What is the best way to terminate a "for await ...of" loop if it fails to finish within a specified timeframe?

Is it possible to stop an asynchronous loop if it doesn't finish within a set time frame? I'm working with the following code:

(async()=>{
  for await(let t of asynDataStreamOrGenerator){
    //some data processing
  }
  //some other code I need to run based on whatever data collected by
  //asyncDataStreamOrGenerators within given time period
})()

If the loop exceeds a certain time limit, how can I exit the loop and continue processing the request?

javascriptecmascript-6es6-promise

Answer №1

(Check out the community wiki answer I shared for another approach.)

In one of your comments, you mentioned:

I am working on a consensus algorithm where each source must send a response within a specified time frame. If some participants fail to respond (i.e., do not send values), the loop will hang indefinitely!

This situation sounds like it requires a timeout mechanism. Typically, timeouts are implemented using Promise.race with a promise wrapped around a timer function (setTimeout or similar). The Promise.race function resolves as soon as any of the promises passed to it resolve, regardless of the others' outcomes.

To achieve this, you should consider looping in a different way instead of using for-await-of, and directly utilizing the promise returned by the result object. Suppose you have a utility function like:

const delay = (ms, value) => new Promise(resolve => {
    setTimeout(resolve, ms, value);
});

This function returns a promise that resolves after X milliseconds with a provided value (if any).

Next:

(async () => {
    const TIMEOUT = 500; // Milliseconds
    const GOT_TIMEOUT = {};
    const it = asynDataStreamOrGenerator[Symbol.asyncIterator]();
    try {
        while (true) {
            const p = it.next();
            const result = await Promise.race([p, delay(TIMEOUT, GOT_TIMEOUT)]);
            if (result === GOT_TIMEOUT) {
                // No response received within the allotted time
                console.log("Timeout");
            } else {
                // Response received
                if (result.done) {
                    // Iteration completed
                    console.log("Iteration complete");
                    break;
                }
                // Perform data processing on 'result.value'...
                console.log(`Process ${result.value}`);
            }
        }
    } finally {
        try {
            it.return?.(); // Close the iterator if necessary
        } catch { }
    }
})();

Live Example illustrating random durations for the async operations, but intentionally causing a timeout on the third iteration:

const delay = (ms, value) => new Promise(resolve => {
    setTimeout(resolve, ms, value);
});

async function* example() {
    for (let i = 1; i <= 6; ++i) {
        const ms = i === 3 ? 600 : Math.floor(Math.random() * 100);
        await delay(ms);
        yield i;
    }
}

(async () => {
    const asynDataStreamOrGenerator = example();
    const TIMEOUT = 500; // Milliseconds
    const GOT_TIMEOUT = {};
    const it = asynDataStreamOrGenerator[Symbol.asyncIterator]();
    try {
        while (true) {
            const p = it.next();
            const start = Date.now();
            const result = await Promise.race([p, delay(TIMEOUT, GOT_TIMEOUT)]);
            const elapsed = Date.now() - start;
            if (result === GOT_TIMEOUT) {
                // Timeout occurred
                console.log(`Got timeout in ${elapsed}ms`);
            } else {
                // Response received
                if (result.done) {
                    // Iteration completed
                    console.log(`Got iteration complete result in ${elapsed}ms`);
                    break;
                }
                // Perform data processing on 'result.value'...
                console.log(`Got result ${result.value} to process in ${elapsed}ms`);
            }
        }
    } finally {
        try {
            it.return?.();
        } catch { }
    }
})();
.as-console-wrapper {
    max-height: 100% !important;
}

A variation of the above scenario can be seen where a timeout is triggered during the first iteration only, addressing your concern regarding this particular case:

const delay = (ms, value) => new Promise(resolve => {
    setTimeout(resolve, ms, value);
});

async function* example() {
    for (let i = 1; i <= 6; ++i) {
        const ms = i === 1 ? 600 : Math.floor(Math.random() * 100);
        await delay(ms);
        yield i;
    }
}

