Multiple API calls with fetch in chain and Promise.all

Question

I'm making an API call in my function to fetch some data. Then I need to make multiple API calls for each item of the data returned, based on a specific value from the first call. I have a problem when I'm rendering state, the values added from multiple promises are not present during rendering.

So far I have something like this:

fetch(url)
.then(resp => resp.json())
.then(data => {
  //do some calculations and populate new array and return it
})
.then(data => {
   const newData = [...data];
   Promise.all(functionWhichReturnsArrayOfPromises).then(el => {
      // more calculation
      newData.push(el.someValue);
   })
   return newData;
})
.then(data => this.setState({data: data}))

Function which returns array of promises looks something like that:

fetchMoreData(arr) {
   const promises = arr.map(el => {
      return fetch(someUrl)
      .then(data => data.json())
      .then(data => data)
   })
   return promises;
}

I think that my chaining the Promise.all inside another promise is not good, is there a recommended and more elegant way to do this ?

Answer 1

I think I'm probably missing something you're trying to do. If you simply need to have access to the the individual items when their "more data" Promises resolve, it is easily captured in the closure of that function.

const fetchMoreData = (thingy) => 
  fetch (`https://example.com/api/thingy/${thingy.id}`)
    .then (res => res .json ())
    .then (res => ({...thingy, ...res})) // NOTE: both `thingy` and `res` are in scope

fetch('https://example.com/api/thingy')
  .then (res => res .json ())
  .then (thingies => Promise .all (thingies .map (fetchMoreData)) )
  .then (allThingies => console .log (`Results: ${JSON.stringify(allThingies)}`))
  // .catch ( ... )

<script>
// dummy version of fetch for testing
const fetch = (url, opts) => Promise.resolve({
  json: () => {
    const result = (url .endsWith ('thingy'))
      ? [{id: 1, x: 'a'}, {id: 2, x: 'b'}, {id: 3, x: 'c'}]
      : {more: ['', 'foo', 'bar', 'baz'] [url .slice (url .lastIndexOf ('/') + 1)]} 
    console.log(`fetch('${url}') ~~> ${JSON.stringify(result)}`)
    return result
  }
})</script>

Are you looking to do something more complex that this pattern won't allow?

Update

Based on the comments, this is a version that uses a public REST API and avoids my fetch override:

const fetchMoreData = (overview) => 
  fetch (`https://jsonplaceholder.typicode.com/todos/${overview.id}`)
    .then (res => res .json () ) 
    .then (details => ({overview, details})) // `overview` and `details` both in scope

fetch('https://jsonplaceholder.typicode.com/todos')
  .then (res => res .json ()) 
  // `slice` because we don't need dozens for a demo
  .then (overviews => Promise .all (overviews .slice (0, 3) .map (fetchMoreData)) )
  .then (console.log)
  .catch (err => console.log(`error: ${err}`) )

Note that this API doesn't include only overview material in its group listings, so in fact overview and details contain the same information. But yours can include whatever you like.

Answer 2

You are correct in saying that your approach is not good, and here's why:

.then(data => {
   const newData = [...data];
   Promise.all(fetchMoreData(newData)).then(el => {
      // more calculation
      newData.push(el.someValue);
   })
   return newData;
})

return newData happens before you reach newData.push(el.someValue), and since they reference the same array, that means you're calling setState() and passing an array which is mutated asynchronously, independent of when your component re-renders.

Essentially, you have created a race-condition that will make your component state indeterminate because it is based on whether the fetch() operations complete before or after the React framework decides to re-render the component.

---

To solve this, there are two options that come to mind, so choose whichever is more readable to you, or consistent with your coding style, but first let's address a small refactor to make your helper function more canonical.

An asynchronous function should prefer to return a promise to an array, not an array of promises:

fetchMoreData(arr) {
   const promises = arr.map(el =>
     fetch(someUrl)
       .then(res => res.json())
   );

   return Promise.all(promises);
}

With that in mind, let's continue to the two solutions:

Nest (and return) the promise chain dependent on the previous scope

fetch(url)
  .then(res => res.json())
  .then(data => {
    // do some calculations and populate new array and return it
  })
  .then(array => {
    // nest the promise chain
    return fetchMoreData(array).then(result => {
      // more calculations dependent on array from previous scope
      result.forEach(el => {
        array.push(el.someValue);
      });

      // pass array along
      return array;
    });
  })
  .then(data => {
    this.setState({ data });
  });

Notice that we return fetchMoreData(array).then(...), and within the nested continuation, also return array. This will allow array to be passed along to data in the next chain.

Pass the dependency from the previous scope through a flattened promise chain

fetch(url)
  .then(res => res.json())
  .then(data => {
    // do some calculations and populate new array and return it
  })
  .then(array => {
    const promise = fetchMoreData(array);
    // pass the dependency along
    return Promise.all([array, promise]);
  })
  // destructure dependencies
  .then(([array, result]) => {
    // more calculations dependent on array from previous scope
    result.forEach(el => {
      array.push(el.someValue);
    });

    // pass array along
    return array;
  })
  .then(data => {
    this.setState({ data });
  });

Here, we encapsulate the dependency in another Promise.all() and pass both the array and promise along to the next flattened chain, which we then use array destructuring syntax to separate out again in the callback parameter. From there, we perform the additional calculations and then pass the array along to the final chain.