Value undefined after being resolved from Promise

Question

I think the problem here might be wrong awaits, or the fact that I have a wrapped up callback inside the promise and I'm trying to resolve the promise inside that callback. Not sure how to fix this. Any help is much appreciated!

async function pointInRegion(latitude, longitude) {
  await new Promise(async (resolve, reject) => {
    try {
      const coordinate = [longitude, latitude]
      const pt = turf.point(coordinate); // tried adding await here
      fs.readFile("./regions.geojson", async function (err, data) { // tried adding await here
        if (err) throw err;
        const geojson = JSON.parse(data);
        for (const feature of geojson.features) {
          const inRegion = turf.booleanPointInPolygon(pt, feature) // tried adding await here
          if (inRegion) {
            console.log (feature.properties.name)
            return resolve(feature.properties.name)
          }
        }
      })
    } catch (err) {
      return reject(err)
    }
  })
}

I call it in the main function like:

const calculatedRegion = await pointInRegion(latitude, longitude)
console.log(calculatedRegion) // prints undefined

You aren't returning anything from your `pointInRegion` function — Hunter McMillen, May 28 '23 at 17:50
That is _inside_ of the Promise you create, but you aren't actually returning that Promise. (e.g. try `return new Promise(....`) — Hunter McMillen, May 28 '23 at 17:52
@nickcoding2 it's in another function. Namely `async function (err, data) {` Even then, `resolve` doesn't have a return value. — VLAZ, May 28 '23 at 17:52
[`require('node:fs/promises')`](https://nodejs.org/api/fs.html#promises-api) — robertklep, May 28 '23 at 18:09
[Never pass an `async function` as the executor to `new Promise`](https://stackoverflow.com/q/43036229/1048572)! — Bergi, May 28 '23 at 18:10

Hunter McMillen · Accepted Answer · 2023-05-28T18:07:55.083

A couple of issues:

You aren't returning your Promise to any callers
You don't need async or await here, the consuming function(s) will call await on the return value of pointInRegion

Modifications with comments:

function pointInRegion(latitude, longitude) {
  // no need to await here, calling functions will await
  // return await is also not needed
  return new Promise((resolve, reject) => {
    try {
      const coordinate = [longitude, latitude]
      const pt = turf.point(coordinate);
      fs.readFile("./regions.geojson", function (err, data) { 
        if (err) {
          throw err;
        }

        const geojson = JSON.parse(data);
        for (const feature of geojson.features) {
          const inRegion = turf.booleanPointInPolygon(pt, feature);
          if (inRegion) {
            console.log (feature.properties.name);
            // no return needed for resolve
            resolve(feature.properties.name);
          }
        }
      })
    } catch (err) {
      // no return needed for reject
      reject(err);
    }
  })
}

Awesome, thank you. I guess it makes sense that the problem was me not actually returning the Promise. Thanks! — nickcoding2, May 28 '23 at 18:24

Value undefined after being resolved from Promise

1 Answers1