Promise.all() vs. Multiple Awaits: When to Use Concurrent vs. Sequential Async Calls in JavaScript?

Concurrent vs. Sequential Async Calls: Promise.all() vs. Multiple Awaits

In JavaScript, developers often use asynchronous functions to avoid blocking the main thread and improve performance. Two common patterns for handling asynchronous tasks are using Promise.all() and multiple await statements.

1. Promise.all()

Promise.all([promise1, promise2, ...]) takes an array of promises and returns a new promise that resolves when all the input promises have settled (either resolved or rejected). This allows you to wait for multiple asynchronous operations to complete concurrently.

2. Multiple Awaits

Using multiple await statements allows you to wait for asynchronous operations one after another. This effectively serializes the operations, executing them sequentially.

Timing Differences

The primary difference between Promise.all() and multiple await statements lies in their timing. Promise.all() starts all the asynchronous operations simultaneously, while multiple await statements execute them one at a time.

This timing difference can affect performance in scenarios where some of the asynchronous operations take significantly longer than others. In such cases, using Promise.all() can improve performance by taking advantage of concurrency.

Example

Consider the following code snippet:

const res = (ms) => new Promise((resolve, reject) => setTimeout(resolve, ms));

const example = async () => {
  const start = Date.now();
  const data = await Promise.all([res(3000), res(2000), res(1000)]);
  console.log(`Promise.all finished after: ${Date.now() - start}ms`);
};

example();

When executing this code, the three asynchronous operations (delayed by 3000, 2000, and 1000 milliseconds, respectively) will run simultaneously. As a result, Promise.all will only resolve once all three operations have completed, and the result will be logged 3000 milliseconds after the start of the function.

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