Promise map with array of objects with arrays

Question

I have the following data structure:

[
  {
    "id": 1,
    "houses": [
      {
        "id": 1,
        "name": "house 1"
      },
      {
        "id": 2,
        "name": "house 2"
      }
    ]
  },
  {
    "id": 2,
    "houses": [
      {
        "id": 3,
        "name": "house 3"
      }
    ]
  }
]

And I want to be able to, for each house in each user do something async, so I have a function with this signature and which returns a promise:

sendWelcomeEmail(user, house)

Now, I know how to use Bluebird's Promise.map when I have an array of promises, but in this case, I have an array of objects with arrays. What is the proper way to call Promise.map so I end up calling sendWelcomeEmail for each user and house?

Bruno · Accepted Answer · 2018-05-16 16:03:28Z

1

Promise.all

The Promise.all(iterable) method returns a single Promise that resolves when all of the promises in the iterable argument have resolved or when the iterable argument contains no promises. It rejects with the reason of the first promise that rejects.

const data = [{
    "id": 1,
    "houses": [{
        "id": 1,
        "name": "house 1"
      },
      {
        "id": 2,
        "name": "house 2"
      }
    ]
  },
  {
    "id": 2,
    "houses": [{
      "id": 3,
      "name": "house 3"
    }]
  }
];

const fakeSendWelcomeEmail = (id, house) => Promise.resolve(`${id} / ${house.name}`);

// Transforming the data into a flat array that contains the return value of fakeSendWelcomeEmail .
// So welcomeResults is an array of Promise.
const welcomeResults = data.reduce((res, user) => {
  return res.concat(user.houses.map((house) => {
    return fakeSendWelcomeEmail(user.id, house)
  }));
}, [])

Promise.all(welcomeResults)
  .then((results) => {
    console.log(results);
  })

I've written a Promise polyfill myself, if you're curious about how stuff works under the hood, or just learn a bit more about Promise.

edited May 16, 2018 at 16:03

answered May 16, 2018 at 15:54

Bruno

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7 Comments

Hommer Smith Over a year ago

Bruno do you mind explaining what does this do? Promise.all is supposed to receive an iterable of promises, and in this case you are just passing an array to it.

Fabio Lolli Over a year ago

@HommerSmith it accepts an Array of anything - if the element is a Promise, it waits for it to resolve, if it's a value, it resolves with that value

Hommer Smith Over a year ago

Why do you prefer using Promise.all vs Promise.map? The good thing with map is that you can have concurrency limits...

Bruno Over a year ago

I am improving my answer, let me know there's something is still not clear

Bruno Over a year ago

@HommerSmith Promise.all is part of the Promise's standard. Promise.map implementation depends on the particular library you're using.

|

yBrodsky · Accepted Answer · 2018-05-16 15:53:25Z

0

You will need to combine reduce with map.

const promises = mainArray.reduce((acc, cur) => {
  return acc.concat(cur.houses.map(house => sendWelcomeEmail(house)));
}, []);

promises will be an array with 3 promises as per your example.

answered May 16, 2018 at 15:53

yBrodsky

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