From df038ab377acaacb113171ec360dacaf1141faea Mon Sep 17 00:00:00 2001
From: Mattias Buelens <mattias@buelens.com>
Date: Sat, 26 Dec 2020 15:06:00 +0100
Subject: [PATCH] Implement CreateAsyncFromSyncIterator abstract op

---
 .../lib/abstract-ops/ecmascript.js            | 21 +++++++++++++++++--
 1 file changed, 19 insertions(+), 2 deletions(-)

diff --git a/reference-implementation/lib/abstract-ops/ecmascript.js b/reference-implementation/lib/abstract-ops/ecmascript.js
index d277aa724..edb69dfac 100644
--- a/reference-implementation/lib/abstract-ops/ecmascript.js
+++ b/reference-implementation/lib/abstract-ops/ecmascript.js
@@ -56,6 +56,23 @@ exports.GetMethod = (V, P) => {
   return func;
 };
 
+exports.CreateAsyncFromSyncIterator = syncIteratorRecord => {
+  // Instead of re-implementing CreateAsyncFromSyncIterator and %AsyncFromSyncIteratorPrototype%,
+  // we use yield* inside an async generator function to achieve the same result.
+
+  // Wrap the sync iterator inside a sync iterable, so we can use it with yield*.
+  const syncIterable = {
+    [Symbol.iterator]: () => syncIteratorRecord.iterator
+  };
+  // Create an async generator function and immediately invoke it.
+  const asyncIterator = (async function* () {
+    return yield* syncIterable;
+  }());
+  // Return as an async iterator record.
+  const nextMethod = asyncIterator.next;
+  return { iterator: asyncIterator, nextMethod, done: false };
+};
+
 exports.GetIterator = (obj, hint = 'sync', method) => {
   assert(hint === 'sync' || hint === 'async');
   if (method === undefined) {
@@ -63,8 +80,8 @@ exports.GetIterator = (obj, hint = 'sync', method) => {
       method = exports.GetMethod(obj, Symbol.asyncIterator);
       if (method === undefined) {
         const syncMethod = exports.GetMethod(obj, Symbol.iterator);
-        const syncIterator = exports.GetIterator(obj, 'sync', syncMethod);
-        return syncIterator; // TODO sync to async iterator
+        const syncIteratorRecord = exports.GetIterator(obj, 'sync', syncMethod);
+        return exports.CreateAsyncFromSyncIterator(syncIteratorRecord);
       }
     } else {
       method = exports.GetMethod(obj, Symbol.iterator);