Firebase cloud function: http function returns null

Question

Here is what I am trying to do.

I am introducing functionality to enable users to search for local restaurants.
I created a HTTP cloud function, so that when the client delivers a keyword, the function will call an external API to search for the keyword, fetch the responses, and deliver the results.
In doing #2, I need to make two separate url requests and merge the results.

When I checked, the function does call the API, fetch the results and merge them without any issue. However, for some reason, it only returns null to the client.

Below is the code: could someone take a look and advise me on where I went wrong?

        exports.restaurantSearch = functions.https.onCall((data,context)=>{
            const request = data.request;
            const k = encodeURIComponent(request);
            const url1 = "an_url_to_call_the_external_API"+k; 
            const url2 = "another_url_to_call_the_external_API"+k;
            const url_array = [ url1, url2 ];
            
            const result_array = [];
            const info_array = [];

            url_array.forEach(url=>{
                return fetch(url, {headers: {"Authorization": "API_KEY"}})
                .then(response=>{
                    return response.json()
                })
                .then(res=>{
                    result_array.push(res.documents);
                    if (result_array.length===2) {
                        const new_result_array_2 = [...new Set((result_array))];
                        new_result_array_2.forEach(nra=>{
                            info_array.push([nra.place_name,nra.address_name])
                        })
                        //info_array is not null at this point, but the below code only return null when checked from the client
                        return info_array;
                    }
                })
            .catch(error=>{
                console.log(error)
                return 'error';
            })
        })
        });

Thanks a lot in advance!

Dharmaraj · Accepted Answer · 2021-08-22 11:01:41Z

You should use Promise.all() instead of running each promise (fetch request) separately in a forEach loop. Also I don't see the function returning anything if result_array.length is not 2. I can see there are only 2 requests that you are making but it's good to handle all possible cases so try adding a return statement if the condition is not satisfied. Try refactoring your code to this (I've used an async function):

exports.restaurantSearch = functions.https.onCall(async (data, context) => {
  // Do note the async                            ^^^^^
  const request = data.request;
  const k = encodeURIComponent(request);
  const url1 = "an_url_to_call_the_external_API" + k;
  const url2 = "another_url_to_call_the_external_API" + k;
  const url_array = [url1, url2];

  const responses = await Promise.all(url_array.map((url) => fetch(url, { headers: { "Authorization": "API_KEY" } })))
  const responses_array = await Promise.all(responses.map((response) => response.json()))
  console.log(responses_array)
  const result_array: any[] = responses_array.map((res) => res.documents)

  // Although this if statement is redundant if you will be running exactly 2 promises
  if (result_array.length === 2) {
    const new_result_array_2 = [...new Set((result_array))];
    const info_array = new_result_array_2.map(({place_name, address_name}) => ({place_name, address_name}))
    return {data: info_array}
  }
  return {error: "Array length incorrect"}
});

If you'll be running 2 promises only, other option would be:

// Directly adding promises in Promise.all() instead of using map
const [res1, res2] = await Promise.all([fetch("url1"), fetch("url2")])
const [data1, data2] = await Promise.all([res1.json(), res2.json()])

Also check Fetch multiple links inside of forEach loop

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