Promise.allSettled Polyfill in JavaScript - Step by Step Explanation

Table of content:

1. Understand Promise.allSettled Static Method
2. Promise.allSettled Polyfill in JavaScript - Step by Step Explanation
3. Handle Multiple Aync API Calls using Promise.allSettled

Understand Promise.allSettled Static Method

Static Promise.allSettled([Promises]) function works similar to Promise.all() method to execute all the promises parallelly but rather than returning a failure response in case of one or more rejected promises, allSettled method takes array of promises as an input, and returns a promise as a response which got fulfilled when all the promises got settled (either fulfilled or rejected).

The fulfilled response contains an array of object which includes the results for all the input promises. All the fulfilled responses contains the status as "fulfilled" and the resolved value for example {status: "fulfilled", value: "<>"} , and all the rejected response contains status as "rejected" along with the "reason" key which included the rejected reason example {status: "rejected", reason: "<>"}

Example

// Promise.allSettled Example

// Creating three promises
const promise1 = Promise.resolve('First promise fulfilled');
const promise2 = Promise.resolve('Second promise fulfilled');
const promise3 = Promise.reject('Third promise rejected');

// Using Promise.allSettled to handle all promises, regardless of whether they are fulfilled or rejected
Promise.allSettled([promise1, promise2, promise3])
  .then(results => {
    console.log(results);
  });

Output

[
  { status: 'fulfilled', value: 'First promise fulfilled' },
  { status: 'fulfilled', value: 'Second promise fulfilled' },
  { status: 'rejected', reason: 'Third promise rejected' }
]

Promise.allSettled Polyfill in JavaScript - Step by Step Explanation

Step 1: Promise.allSettled() returns a promise, so as part of the first step we need to retuned a promise but since allSettled also need to process all the promises parallelly we need to use the Promise.all() method internally for parallel execution of promises.

Step 2:  As part of the second step, all the input promises needs to be iterated using a map (because it returns an processed array, which will going to be the input for Promise.all method).

Step 3: In the next step we just need to resolve the each promise, passed in each iteration of map. If the promise got fulfilled it adds {status: "fulfilled", value: "<v>"} to the array other wise in case of rejected promise, it will go to a catch block which adds rejected object {status: "rejected", reason: "<reason>"} 

Promise.allSettled Polyfill Implementation Code:

Promise.customAllSettled = function(promises) {
    return Promise.all(
      promises.map(promise =>
        Promise.resolve(promise)
          .then(value => ({ status: 'fulfilled', value }))
          .catch(reason => ({ status: 'rejected', reason }))
      )
    );
};

Example of Handling Multiple Aync API calls using Promise.allSettled

One of the most common case of using Promise.allSettled is to execute parallel API calls, where successful execution of all the API call is not mandatory. allSettled() method provides a convenient way to handle fulfilled and rejected API calls, so that application can retry the failed api calls again.

// Example: Multiple DOG API calls handled using Promise.allSettled()

// Helper function to fetch a random dog image from the Dog API
function fetchDogImage(urlPath) {
  return fetch(`https://dog.ceo/api/breeds/image/${urlPath}`)
    .then(response => {
      if (!response.ok) {
        throw new Error('Failed to fetch dog image');
      }
      return response.json();
    })
}

// Simulating multiple Dog API calls, with some failures
const apiCalls = [
  fetchDogImage('random'), // Successful call
  fetchDogImage('random'), // Successful call
  fetchDogImage('random'), // Successful call
  fetchDogImage('failed'), // Simulate a failure here by causing an error in the URL (incorrect API endpoint)
  fetchDogImage('random')  // Successful call
];

// Use Promise.allSettled to handle all API calls
Promise.allSettled(apiCalls)
  .then(results => {  
    results.forEach((result, index) => {
      if (result.status === 'fulfilled') {
        console.log(`API call ${index + 1}: Success! Dog image URL`, result.value);
      } else {
        console.log(`API call ${index + 1}: Failed. Reason:`, result.reason);
      }
    });
  })
  .catch(error => {
    console.error('Unexpected error occurred:', error);
  });

💻Checkout the Live Code here - JS Bin

Output

API call 1: Success! Dog image URL {message: 'https://images.dog.ceo/breeds/briard/n02105251_12.jpg', status: 'success'}
API call 2: Success! Dog image URL {message: 'https://images.dog.ceo/breeds/akita/512px-Akita_inu.jpg', status: 'success'}
API call 3: Success! Dog image URL {message: 'https://images.dog.ceo/breeds/bulldog-english/jager-2.jpg', status: 'success'}
API call 4: Failed. Reason: Error: Failed to fetch dog image
API call 5: Success! Dog image URL {message: 'https://images.dog.ceo/breeds/african/n02116738_4734.jpg', status: 'success'}

Final Note:

Promise.allSettled() static method provides a powerful way to call the promises parallelly which results as a response of all the settled promises whether those are fulfilled or rejected. This helps in the partial execution of the promises, and particularly helpful where out of many promises one of the promise got rejected due to some reason.

This is the preferred way to call APIs when the APIs are independent and failure of an API doesn't effect the other API calls

Topics to Learn Next

1. Notes to Master Promise Methods in JavaScript: all(), allSettled(), race() and any()
2. Promise Polyfill in JavaScript - Step by Step Explanation 
3. Promise.all Polyfill in JavaScript - Detailed Explanation [For Interviews]
4. Promise.allSettled in JavaScript explained
5. Promise.race polyfill in JavaScript explained
6. Promise.any polyfill in JavaScript explained