Killing Your App with .map()? Here’s the Modern Fix

When working with large datasets in JavaScript, many developers instinctively reach for .map() to transform arrays.

It’s clean, elegant, and easy to use but it can quietly become a performance bottleneck.

In this post, we’ll explore why using largeArray.map(...) can be problematic for large arrays, especially in resource-constrained environments, and how you can avoid those pitfalls using lazy and batched processing patterns.

🔍 The Problem with largeArray.map(...)

const results = largeArray.map(item => process(item));

Let’s understand on “Why it’s risky?” —

• Memory Explosion: .map() eagerly evaluates and returns a brand new array, holding all transformed elements in memory at once.

• Garbage Collection Pressure: Thousands or millions of intermediate objects can quickly strain the JS garbage collector, leading to frequent pauses and long GC cycles.

• Lack of Control: There's no built-in way to pause, resume, or handle the processing in chunks.

• Inflexible with Async: Using .map() in combination with async/await requires an awkward Promise.all, which further amplifies memory usage.

💡 The Lazy & Efficient Alternatives

Let’s look at three utility functions that fix these problems:

1. lazyMap() – Sync Lazy Mapping

/**
 * Generator function that lazily maps over an array
 * Use case: Memory-efficient processing of large arrays by processing one item at a time
 * @param {Array} arr - Input array to map over
 * @param {Function} fn - Mapping function to apply to each element
 * @yields {*} The result of applying fn to each array element
 */
function *lazyMap(arr, fn) {
  for (const item of arr) {
    yield fn(item);
  }
}

✔️ Transforms one item at a time
✔️ No new array in memory
✔️ Excellent for streaming or pipelining

2. lazyMapAsync() – Async Lazy Mapping

/**
 * Async generator function that lazily maps over an array with async operations
 * Use case: Memory-efficient processing of large arrays with asynchronous operations
 * @param {Array} arr - Input array to map over
 * @param {Function} fn - Async mapping function to apply to each element
 * @yields {Promise<*>} The result of applying async fn to each array element
 */
async function *lazyMapAsync(arr, fn) {
  for (const item of arr) {
    yield await fn(item);
  }
}

✔️ Processes each item sequentially, ideal for:

• API requests

• File reads

• Rate-limited operations

3. processPromises() – Batched Promise Handling

/**
 * Utility function for handling Promise.all and Promise.allSettled operations
 * Use case: Batch processing of promises with configurable error handling and concurrency
 * @param {Array<Promise>} promises - Array of promises to process
 * @param {Object} options - Configuration options
 * @param {boolean} options.settled - Whether to use Promise.allSettled (true) or Promise.all (false)
 * @param {number} options.batchSize - Number of promises to process concurrently
 * @returns {Promise<Array>} Results of the promise operations
 */
async function processPromises(promises, { settled = false, batchSize = 10 } = {}) {
  const results = [];

  for (let i = 0; i < promises.length; i += batchSize) {
    const batch = promises.slice(i, i + batchSize);
    const batchResults = settled
      ? await Promise.allSettled(batch)
      : await Promise.all(batch);

    results.push(...batchResults);
  }

  return results;
}

✔️ Prevents Promise.all([...]) from overwhelming memory
✔️ Offers optional .allSettled() behavior
✔️ Lets you throttle execution with batchSize

📊 Performance & Memory Usage Comparison

🛠️ Real-World Use Cases

Case #1: Large API Calls

for await (const data of lazyMapAsync(userIds, fetchUserData)) {
  console.log({ data });
}

Case #2: Processing 100k+ Files

const filenames = getFileNames();
for (const processed of lazyMap(filenames, readAndTransform)) {
  save(processed);
}

Case #3: Uploading Data in Batches

const uploadPromises = records.map(r => uploadToCloud(r));
const results = await processPromises(uploadPromises, { batchSize: 50 });

🔄 Transitioning from .map() to Lazy Patterns

Final Thoughts

You don’t need to abandon .map() entirely but for large arrays, streaming workloads, and async-heavy tasks, lazy and batched processing offers:

• 🧘 Smoother memory profiles

• 🔥 Scalable performance

• 🛡️ More resilient async handling

Start small, wrap one expensive .map() with a lazy iterator. Then observe how your memory footprint and performance improve!

Over to You

Are you still using .map() for large datasets? Try these utilities and share your performance benchmarks!