Promise.all() in Promise.all() - Javascript

Asynchronous programming is essential in modern web development, and Promises are a powerful tool for managing asynchronous operations in Javascript. One of the most useful methods in the Promise API is Promise.all(), which allows you to execute multiple Promises in parallel and get the results when all of them are resolved.

But what if you need to execute multiple Promise.all() operations in parallel and get the results when all of them are resolved? This is where Promise.all() in Promise.all() comes in handy.

Syntax

Promise.all([
  Promise.all([Promise1, Promise2]),
  Promise.all([Promise3, Promise4])
])
  .then(([results1, results2]) => {
    // handle results1 and results2
  })
  .catch(error => {
    // handle errors
  });

As you can see, the syntax for Promise.all() in Promise.all() is quite simple. You just need to create an array of Promises for each Promise.all() operation and then pass them as arguments to the outer Promise.all() method. The then() callback function receives an array of results for each Promise.all() operation.

Example

Let's say we have an API that allows us to get the book recommendations for a set of users, and we want to get the recommendations for multiple users in parallel. We can use Promise.all() in Promise.all() to achieve this:

const users = ['user1', 'user2', 'user3'];
const api = 'https://example.com/api';

Promise.all(
  users.map(user => 
    fetch(`${api}/recommendations/${user}`)
      .then(response => response.json())
  )
)
  .then(recommendations => {
    return Promise.all(
      recommendations.map(userRecs => 
        Promise.all(userRecs.map(bookId => 
          fetch(`${api}/books/${bookId}`)
            .then(response => response.json())
        ))
      )
    );
  })
  .then(results => {
    console.log(results);
  })
  .catch(error => {
    console.error(error);
  });

In this example, we first use Promise.all() to fetch the recommendations for each user in parallel. Then, we use another Promise.all() to fetch the details of each recommended book in parallel for all users. The final then() callback function receives an array of arrays, where each inner array contains the book details for a single user.

Conclusion

Promise.all() in Promise.all() is a powerful technique for executing multiple asynchronous operations in parallel and getting the results when all of them are resolved. It allows you to build complex asynchronous workflows with ease, and makes your code more readable and maintainable.