(async () => {
    const asynDataStreamOrGenerator = example();
    const TIMEOUT = 500; // Milliseconds
    const GOT_TIMEOUT = {};
    const it = asynDataStreamOrGenerator[Symbol.asyncIterator]();
    try {
        while (true) {
            const p = it.next();
            const start = Date.now();
            const result = await Promise.race([p, delay(TIMEOUT, GOT_TIMEOUT)]);
            const elapsed = Date.now() - start;
            if (result === GOT_TIMEOUT) {
                // Timeout detected
                console.log(`Got timeout in ${elapsed}ms`);
            } else {
                // Response obtained
                if (result.done) {
                    // Iteration complete
                    console.log(`Got iteration complete result in ${elapsed}ms`);
                    break;
                }
                // Perform data processing on 'result.value'...
                console.log(`Got result ${result.value} to process in ${elapsed}ms`);
            }
        }
    } finally {
        try {
            it.return?.();
        } catch { }
    }
})();
.as-console-wrapper {
    max-height: 100% !important;
}

If you aim to continue collecting subsequent values without waiting for the processing, avoid using await during processing tasks (consider queuing up an array of promises representing these tasks and resolving them collectively using Promise.all at the end).

Alternatively, if you intend to terminate the entire operation prematurely:

(async () => {
    const TIMEOUT = 500; // Milliseconds
    const GOT_TIMEOUT = {};
    const results = [];
    const it = asynDataStreamOrGenerator[Symbol.asyncIterator]();
    try {
        while (true) {
            const p = it.next();
            const result = await Promise.race([p, delay(TIMEOUT, GOT_TIMEOUT)]);
            if (result === GOT_TIMEOUT) {
                // No timely response received, bail out
                console.log("Timeout");
                break;
            }
            // Response acquired
            if (result.done) {
                // Iteration complete
                console.log("Iteration complete");
                break;
            }
            console.log(`Received ${result.value}`);
            results.push(result.value);
        }
    } finally {
        try {
            it.return?.();
        } catch { }
    }
    // ...processing logic for items stored in `results`...
    for (const value of results) {
        console.log(`Process ${value}`);
    }
})();

Live Example:

const delay = (ms, value) => new Promise(resolve => {
    setTimeout(resolve, ms, value);
});

async function* example() {
    for (let i = 1; i <= 6; ++i) {
        const ms = i === 3 ? 600 : Math.floor(Math.random() * 100);
        await delay(ms);
        yield i;
    }
}

(async () => {
    const asynDataStreamOrGenerator = example(); // For illustrative purposes
    const TIMEOUT = 500; // Milliseconds
    const GOT_TIMEOUT = {};
    const results = [];
    const it = asynDataStreamOrGenerator[Symbol.asyncIterator]();
    try {
        while (true) {
            const p = it.next();
            const start = Date.now();
            const result = await Promise.race([p, delay(TIMEOUT, GOT_TIMEOUT)]);
            const elapsed = Date.now() - start;
            if (result === GOT_TIMEOUT) {
                // Timed out, exiting early
                console.log(`Got timeout after ${elapsed}ms`);
                break;
            }
            // Response obtained
            if (result.done) {
                // Iteration complete
                console.log(`Got iteration complete after ${elapsed}ms`);
                break;
            }
            console.log(`Got value ${result.value} after ${elapsed}ms`);
            results.push(result.value);
        }
    } finally {
        try {
            it.return?.();
        } catch { }
    }
    // ...processing logic for items stored in `results`...
    for (const value of results) {
        console.log(`Process ${value}`);
    }
})();

.as-console-wrapper {
    max-height: 100% !important;
}

An alternative scenario could involve triggering a timeout on the initial pass but not thereafter (since bailing out on the first timeout means subsequent ones won't occur):

const delay = (ms, value) => new Promise(resolve => {
    setTimeout(resolve, ms, value);
});

async function* example() {
    for (let i = 1; i <= 6; ++i) {
        const ms = i === 1 ? 600 : Math.floor(Math.random() * 100);
        await delay(ms);
        yield i;
    }
}

