For loop with fetch returning empty array

You can try using Promise.all to turn your array of fetch calls into an aggregate promise that resolves to an array of responses when all have arrived. If any fail, the whole thing fails (use Promise.allSettled if you don't want all-or-nothing semantics). Don't forget to catch the error.

Although the code doesn't show it, be sure to check response.ok to make sure the request actually succeeded before calling .json(). Throwing an error if !repsonse.ok and handling it in the .catch block is a typical strategy. Writing a wrapper on fetch is not a bad idea to avoid verbosity.

Lastly, note that Array#slice replaces the for loop. For arrays with fewer than 8 elements, it'll slice as many as are available without issue.

// mock everything
const fetch = (() => {
  const responses = [
    {
      json: async () => 
        [...Array(10)].map((e, i) => ({reference_image_id: i}))
    },
    ...Array(10)
      .fill()
      .map((_, i) => ({json: async () => i})),
  ];
  return async () => responses.shift();
})();
const req = {query: {name: "doberman"}};
const key = "foobar";
const res = {send: response => console.log(`sent ${response}`)};
// end mocks

fetch(`https://api.thedogapi.com/v1/breeds/search?name=${req.query.name}&apikey=${key}`)
  .then(response => response.json())
  .then(data => 
    Promise.all(data.slice(0, 8).map(e =>
      fetch(`https://api.thedogapi.com/v1/images/${e.reference_image_id}`)
        .then(response => response.json())
    ))
  )
  .then(results => res.send(results))
  .catch(err => console.error(err))
;

Here is an example of an async function unsing await:

async function fun(queryName, key){
  const a = [], p, j = [];
  let firstWait = await fetch(`https://api.thedogapi.com/v1/breeds/search?name=${req.query.name}&apikey=${key}`);
  let firstJson = await firstWait.json(); // must be an Array
  for(let i=0,n=8,j,l=firstJson.length; i<n && i<l; i++){
    a.push(fetch('https://api.thedogapi.com/v1/images/'+firstJson[i].reference_image_id));
  }
  p = await Promise.all(a);
  for(let v of p){
    j.push(v.json());
  }
  return Promise.all(j);
}
// assumes req, req.query, req.query.name, and key are already defined
fun(req.query.name, key).then(a=>{
  // a is your JSON Array
});

JSON

Here's my hot take: Stop using low-level functions like fetch every time you want to get JSON. This tangles up fetching logic every time we want to get a bit of JSON. Write getJSON once and use it wherever you need JSON -

const getJSON = s =>
  fetch(s).then(r => r.json())

const data =
  await getJSON("https://path/to/some/data.json")

// ...

URL and URLSearchParams

Another hot take: Stop writing all of your URLs by hand. This tangles URL writing/rewriting with all of your api access logic. We can setup a DogApi endpoint once, with a base url and an apikey -

const DogApi =
  withApi("https://api.thedogapi.com/v1", {apikey: "0xdeadbeef"})

And now whenever we need to touch that endpoint, the base url and default params can be inserted for us -

const breed = 
  // https://api.thedogapi.com/v1/breeds/search?apikey=0xdeadbeef&name=chihuahua
  await getJSON(DogApi("/breeds/search", {name}))
    

// ...

withApi has a simple implementation -

const withApi = (base, defaults) => (pathname, params) =>
{ const u = new URL(url) // <- if you don't know it, learn URL
  u.pathname = pathname 
  setParams(u, defaults)
  setParams(u, params)
  return u.toString()
}

function setParams (url, params = {})
{ for (const [k,v] of Object.entries(params))
    url.searchParams.set(k, v)  // <- if you don't know it, learn URLSearchParams
  return url
}

fruits of your labor

Now it's dead simple to write functions like imagesForBreed, and any other functions that touch JSON or your DogApi -

async function imagesForBreed (name = "")
{ if (name == "")
    return []

  const breed = 
    await getJSON(DogApi("/breeds/search", {name}))

  const images =
    data.map(v => getJSON(DogAPI(`/images/${v.reference_image_id}`))
  
  return Promise.all(images)
}

And your entire Express handler is reduced to a single line, with no need to touch .then or other laborious API configuration -

async function fooHandler (req, res)
{ 
   res.send(imagesForBreed(req.query.name))
}