Yielding a promise from async generator

[agentcooper]

This is a bug report.

It seems that the generated code for async generators is incorrect.

Search Terms: yield, promise, async, generator.

• TypeScript version: master
• Node version: 10.0.0

// example.ts

function withDelay(n) {
  return new Promise(resolve => setTimeout(() => resolve(n), 1000));
}

async function* makeGenerator() {
  yield 1;
  yield withDelay(2);
  yield 3;
  yield withDelay(4);
  yield 5;
}

async function pull(gen) {
  const { done, value } = await gen.next();
  console.log('out', value);
  if (!done) {
    await pull(gen);
  }
}

pull(makeGenerator());

Expected behavior:

node example.ts # no TS specific stuff in the code

Output:

out 1
out 2
out 3
out 4
out 5
out undefined

Same behavior in Chrome 66 and Firefox 58.

Actual behavior:

node built/local/tsc.js --lib 'esnext,esnext.asynciterable' example.ts
node example.js

Output:

out 1
out Promise { <pending> }
out 3
out Promise { <pending> }
out 5
out undefined

Playground link

Possible fix:

I was able to fix this by changing __asyncGenerator:

diff --git a/src/compiler/transformers/esnext.ts b/src/compiler/transformers/esnext.ts
index 68861a5bb4..9a8721e279 100644
--- a/src/compiler/transformers/esnext.ts
+++ b/src/compiler/transformers/esnext.ts
@@ -909,7 +909,15 @@ namespace ts {
                 function step(r) { r.value instanceof __await ? Promise.resolve(r.value.v).then(fulfill, reject) : settle(q[0][2], r);  }
                 function fulfill(value) { resume("next", value); }
                 function reject(value) { resume("throw", value); }
-                function settle(f, v) { if (f(v), q.shift(), q.length) resume(q[0][0], q[0][1]); }
+                function settle(f, v) {
+                    if (v.value instanceof Promise) {
+                        v.value.then(function(resolvedValue) {
+                            settle(f, { value: resolvedValue, done: v.done });
+                        });
+                    } else {
+                        if ((f(v), q.shift(), q.length)) resume(q[0][0], q[0][1]);
+                    }
+                }
             };`
     };

Would you be ok with such pull request?

[j-oliveras]

As a workaround, adding await before calls to withDelay works with both (original and generated code):

function withDelay(n) {
  return new Promise(resolve => setTimeout(() => resolve(n), 1000));
}

async function* makeGenerator() {
  yield 1;
  yield await withDelay(2);
  yield 3;
  yield await withDelay(4);
  yield 5;
}

async function pull(gen) {
  const { done, value } = await gen.next();
  console.log('out', value);
  if (!done) {
    await pull(gen);
  }
}

pull(makeGenerator());

[nmain]

Is this related to #21115?

[falsandtru]

Duplicate of #23731

[mhegazy]

thanks @agentcooper!