function isPromiseLike
Since 0.3.0
isPromiseLike<T>(value: T | PromiseLike<T>): value is PromiseLike<T>

Reports whether a value is thenable, by the same duck-typed test the platform serve layers use rather than instanceof Promise.

The distinction is load-bearing on the request path (M87). instanceof asks whether a value was built by this realm's Promise constructor, and answers false for a promise from another realm (a vm context, a worker) and for every userland promise library. Those values satisfy RouteHandler's declared Promise<HandlerResult> structurally, so TypeScript accepts them and only the runtime check can tell them apart — and a request path that treats one as "already finished" sends the response while the handler is still running, with no error anywhere.

Both @setu-ts/kernel and @setu-ts/runtime decide "did this need awaiting?" on the hot path, so the rule lives here and neither can drift from the other about what counts as asynchronous.

Examples

Example 1

const result = handler(ctx);
// `Promise.resolve` returns a native promise unchanged, and adopts a
// foreign thenable, so the fast path allocates nothing.
return isPromiseLike(result) ? Promise.resolve(result) : undefined;

Type Parameters

T

The value the thenable resolves to, preserved by the guard so the narrowed branch keeps its type rather than widening to unknown

Parameters

value: T | PromiseLike<T>

Any value a handler or framework layer produced

Return Type

value is PromiseLike<T>

true when the value exposes a callable then

Usage

import { isPromiseLike } from "common/src/index.ts";