Understanding Promise.allSettled in JavaScript

Promise.allSettled() Example:

Write a JavaScript program that takes an array of Promises and logs both resolved and rejected results using Promise.allSettled().

Solution-1: Basic Use of Promise.allSettled()

Code:

// Define multiple Promises
const promise1 = Promise.resolve("Resolved from promise1");
const promise2 = Promise.reject("Error from promise2");
const promise3 = Promise.resolve("Resolved from promise3");

// Use Promise.allSettled to handle both resolved and rejected results
Promise.allSettled([promise1, promise2, promise3])
  .then((results) => {
    results.forEach((result) => {
      if (result.status === "fulfilled") {
        console.log("Fulfilled:", result.value);
      } else if (result.status === "rejected") {
        console.log("Rejected:", result.reason);
      }
    });
  });

Output:

"Fulfilled:"
"Resolved from promise1"
"Rejected:"
"Error from promise2"
"Fulfilled:"
"Resolved from promise3"

Explanation:

• Three Promises are defined:
• promise1 resolves with a message.
• promise2 rejects with an error.
• promise3 resolves with another message.
• Promise.allSettled() handles all the Promises and provides a detailed status report.
• The results are iterated:
• If status is "fulfilled", the resolved value is logged.
• If status is "rejected", the rejection reason is logged.

Solution-2: Using Promise.allSettled() with Dynamic Promise Creation

Code:

// Function to simulate an asynchronous task
const asyncTask = (taskName, time, shouldReject = false) => {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      if (shouldReject) {
        reject(`${taskName} failed`);
      } else {
        resolve(`${taskName} succeeded`);
      }
    }, time);
  });
};

// Create an array of dynamic Promises
const tasks = [
  asyncTask("Task 1", 1000),
  asyncTask("Task 2", 2000, true),
  asyncTask("Task 3", 1500)
];

// Use Promise.allSettled to handle the results
Promise.allSettled(tasks)
  .then((results) => {
    console.log("All tasks completed:");
    console.log(results);
  });

Output:

"All tasks completed:"
[[object Object] {
  status: "fulfilled",
  value: "Task 1 succeeded"
}, [object Object] {
  reason: "Task 2 failed",
  status: "rejected"
}, [object Object] {
  status: "fulfilled",
  value: "Task 3 succeeded"
}]

Explanation:

• A helper function asyncTask creates asynchronous operations with customizable delay and rejection.
• Three tasks are dynamically created:
• Task 1 succeeds after 1 second.
• Task 2 fails after 2 seconds.
• Task 3 succeeds after 1.5 seconds.
• Promise.allSettled() handles all the Promises and provides a summary of the results.
• The complete results are logged as an array of objects with status and either value or reason.

See the Pen promises-and-async-await-exercise-5 by w3resource (@w3resource) on CodePen.

For more Practice: Solve these Related Problems:

• Write a JavaScript program that uses Promise.allSettled() to process several API call simulations and categorizes the results into successes and failures.
• Write a JavaScript function that applies Promise.allSettled() on an array of Promises that resolve or reject randomly.
• Write a JavaScript function that aggregates the outcomes of mixed asynchronous tasks using Promise.allSettled() and returns a summary object.
• Write a JavaScript function that demonstrates the use of Promise.allSettled() by processing immediate and delayed Promises together.

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