patmigliaccio.com/resolving-nested-promises 4/4/19

resolving-nested-promises-9.js

  function resolveUsers(users) {
    const resolveComments = createCommentsResolver();
  
    return Promise.all(
      users.map(user => {
        return resolveComments(user)
      })
    );
  }

  /**
   * Creates a `resolveComments` function which retrieves the full 
   *  comments for the specified user and caches the comment request
   *  if it has already been made.
   *
   * @returns {Function} `resolveComments`
   */
  function createCommentsResolver() {
    let cache = {};

    return async function resolveComments(user) {
      // Retrieves all comments based on `commentId`.
      user.comments = await Promise.all(
        user.commentIds.map(commentId => {
          // Adds comment to cache, if it doesn't already exist.
          if (!cache[commentId]) {
            cache[commentId] = comments.getCommentById(commentId)
          }
    
          return cache[commentId]
        })
      );
  
      return user;
    }
  }

  const activeUsers = await post.getActiveUsers();
  const resolvedUsers = await resolveUsers(activeUsers);