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no-await-in-loop

Performing await operations sequentially in a loop prevents taking advantage of the parallelization benefits of async/await.
Rule Type: Problem
Fixable: No

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Why This Rule Exists

Using await inside a loop means each iteration must complete before the next one starts. Usually, you can refactor this code to create all promises at once, then use Promise.all() to wait for them all to complete in parallel.

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Sequential vs Parallel

// Bad: Sequential - each iteration waits for the previous one
async function foo(things) {
  const results = [];
  for (const thing of things) {
    results.push(await doAsyncWork(thing)); // 3 seconds total for 3 items
  }
  return results;
}

// Good: Parallel - all operations start immediately
async function foo(things) {
  const promises = [];
  for (const thing of things) {
    promises.push(doAsyncWork(thing)); // 1 second total for 3 items
  }
  return await Promise.all(promises);
}

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Rule Details

This rule disallows the use of await within loop bodies.

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Error Handling Benefits

Parallel execution also improves error handling by preventing unhandled promise rejections: 
// Bad: Later rejections become unhandled
async function foo() {
    const arrayOfPromises = somethingThatCreatesAnArrayOfPromises();
    for (const promise of arrayOfPromises) {
        const value = await promise; // If this throws, remaining promises become unhandled
        console.log(value);
    }
}

// Good: All rejections can be caught
async function foo() {
    const arrayOfPromises = somethingThatCreatesAnArrayOfPromises();
    const arrayOfValues = await Promise.all(arrayOfPromises); // Catches all rejections
    for (const value of arrayOfValues) {
        console.log(value);
    }
}

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Examples

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Incorrect Code

// Awaiting in a for-of loop
async function foo(things) {
  const results = [];
  for (const thing of things) {
    results.push(await doAsyncWork(thing)); // Error
  }
  return results;
}

// Awaiting in a for loop
async function bar(items) {
  for (let i = 0; i < items.length; i++) {
    await processItem(items[i]); // Error
  }
}

// Await using in for loop
async function baz(things) {
  for (const thing of things) {
    await using resource = getAsyncResource(thing); // Error
  }
}

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Correct Code

// Create all promises, then await them together
async function foo(things) {
  const promises = [];
  for (const thing of things) {
    promises.push(doAsyncWork(thing)); // No await here
  }
  const results = await Promise.all(promises);
  return results;
}

// Or use map for a more concise version
async function foo(things) {
  return await Promise.all(
    things.map(thing => doAsyncWork(thing))
  );
}

// Await outside the loop
async function bar(items) {
  const promises = items.map(item => processItem(item));
  await Promise.all(promises);
}

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When Not to Use It

There are legitimate cases where sequential await is necessary:

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Serial Operations

It’s safe to disable this rule when iterations actually depend on each other.

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Rate Limiting

// Intentionally sequential to avoid rate limits
async function makeUpdatesToRateLimitedApi(thingsToUpdate) {
    for (const thing of thingsToUpdate) {
        await updateThingWithRateLimitedApi(thing); // Must be sequential
    }
}

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Resource Constraints

// Sequential to avoid overwhelming system resources
async function streamingProcess() {
    for (let i = 0; i < 10; i++) {
        await ramIntensiveAction(data[i]); // Can't run all at once
    }
}

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Dependent Iterations

// Output of one iteration is input to next
async function loopIterationsDependOnEachOther() {
    let previousResult = null;
    for (let i = 0; i < 10; i++) {
        const result = await doSomething(i, previousResult);
        if (someCondition(result)) {
            break;
        }
        previousResult = result;
    }
}

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Intentional Delays

// Countdown that must be sequential
async function printCountdown() {
    for (let i = 10; i >= 0; i--) {
        await new Promise(resolve => setTimeout(resolve, 1000));
        console.log(i);
    }
}

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File System Operations

// Writing to file must be sequential
async function writeNumbersToFile() {
    for (let i = 0; i < 100; i++) {
        await fileWriteStream.write(i + "\n"); // Parallel would garble output
    }
}

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Disabling for Specific Cases

When you have a legitimate use case, disable the rule with a comment: 
/* eslint-disable no-await-in-loop */
for (const item of items) {
    // Rate limiting requires sequential processing
    await processWithRateLimit(item);
}
/* eslint-enable no-await-in-loop */