How to chain Promises while keeping the data?

Question

I have two API-calls chained like so:

let weatherPromise;

if (crds) {
    weatherPromise = mapbox.getLocation(crds)
    .then(locData => darksky.getWeather(crds))
    .then(weatherData => Object.assign(weatherData, {city: locData.city}));
} else if (term) {
    weatherPromise = mapbox.getCoords(term)
    .then(locData => darksky.getWeather(locData.crds));
} else {
    weatherPromise = Promise.reject({error: 'Aborted due to unexpected POST-Body'});
}

weatherPromise.then(weatherData => {
    io.emit('update', weatherData);
    console.log(`Answer sent: ${JSON.stringify(weatherData)}`);
}, reject => {
    io.emit('update', reject);
    console.error(reject.error);
});

But it is not working at all. The error-handling is all over the place (mainly just logging undefined and failing) and for example locData doesn't seem to be accessible in that second then() anymore. I tried nesting Promises and it worked fine but I would like to avoid that anti-pattern if I can.

You can use `async/await` along with `try` and `catch` blocks for error handling. — Rohit Kashyap, Aug 26 '19 at 20:59
You can, and also nesting promises is not an anti-pattern. This is certainly opinionated. — Rohit Kashyap, Aug 26 '19 at 21:01
@RohitKashyap Seems to be somewhat agreed on: https://stackoverflow.com/a/25569299/7910454 — leonheess, Aug 26 '19 at 21:04
it seems like what you want to do here is _join_ promises, not chain them; consider doing something like `Promise.all([mapbox.getLocation(crds), darksky.getWeather(crds)]).then([locData, weatherData] => ...)` — Hamms, Aug 26 '19 at 21:27
It's an antipattern to *use a promise inside of a promise constructor*, but nesting the usage of promises is totally fine and mandatory in some cases. I'd just use `async` / `await` for readability & maintainability though. — Jonas Wilms, Aug 26 '19 at 21:30
@JonasWilms I'm struggling with implementing `async / await` here. Could you provide an example? In case it is relevant: The code I'm actually working with is [here](https://github.com/MiXT4PE/wthr/blob/master/index.js) starting in line 44. Thank you in advance — leonheess, Aug 27 '19 at 21:48

score 2 · Accepted Answer · answered Aug 27 '19 at 22:31

You can nest `.thens´, but in this case, I'd clearly go with async / await:

async function retrieveWeather() {
   if (crds) {
      const locData = await mapbox.getLocation(crds);
      const weatherData = await darksky.getWeather(crds);

      return Object.assign(weatherData, {city: locData.city});
   } else if (term) {
     const locData = await mapbox.getCoords(term);
     const weatherData = await darksky.getWeather(locData.crds);

     return weatherData;
   } else {
     throw {error: 'Aborted due to unexpected POST-Body'};
   }
}

(async function sendWeather() {
  try {
    const weatherData = await retrieveWeather();
    io.emit('update', weatherData);
    console.log(`Answer sent: ${JSON.stringify(weatherData)}`);
  } catch(error) {
     io.emit('update', error);
     console.error(reject.error);
  }
})();

@MiXT4PE async functions also "just return a promise". Thats what await works on. — Jonas Wilms, Aug 29 '19 at 19:26

How to chain Promises while keeping the data?

1 Answers1