Return best weather service that answers within a timed limit

fetchWeather.js

function fetchWeather(services, timeout) {
  let bestData = null;
  const controllers = services.map(() => new AbortController());
  const cancelFrom = index => controllers.slice(index).map(c => c.abort());
  const fetches = services.map((url, index) =>
    fetch(url, {signal: controller[index].signal})
      .then((res) => {
        bestData = res;
        cancelFrom(index+1);
        return res;
      })
  );
  const limit = new Promise(r => setTimeout(r, timeout))
    .then(() => cancelFrom(0));
  return Promise.race([limit, fetches[0]])
    .then(() => bestData)
};

I'm not sure about line 15, what if api 0 instantly rejects and all others are slow (but could return data eventually) then this would return null. Might make sense to race limit with a promise from promise.all somehow

what if api 0 instantly rejects

Hm yeah it wouldn't actually return null, as the then() wouldn't be invoked if the race rejects, but it would throw an unhandled rejection exception. You'd need something to deal with that case, true.

You could solve it by adding a catch handler to line 6 returning the next promise in the chain - a bit ugly, but it might work!

const fetches = services.map((url, index) =>
    fetch(url, {signal: controller[index].signal})
      .then(
        res => {
          bestData = res;
          cancelFrom(index+1);
        },
        () => services[index + 1],
      )
  );