Understanding Async/Await with Iteration: Sequential vs. Parallel Processing

Introduction

Asynchronous programming is a crucial part of JavaScript, especially when dealing with operations that involve I/O, such as reading files or making network requests. The introduction of async/await syntax in ES2017 made writing asynchronous code more intuitive and manageable. However, combining async/await with array iteration methods like forEach, for...of, map, and reduce can be tricky. This tutorial will guide you through using async/await effectively with these iteration patterns to achieve both sequential and parallel processing.

Understanding Async/Await

The async keyword defines a function as asynchronous, allowing it to use the await keyword within its body. The await keyword pauses the execution of an async function until a Promise is resolved or rejected. This pattern simplifies handling Promises by writing asynchronous code in a synchronous-like manner.

Common Pitfalls with forEach

Using async/await inside a forEach loop can lead to unexpected behaviors because forEach does not wait for the promises returned by the iteration function:

const files = ['file1.txt', 'file2.txt', 'file3.txt'];

files.forEach(async (file) => {
  const contents = await readFileAsync(file);
  console.log(contents);
});

console.log('All files processed?');

In this example, forEach does not pause to wait for each file read operation. As a result, the message "All files processed?" is logged before any of the files have been read.

Sequential File Processing

To process files sequentially using async/await, you should use a loop that inherently supports awaiting promises:

Using for…of Loop

The for...of loop allows waiting for each file operation to complete before moving on to the next one:

async function printFiles() {
  const files = await getFilePaths();
  
  for (const file of files) {
    const contents = await readFileAsync(file, 'utf8');
    console.log(contents);
  }
}

This ensures that each readFileAsync call completes before the next iteration begins.

Parallel File Processing

When you want to read multiple files simultaneously and wait for all operations to complete, consider using Promises effectively:

Using map with Promise.all

You can use map in combination with Promise.all to achieve parallel processing. This approach waits for all promises to resolve before proceeding:

async function printFiles() {
  const files = await getFilePaths();
  
  const fileReadPromises = files.map(async (file) => {
    const contents = await readFileAsync(file, 'utf8');
    console.log(contents);
  });

  // Wait for all promises to resolve
  await Promise.all(fileReadPromises);
}

Using Array.prototype.reduce for Order Preservation

If maintaining the order of file processing is important, use reduce:

async function printFiles() {
  const files = await getFilePaths();
  
  await files.reduce(async (prevPromise, currentFile) => {
    await prevPromise;
    const contents = await readFileAsync(currentFile, 'utf8');
    console.log(contents);
  }, Promise.resolve());
}

This pattern ensures each file is processed in sequence while maintaining a promise chain.

Simplified Async Iteration with for-await-of

ES2018 introduced for-await-of, which simplifies handling asynchronous iterables:

async function printFiles() {
  const files = await getFilePaths();

  for await (const contents of files.map(file => readFileAsync(file, 'utf8'))) {
    console.log(contents);
  }
}

This allows for iterating over promises as if they were a synchronous array.

Conclusion

Choosing the right iteration pattern when using async/await is crucial to ensure your asynchronous operations behave as expected. Whether you need sequential or parallel processing, understanding how different loops and methods handle Promises will help you write more effective JavaScript code.