Is "yield await" redundant in JavaScript async generator functions?

Question

Promises yielded within async function* declarations seem to be automatically awaited before the iterator result is sent to the caller (source: MDN). Does this mean that yield await somePromise is redundant and yield somePromise will produce the same behavior in all cases? Or, are there exceptions to watch out for?

P.S. I thought there might be some behavioral differences related to error handling contexts and try/catch. From my experimentation though, it seems that a promise yielded in a try block will always be caught and handled within the async generator, regardless if there is an explicit await.

Yes

Bergi
– Bergi

2023-08-31 01:31:54 +00:00
Commented Aug 31, 2023 at 1:31 — Bergi
– Bergi, Commented Aug 31, 2023 at 1:31

Hao Wu · Accepted Answer · 2023-08-31 02:18:13Z

2

The await is quite redundant here. Using await also delays the resolve timing in the microtask queue.

Promise.resolve()
  .then(() => console.log(1))
  .then(() => console.log(2))
  .then(() => console.log(3))
  .then(() => console.log(4))
  .then(() => console.log(5));
  
(async () => {
  const func = async function*() {
    yield await Promise.resolve('yield await');
  };
  
  for await (let i of func()) {
      console.log(i);
  }
})();

Promise.resolve()
  .then(() => console.log(1))
  .then(() => console.log(2))
  .then(() => console.log(3))
  .then(() => console.log(4))
  .then(() => console.log(5));
  
(async () => {
  const func = async function*() {
    yield Promise.resolve('yield');
  };
  
  for await (let i of func()) {
      console.log(i);
  }
})();

answered Aug 31, 2023 at 2:18

Hao Wu

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