The Promise.all( ) Dilemma: When it helps and When it hurts

In modern JavaScript development, handling asynchronous operations is a common task. Whether it’s making API requests, querying databases, or reading files, working with async code is almost unavoidable. One of the common tools developers come across is Promise.all(). However, many of us, myself included, can fall into the trap of trying to use Promise.all() just because it’s there without really understanding whether it’s the best solution for our particular use case.

1. Jumping on the Promise.all() Bandwagon

As a developer, it’s easy to come across new features or tools and assume that they should be implemented everywhere. I found myself in this situation with Promise.all(). Having read about how it runs multiple promises in parallel and waits for all of them to complete before continuing, I was eager to integrate it into my code. Without fully understanding whether it was necessary, I jumped on the bandwagon and applied it wherever I could.

It’s easy to assume that since it’s a powerful tool, it must be better than the simpler alternatives. But as I soon realized, blindly applying Promise.all() without considering the context doesn’t always lead to the most efficient or readable code.

2. The Basics of Asynchronous JavaScript

Before we dive deeper into when Promise.all() is useful, let’s first look at how asynchronous functions work in JavaScript. When you write an async function and use await, JavaScript allows that operation to happen without blocking the rest of your code. This means that you can initiate one operation and move on to others while waiting for the result.

However, if you’re not careful, you might end up making operations unnecessarily dependent on each other when they could be run independently. I found myself in this situation with Promise.all(), thinking it was always a good idea to run all my async functions in parallel.

Example: Sequential Execution of Asynchronous Functions

const fetchData = async () => {
  const data1 = await getChart();   // Request #1
  const data2 = await getDetails(); // Request #2
};

Even though data1 and data2 are fetched one after the other in code, the browser still initiates both requests asynchronously and almost simultaneously. In fact, when I checked the Network tab, I saw both requests start around the same time. This made me realize that JavaScript was already handling things in parallel, and Promise.all() wasn’t necessarily required.

3. When Should You Use Promise.all()?

Despite my initial rush to use Promise.all() everywhere, there are situations where it truly shines. It is especially useful when you need to wait for multiple asynchronous operations to complete before moving forward.

Why Use Promise.all()?

1. Waiting for All Promises: If the results of multiple asynchronous tasks are dependent on each other, or if you need all of them to complete before proceeding, Promise.all() is ideal.
2. Error Handling: Promise.all() fails fast — meaning if any promise fails, the entire operation is rejected. This can be useful when you want to ensure everything succeeds together or nothing moves forward.
3. Combined Results: If you need the results of all promises at once (e.g., combining user data with purchase history), Promise.all() is the perfect solution.

Example: Using Promise.all()

const fetchData = async () => {
  const [data1, data2] = await Promise.all([getChart(), getDetails()]);
  console.log('Both requests completed'); // This runs only when both requests finish
};

In this example, both getChart() and getDetails() run in parallel, and the function waits until both are finished before moving forward. Promise.all() is perfect in situations like this where both requests are related and need to complete together.

4. Why I Didn’t Need Promise.all() in My Case

After applying Promise.all() a few times, I started to notice that it wasn’t always making my code better. In fact, I was overcomplicating things. I had two independent API requests—getChart() and getDetails()—each with its own loading spinner and results, and yet I was bundling them together unnecessarily.

By using Promise.all(), I was forcing the code to wait for both requests to complete before handling either result, even though the requests were independent and didn’t rely on each other. In cases like this, Promise.all() only adds complexity without any real benefit.

5. When Promise.all() Might Be Overkill

Sometimes, Promise.all() is overkill. If your async functions are independent, meaning one doesn’t rely on the other to complete, then there’s no need to bundle them together. They can run in parallel just fine without waiting on each other, and you can handle their results independently. This realization hit me when I saw that JavaScript already handles asynchronous tasks efficiently without needing to group everything together.

When to Avoid Promise.all()

1. Independent Async Functions: If your requests don’t depend on each other, they can complete at different times, and you can handle their results separately. There’s no need to wait for all of them to finish together.
2. Individual Loading States: In my case, I had individual loading spinners for each request, but I was unnecessarily holding everything up by waiting for both requests. It’s better to handle each request separately and update the UI as soon as each one finishes.

Example: Independent Requests Without Promise.all()

useEffect(() => {
  getChart();   // Trigger first async request
  getDetails(); // Trigger second async request
}, []);

In this setup, both requests run in parallel without needing Promise.all(). You can show individual loading states and handle each result independently, which is exactly what I needed for my project.

6. Real-World Scenarios for Using or Avoiding Promise.all()

Let’s look at how these concepts apply to real-world scenarios.

Scenario 1: Fetching Related Data (Use Promise.all())

Imagine you’re building a dashboard where you need to show user information and user purchase history together. In this case, you’d want to wait for both pieces of information to load before rendering the UI. Here, Promise.all() is the right choice:

const fetchData = async () => {
  const [userInfo, purchaseHistory] = await Promise.all([
    fetchUserInfo(),
    fetchUserPurchaseHistory()
  ]);
  console.log('Both user info and purchase history loaded');
};

Scenario 2: Independent API Calls (Avoid Promise.all())

Now, let’s say you’re fetching chart data and table data for a dashboard, and these two pieces of information are independent of each other. You might want to show a spinner for the chart and a separate one for the table. In this case, there’s no need to wait for both requests to complete together:

useEffect(() => {
  getChart();   // Fire chart request
  getDetails(); // Fire table request
}, []);

Both requests are independent, and you handle each of them separately, updating the UI as soon as each one completes. Promise.all() isn’t necessary here.

Conclusion: Don’t Jump on the Bandwagon

Promise.all() is a powerful tool, but it’s not always the best solution. I jumped on the bandwagon initially, assuming that using it everywhere would make my code better. However, I quickly learned that in cases where async functions are independent and have their own loading states, Promise.all() can actually make things more complicated.

Key Takeaways:

• Use Promise.all() when you need to wait for multiple promises to resolve before proceeding.
• Avoid Promise.all() when async tasks are independent, and you can handle them individually without unnecessary waiting.

Ultimately, it’s important to understand when and why to use a feature like Promise.all() instead of just assuming it’s always beneficial. After stepping back and re-evaluating my use case, I found that sticking with independent async calls was the right approach.