(async () => {
    const asynDataStreamOrGenerator = example(); // For demonstrative purposes
    const TIMEOUT = 500; // Milliseconds
    const GOT_TIMEOUT = {};
    const results = [];
    const it = asynDataStreamOrGenerator[Symbol.asyncIterator]();
    try {
        while (true) {
            const p = it.next();
            const start = Date.now();
            const result = await Promise.race([p, delay(TIMEOUT, GOT_TIMEOUT)]);
            const elapsed = Date.now() - start;
            if (result === GOT_TIMEOUT) {
                // No timely response received, aborting
                console.log(`Got timeout after ${elapsed}ms`);
                break;
            }
            // Response obtained
            if (result.done) {
                // Iteration complete
                console.log(`Got iteration complete after ${elapsed}ms`);
                break;
            }
            console.log(`Got value ${result.value} after ${elapsed}ms`);
            results.push(result.value);
        }
    } finally {
        try {
            it.return?.();
        } catch { }
    }
    // ...processing logic for items stored in `results`...
    for (const value of results) {
        console.log(`Process ${value}`);
    }
})();

.as-console-wrapper {
    max-height: 100% !important;
}

You may need to adjust this based on your specific requirements, but it provides a starting point for further development.

Throughout all scenarios described above:

  • If optional chaining (${it.return?.();}) isn't supported in your environment, replace it with standard checks (${if (it.return) { it.return(); }}).
  • If optional catch bindings (${catch { }}) aren't supported, replace them with explicit error handling (${catch (e) { }}).

Answer №2

You can utilize a timeout Promise (referred to as timer in the code) and employ Promise.race during each iteration.

The snippet provided will output results up to approximately 30, even though the generator could potentially produce more values.

async function wait(ms) {
  return new Promise(r=>setTimeout(r, ms))
}

async function* asynDataStreamOrGenerator() {
  for (let i = 0; i < 100; ++i) {
    await wait(30)
    yield i;
  }
}

async function* iterate_until(generator, timeout) {
  let timer = wait(timeout).then(_=>Promise.reject("TimeOut"))
  for (;;) {
    let it = generator.next()
    let result = await Promise.race([timer, it])
    if (result.done) break;
    yield result.value;
  }
}

{(async () => {
  try {
    for await (let t of iterate_until(asynDataStreamOrGenerator(), 1000)) {
      console.log(t)
    }
  } catch (e) { /* handle timeout error here */}
})()}

JSFiddle Link

Answer №3

Heretic Monkey had an interesting suggestion in the comments section of the query, proposing to wrap the async iterable in another one with a timeout implementation. This method allows the code using the wrapper to leverage for-await-of.

If you wish to proceed after a timeout, the implementation would look like this:

async function* handleTimeout(asyncIterable, duration, value) {
    const iterator = asyncIterable[Symbol.asyncIterator]();
    try {
        while (true) {
            const result = await Promise.race([
                iterator.next(),
                delay(duration, value)
            ]);
            if (result === value) {
                yield value;
            } else if (result.done) {
                break;
            } else {
                yield result.value;
            }
        }
    } finally {
        iterator.return?.();
    }
}

Usage example:

for await (const t of handleTimeout(dataStreamOrGenerator, TIMEOUT_DURATION, TIMEOUT_VALUE)) {
    if (t === TIMEOUT_VALUE) {
        console.log("Operation timed out");
    } else {
        console.log(`Process ${t}`);
    }
}
console.log("Complete");

For situations where you do not want to continue after a timeout, an error can be thrown to stop the iteration:

async function* handleTimeout(asyncIterable, duration) {
    const VALUE = {};
    const iterator = asyncIterable[Symbol.asyncIterator]();
    try {
        while (true) {
            const result = await Promise.race([
                iterator.next(),
                delay(duration, VALUE)
            ]);
            if (result === VALUE) {
                throw new Error(`Operation timed out after ${duration}ms`);
            } else if (result.done) {
                break;
            } else {
                yield result.value;
            }
        }
    } finally {
        iterator.return?.();
    }
}

Utilizing it:

try {
    for await (const t of handleTimeout(dataStreamOrGenerator, TIMEOUT)) {
        console.log(`Process ${t}`);
    }
    console.log("Complete");
} catch (e) {
    console.error(e.message);
}

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