# closeout/once > Every export of closeout/once, with its signature and doc comment: once. Source: https://closeout.interlace.tools/docs/api/once `once(fn)` — a function that runs at most once and returns its first result thereafter (design R5). Two packages, 262 M downloads a week between them, for this: `onetime` (162.3 M/wk) calls `mimic-fn` (99.7 M/wk) so that the wrapper it returns still answers to the wrapped function's `name` and `length`. Both are inside this layer's own incumbent tree — `restore-cursor` → `onetime` → `mimic-fn` — so the drop-in recipe is incomplete without it, which is the only reason a once-wrapper lives in a package about exiting. ## What "preserving" means here, and why each half is load-bearing **`name` and `length`.** Not cosmetics: a wrapper whose `name` is `''` turns every stack frame and every deadline report into `(anonymous)`, and this package's own report names handlers by `fn.name`. Wrapping a handler in `once()` must not be the reason a hang becomes unattributable. **`this`.** A plain arrow would swallow the receiver, so `obj.method = once(obj.method)` would call the method against `undefined` — the failure arrives later, somewhere else, and looks nothing like the line that caused it. The wrapper is therefore a `function` expression that forwards its own `this`, which is the one thing an arrow cannot do. Zero dependencies, and about as many lines as `mimic-fn`'s README. ```ts import { once } from 'closeout/once'; ``` ## Functions ### once Wrap `fn` so it runs at most once. Later calls do not run it again and do not throw: they return the first result, which is what makes this safe on an exit path where two triggers race. The arguments of a second call are ignored, deliberately and visibly — a "once" that quietly re-ran for different arguments would be a memoiser, and a memoiser on a shutdown handler is a bug with a friendly name. ```ts function once(fn: T): T; ``` | Parameter | Type | | :-- | :-- | | `fn` | `T` | **Returns** `T`