🚀 Inline

Collapse nested yield* delegation into a single flat iterator for performance-critical code paths.

On rare occasions, yield* syntax can cause a performance degradation, for example when there are many, many levels of recursion. The reason is because when javascript composes generators via yield*, each level of nesting creates an intermediate generator frame. The engine must unwind through every one of these frames on each call to .next(), .return(), or .throw(). For deeply nested or recursive operations, the cost of resuming a single yield point is O(depth).

Most of the time, this overhead is negligible and you should use plain yield* — it's type-safe, gives you clear stack traces, and composes naturally. But if you've profiled your code and identified deep yield* nesting as a bottleneck (e.g. recursive operations or tight inner loops with many layers of delegation), inline() lets you opt into a flat execution model where the cost is O(1) regardless of depth.

Instead of delegating with yield*:

let value = yield* someOperation();

Use inline() with a plain yield:

import { inline } from "@effectionx/inline";

let value = (yield inline(someOperation())) as SomeType;

The trade-off is that the return type is unknown (requiring a cast), and you lose the natural generator stack trace.

Note: Source map support for the build-time transforms is not yet available but is planned for a future release.

Build-time Transform

Instead of manually converting each yield* call, you can apply the inline optimization automatically at build time. The transform rewrites every yield* expression inside generator functions into the equivalent yield inline(...) call. This means you can benefit from type-safety and helpful stack traces while developing, but ship optimal code to production.

esbuild

import { build } from "esbuild";
import { inlinePlugin } from "@effectionx/inline/esbuild";

await build({
  entryPoints: ["src/index.ts"],
  bundle: true,
  plugins: [inlinePlugin()],
});

SWC

A compiled WASM plugin is available for use with @swc/core or any SWC-based toolchain (e.g. Next.js, Parcel):

import { transformSync } from "@swc/core";

let result = transformSync(source, {
  jsc: {
    experimental: {
      plugins: [["@effectionx/inline/swc", {}]],
    },
  },
});

Both transforms produce identical output: they add import { inline as $$inline } from "@effectionx/inline" and rewrite yield* expr() into (yield $$inline(expr())).

You can opt out of the transform for specific functions with a /** @noinline */ JSDoc annotation, or for an entire file by adding "no inline"; as the first statement.