krl krl - 2 months ago 6
Node.js Question

Poll for a result n times (with delays between attempts) before failing

We need to write a Node.js function that polls a certain API endpoint for a result of a previously requested calculation. The result takes a random time to be generated and may not me generated at all. We'd like to get it as soon as possible, but also I don't want to wait for too long, which means that after a certain number of API calls we'd like the function to fail (reject a Promise).

There is one way communication between our code and the API.

const Bluebird = require('bluebird');

function getResult() {
return new Bluebird(async function (resolve, reject) {

let counter = 0;

while (counter < 10) {
await Bluebird.delay(1000);

const res = await apiCall();
if (res.data) {
resolve(res.data);
} else {
counter += 1;
}
}

reject('timeout');
});
}


Is this the correct approach?

Answer

No. This is the async/await version of the Promise constructor antipattern! It won't even stop the loop when you call resolve, or reject when an exception is thrown (e.g. when res is null).
You should use

async function getResult() {
  for (let counter = 0; counter < 10; counter += 1) {
    await Bluebird.delay(1000);
    const res = await apiCall();
    if (res.data) {
      return res.data;
    }
  }
  throw new Error('timeout');
}

If you want to ensure that a Bluebird promise is returned, not a native one, wrap it in Bluebird.method or tell your transpiler to use Bluebird.