Why Promise.race and Promise.allSettled in Node.js? - Purpose & Use Cases

The Big Idea

Discover how to handle many tasks at once without messy code or missed results!

The Scenario

Imagine you have several tasks running at the same time, like downloading files or fetching data from different websites, and you want to know when the first one finishes or when all of them have completed, no matter if they succeed or fail.

The Problem

Manually tracking each task's completion is tricky and messy. You have to write lots of code to check each task's status, handle errors separately, and decide when to move on. This can easily lead to bugs and slow down your program.

The Solution

Promise.race and Promise.allSettled let you handle multiple tasks easily. Promise.race gives you the result of the first task that finishes, while Promise.allSettled waits for all tasks to finish and tells you their outcomes, whether success or failure.

Before vs After

✗ Before

let finished = false;
tasks.forEach(task => {
  task.then(result => {
    if (!finished) {
      finished = true;
      console.log('First done:', result);
    }
  }).catch(() => {});
});

✓ After

Promise.race(tasks).then(result => console.log('First done:', result));

What It Enables

You can easily manage many asynchronous tasks and react quickly to the first result or wait for all results without complicated code.

Real Life Example

When loading images on a webpage, you might want to show the first image that loads quickly (Promise.race) or wait until all images have either loaded or failed before showing a gallery (Promise.allSettled).

Key Takeaways

Manually tracking multiple async tasks is complex and error-prone.

Promise.race returns the first finished task's result automatically.

Promise.allSettled waits for all tasks and reports their success or failure.

Practice

1. What does Promise.race do when given multiple promises?

easy

A. Cancels all promises except the first one.

B. Waits for all promises to finish and returns their results.

C. Returns only the results of promises that resolved successfully.

D. Returns the result or error of the first promise that finishes.

Solution

1. Step 1: Understand Promise.race behavior

   Promise.race returns as soon as any promise settles (resolves or rejects).

2. Step 2: Compare with other Promise methods

   Unlike Promise.all or allSettled, it does not wait for all promises.

3. Final Answer:

   Returns the result or error of the first promise that finishes. -> Option D

4. Quick Check:

   Promise.race = first finished promise result [OK]

Hint: Remember race means first to finish wins [OK]

Common Mistakes:

• Thinking it waits for all promises
• Assuming it only returns successful results
• Believing it cancels other promises

2. Which of the following is the correct syntax to use Promise.allSettled with an array of promises named tasks?

easy

A. Promise.allSettled(tasks).finally(() => console.log('done'));

B. Promise.allSettled(tasks).catch(error => console.log(error));

C. Promise.allSettled(tasks).then(results => console.log(results));

D. Promise.allSettled(tasks).resolve(results => console.log(results));

Solution

1. Step 1: Recall Promise.allSettled usage

   Promise.allSettled returns a promise that resolves with an array of results.

2. Step 2: Check correct method chaining

   We use .then() to handle the resolved results, not .catch() or .resolve().

3. Final Answer:

   Promise.allSettled(tasks).then(results => console.log(results)); -> Option C

4. Quick Check:

   Use .then() to get allSettled results [OK]

Hint: Use .then() to handle Promise.allSettled results [OK]

Common Mistakes:

• Using .catch() instead of .then() for results
• Trying to use .resolve() method on promise
• Assuming .finally() receives results

3. Consider the code:

const p1 = new Promise(res => setTimeout(() => res('A'), 100));
const p2 = new Promise((_, rej) => setTimeout(() => rej('Error'), 50));
Promise.race([p1, p2])
  .then(console.log)
  .catch(console.error);

What will be printed?

medium

A. 'Error'

B. 'A'

C. An array of results

D. Nothing, code throws syntax error

Solution

1. Step 1: Identify which promise settles first

   p2 rejects after 50ms, p1 resolves after 100ms, so p2 finishes first.

2. Step 2: Understand Promise.race behavior on rejection

   Promise.race rejects immediately with the first rejection, so .catch logs 'Error'.

3. Final Answer:

   'Error' -> Option A

4. Quick Check:

   First finished promise is rejection 'Error' [OK]

Hint: Check which promise settles first, resolve or reject [OK]

Common Mistakes:

• Assuming resolve wins over reject
• Expecting an array instead of single result
• Thinking code throws syntax error

4. What is wrong with this code snippet?

const promises = [Promise.resolve(1), Promise.reject('fail')];
Promise.allSettled(promises).catch(console.error);

medium

A. Promise.allSettled requires async/await syntax.

B. Promise.allSettled never rejects, so .catch will never run.

C. Promises array must contain only resolved promises.

D. You must use .then() before .catch() with allSettled.

Solution

1. Step 1: Recall Promise.allSettled behavior

   It waits for all promises and never rejects, it always resolves with results.

2. Step 2: Understand .catch usage here

   Since it never rejects, .catch will never be called, so error handling is ineffective.

3. Final Answer:

   Promise.allSettled never rejects, so .catch will never run. -> Option B

4. Quick Check:

   allSettled always resolves, .catch unused [OK]

Hint: allSettled never rejects, so .catch is useless here [OK]

Common Mistakes:

• Thinking allSettled rejects on any promise failure
• Believing async/await is required
• Assuming .then() must come before .catch() always

5. You want to run three tasks: task1, task2, and task3. You want to get the first task that finishes successfully, but if all fail, you want to know all errors. Which approach correctly achieves this?

hard

A. Use Promise.race on all tasks, then if it rejects, run Promise.allSettled to get all errors.

B. Use Promise.allSettled first, then pick the first successful result from the array.

C. Use Promise.all on all tasks and catch errors to get all results.

D. Use Promise.race and ignore errors from rejected promises.

Solution

1. Step 1: Understand requirement for first success or all errors

   Promise.race gives first finished promise, but may reject if first is failure.

2. Step 2: Combine Promise.race and Promise.allSettled

   If Promise.race rejects, then run Promise.allSettled to collect all errors from all tasks.

3. Step 3: Evaluate other options

   Promise.allSettled alone waits for all, no early success; Promise.all fails fast; ignoring errors loses info.

4. Final Answer:

   Use Promise.race on all tasks, then if it rejects, run Promise.allSettled to get all errors. -> Option A

5. Quick Check:

   race first success, allSettled for all errors [OK]

Hint: Race first success, fallback to allSettled for errors [OK]

Common Mistakes:

• Using allSettled only and waiting too long
• Ignoring rejected promises in race
• Using all which fails on first rejection