Coding Challenge Practice - Question 93

The task is to implement the promise.allSettled method.

The boilerplate code

function allSettled(promises) {
  // your code here
}

Promise.allSettled receives an array of promises, waits for all of them to settle and never rejects.

Promise.allSettled returns a promise

return new Promise((resolve) => {
}

Store the final output objects, and count how many promises have finished

const results = [];
let completed = 0;

If the arguments are not iterable

if(!Array.isArray(promises)) {
throw new TypeError("Argument must be iterable")
}

If there are no promises

if (promises.length === 0) {
  resolve(results);
  return;
}

Loop through each promise, store the index to preserve order and store results in their correct position.

promises.forEach((promise, index) => {

If the input is already a promise, use it. If it's a value, convert it to a promise.

Promise.resolve(promise)

When a promise resolves, store it in the correct index

.then(value => {
  results[index] = {
    status: 'fulfilled',
    value
  };
})

When a promise is rejected, it should be captured but not propagated.

.catch(reason => {
  results[index] = {
    status: 'rejected',
    reason
  };
})

When all promises are done, resolve the final array

.finally(() => {
  completed++;
  if (completed === promises.length) {
    resolve(results);
  }
});

The final code

function allSettled(promises) {
  // your code here
  return new Promise((resolve) => {
    if(!Array.isArray(promises)) {
      throw new TypeError("Argument must be iterable")
    }
    const results = [];
    let completed = 0;

    if(promises.length === 0) {
      resolve(results);
      return;
    }

    promises.forEach((promise, index) => {
      Promise.resolve(promise)
      .then(value => {
        results[index] = {
          status: 'fulfilled',
          value
        }
      })
      .catch(reason => {
        results[index] = {
          status: 'rejected',
          reason
        }
      })
      .finally(() => {
        completed++;
        if(completed === promises.length) {
          resolve(results);
        }
      })
    })
  })
}

That's all folks!