fields `value0…`	same
Records	JS objects	same
Int	double wrapped `\|0` → int32	same
Strings	UTF-16 code units	same
Effect	nullary thunk; MagicDo collapses binds	same
TCO	optimizer: self-tail-calls → `while` loops	same
Concurrency	event loop, workers	event loop, worker_threads
Native niche	DOM/UI (Halogen, react bindings)	servers, CLIs, tooling
Library coverage	entire registry	entire registry

Because it's the reference, its quirks become the spec the others must decide whether to honour — most consequentially int32 Int (which overflows on sumTo 1000000) and UTF-16 strings.

Also in this habitat

purs-backend-es (Faubion / Arista) is an optimizing JS generator built on the shared backend-optimizer IR — same habitat, different generator. It belongs on the matrix as a control column.