# Platform Notes

> USearch for JavaScript

### What This Binding Does Not Expose

A JavaScript callback runs only on the thread owning the JavaScript context, and an executor's threads never are.
A thread-safe function would queue onto a loop that sits blocked inside the verb, so the callback could never run.
That is why `attend` runs on the calling thread alone, where its `rowOf` can, and no executor receiver offers it.
It is also why a search `predicate`, a per-candidate callback the engine calls from the threads it fanned out over, is not exposed at all; pull through `searchStream` instead and filter in JavaScript.
The same reasoning excludes the progress handlers every long verb takes.

### Design

- __A thin layer over the C ABI.__
  Every name follows `include/usearch/`: a C verb `usearch_dense_<verb>` is a camel-cased `<verb>` here, every type is its C name without `usearch_` and `_t`, and a verb's argument structure becomes its `Dense*Config`.
  Enums mirror C enums through the C name tables, so a misspelling is refused rather than read as a default.
- __The receiver chooses, the verb stays.__
  The index runs a verb on the calling thread, `on(executor)` blocks on an executor, and `deferred(executor)` submits it, all with the same arguments.
- __Deferred calls use the language's own idioms.__
  `status` polls through `usearch_outcome_status`, `wait()` blocks through `usearch_outcome_wait`, and `await` hands that same wait to the libuv pool through `napi_async_work`, since the library has no wake-up to register.
- __No life of its own.__
  The binding starts no threads and keeps no queues; releasing a `Deferred` rides `using` and `Symbol.dispose`, which waits for the call first.
- __Arenas are lent and reused, never shared.__
  Every output whose length scales with the call lands in an arena the caller passes; a rewind detaches the buffers under the previous call's views, so a stale one throws rather than reading another call's memory.
- __Nothing costs anything until it is used.__
  Submitting a call allocates nothing, and an arena allocates only when a call writes into it.
- __Every wrapper names its C symbol.__
  Each wrapper's documentation names the C function it calls, or says it is binding-only, as `contains`, `keyAt` and `RowsView.of` are.