stream: support flatMap

Support the `flatMap` method from the iterator helper TC39 proposal on
readable streams.

Co-Authored-By: Robert Nagy <ronagy@icloud.com>
PR-URL: https://github.com/nodejs/node/pull/41612
Reviewed-By: Robert Nagy <ronagy@icloud.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>

1 files changed, 8 insertions, 0 deletions

diff --git a/lib/internal/streams/operators.js b/lib/internal/streams/operators.js
index 9c50865f3da..1218942b641 100644
--- a/lib/internal/streams/operators.js
+++ b/lib/internal/streams/operators.js
@@ -229,8 +229,16 @@ async function toArray(options) {
   }
   return result;
 }
+
+async function* flatMap(fn, options) {
+  for await (const val of this.map(fn, options)) {
+    yield* val;
+  }
+}
+
 module.exports.streamReturningOperators = {
   filter,
+  flatMap,
   map,
 };