ES2017 Async Generator Suspended on `yield*` Continues Execution When Returned

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🔎 Search Terms es2017 async generator delegator await loop yield* 🕗 Version & Regression Information This is the behavior in every version I tried, and I reviewed the FAQ for entries about async generators ⏯ Playground Link https://www.typescriptlang.org/play/?declaration=false&module=0&ts=5.8.0-dev.20250122#code/PQgEB4CcFMDNpgOwMbVAGwJYCMC8AiaAZ0WgA8AXfUYAPgCh6AKAQyIE8VQmBKUXWqADe9UGJrBQAQQAmLAA4VoM0LEgB7ALYAuUeLAALChXlFtIAO5WAdBXbziySJkXoWiAObX1kD8BnqyETABu4y2OrqANbAMOjQbNAAtIjqSsF2DkROLhRJAExJAMzWRproemJsnMiqAK4oFJjqiABUoB68wpXiYuyY0Ogy7awcXLANyE0tI3wivQvi-YMqAIwA3D29AL48vJtb4sgtROrx1ujqnfgssEqQ+DybO4y9x4hEFKCYX7gdvABtADK7E0EXQ1mqKAAkvcWBQfABdfY9d6nc6XTosCwsH7fCjWUiUXhPHrY3FfH7WGAUOqQRAAfmsKN2KKAA 💻 Code /// <reference lib="esnext" /> (async () => { // Adapted from: // https://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-3.html async function* g() { yield* (async function* () { yield 1; })(); console.log("after"); } const it = g()[Symbol.asyncIterator](); console.log(await it.next()); await it.return?.(); })(); 🙁 Actual behavior If you run the provided TS code directly in a modern Node process, you get the expected following output: { value: 1, done: false } If you instead run the JS output code, you get: { value: 1, done: false } after Here, the downleveled code behaves differently than the source and continues execution after .return() was called in the iterator. 🙂 Expected behavior The generated code should work like the original. Additional information about the issue This only seems to be an issue when yield*ing values from another generator. The following works just fine: /// <reference lib="esnext" /> (async () => { async function* g() { yield* [1, 2]; console.log('after'); } const it = g()[Symbol.asyncIterator](); console.log(await it.next()); await it.return?.(); })(); If you extract the generator into a constant and put the log in a while loop, the loop never exits: /// <reference lib="esnext" /> (async () => { async function* g() { const x = (async function* () { yield 1; })(); for (let i = 0; i < 15; i++) { yield* x; console.log('after'); } } const it = g()[Symbol.asyncIterator](); console.log(await it.next()); await it.return?.(); })(); { value: 1, done: false } after after after after after ... If you inline the generator from the above example, the "after" is still logged, but the loop isn't run. Only if you extract the inline generator into a separate constant and only yield that within the loop instead, it keeps going. /// <reference lib="esnext" /> (async () => { async function* g() { for (let i = 0; i < 15; i++) { yield* (async function* () { yield 1; })(); console.log('after'); } } const it = g()[Symbol.asyncIterator](); console.log(await it.next()); await it.return?.(); })(); { value: 1, done: false } after The above does not apply to generators that only yield* themselves: /// <reference lib="esnext" /> (async () => { async function* id(x) { yield* x; } async function* g() { const x = (async function* () { yield 1; })(); for (let i = 0; i < 15; i++) { yield* id(id(x)); console.log('after'); } } const it = g()[Symbol.asyncIterator](); console.log(await it.next()); await it.return?.(); })(); { value: 1, done: false } after after after after after ...