Coding Challenge Practice - Question 92

The task is to implement Promise.all()

The boilerplate code

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

Promise.all takes an array of promises and returns one promise. When all promises resolve, it resolves with an array of results and maintains the original order.

Validate if the argument is iterable

return new Promise((resolve, reject) => { 
if (!Array.isArray(promises)) {
  return reject(new TypeError('Argument must be an array'));
}

Store the resolved values, count how many promises have been resolved, and the total number of promises.

const results = [];
let completed = 0;
const total = promises.length;

If the array is empty

if (total === 0) {
  resolve([]);
  return;
}

Loop through each promise and remember its index

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

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

Promise.resolve(promise)

Store the result at the correct index, increase the completion count and resolve final promise when all are done

.then(value => {
  results[index] = value;
  completed++;

  if (completed === total) {
    resolve(results);
  }
}

As soon as any promise rejects, the return promise rejects

.catch(err => {
  reject(err);
});

The final code

function all(promises) {
  // your code here
  return new Promise((resolve, reject) => {
    if(!Array.isArray(promises)) {
      return reject(new TypeError('Argument must be an array'))
    }

    const results = [];
    let completed = 0;
    const total = promises.length;

    if(total === 0) {
      resolve([]);
      return;
    }
    promises.forEach((promise, index) => {
      Promise.resolve(promise)
      .then(value => {
        results[index] = value;
        completed++;

        if(completed === total) {
          resolve(results);
        }
      })
      .catch(err => {
        reject(err);
      })
    })
  })
}

That's all folks!