feat: merge

src/merge.ts

@@ -0,0 +1,32 @@
+const recursivePromiseBuilder = <T>(
+	reader: ReadableStreamDefaultReader<T>,
+	controller: ReadableStreamDefaultController<T>,
+) => {
+	const recursivePromise = async () => {
+		const readResult = await reader.read();
+
+		if (!readResult.done) {
+			controller.enqueue(readResult.value);
+
+			return recursivePromise();
+		}
+	};
+
+	return recursivePromise();
+};
+
+export const merge = <T>(...readableStreams: ReadableStream<T>[]) => {
+	const fallbackedStreams = readableStreams ?? [];
+
+	return new ReadableStream<T>({
+		async pull(controller) {
+			await Promise.all(
+				fallbackedStreams.map((stream) =>
+					recursivePromiseBuilder(stream.getReader(), controller),
+				),
+			);
+
+			controller.close();
+		},
+	});
+};

test/merge.test.ts

@@ -0,0 +1,33 @@
+import { describe, expect, test } from "vitest";
+
+import { fromIterable } from "../src/fromIterable";
+import { merge } from "../src/merge";
+import { pipeline } from "../src/pipeline";
+import { toArray } from "../src/toArray";
+
+describe("merge", () => {
+	test("should merge streams", async () => {
+		const destinationArray = [];
+
+		await pipeline(
+			merge(fromIterable([1]), fromIterable([2]), fromIterable([3])),
+			toArray(destinationArray),
+		);
+
+		expect(destinationArray).toContain(1);
+		expect(destinationArray).toContain(2);
+		expect(destinationArray).toContain(3);
+	});
+
+	test("should merge streams even if first stream is empty", async () => {
+		const destinationArray = [];
+
+		await pipeline(
+			merge(fromIterable([]), fromIterable([2]), fromIterable([3])),
+			toArray(destinationArray),
+		);
+
+		expect(destinationArray).toContain(2);
+		expect(destinationArray).toContain(3);
+	});
+});