Pass variable down through promise chain

Question

I'm slightly new to promises so please excuse my lack of knowledge. The code below highlights what I want to do but clearly I am unable to.

var token = "";
var id = "";
var cms = {
    login: function () {
        return request({
            "method": "POST",
            "uri": process.env.CMS_URL + "/cms/login",
            "json": true,
            "body": {
                "email": "xxxxxxx",
                "password": "xxxxx"
            }
        }).then(response => {
            console.log("Successfully logged in to cms. Access token: " + response);
            token = response;
        }).catch(error => {
            console.log("Couldn't get access token from cms");
        });
    },
    createCollection: function (token) {
        return request({
            "method": "POST",
            "uri": process.env.CMS_URL + "/cms/collection",
            "json": true,
            "headers": {
                "X-Cms-Token": token
            },
            "body": {
                "name": "Acceptance test collection",
                "type": "manual"
            }
        }).then(response => {
            console.log("Successfully created a collection.");
            id = response.id;
        }).catch(error => {
            console.log("Collection already exists");
        });
    },
    deleteCollection: function (token, id) {
        return request({
            "method": "DELETE",
            "uri": process.env.CMS_URL + "/cms/collection" + id,
            "json": true,
            "headers": {
                "X-Cms-Token": token
            }
        }).then(response => {
            console.log("Successfully deleted collection.");
        }).catch(error => {
            console.log("No collection to delete");
            console.log(error);
        });
    }
};
var createCollectionCms = function (token, id) {
    return new Promise((resolve) => {
        cms.login()
            .then(zebedee.createCollection(token))
            .then(zebedee.deleteCollection(token, id));
        setTimeout(resolve, 6000);
    });
}
createCollectionCms();

I need to run each function and pass in "token" and "id". By doing it the way above each function runs at the same time. I need them to execute after each other but pass down the required variables.

Essentially I need to log in, create a "collection", the delete the "collection". This is part of some tests i'm setting up.

I tend to prefer [async.js](https://github.com/caolan/async) for control flow, since it provides a lot of options such as waterfall (which lets you pass the output of one callback as the input to the next). — Paul, Jan 09 '18 at 17:18
Don't use global variables for the results, instead resolve the promises with them. — Bergi, Jan 09 '18 at 17:22
Don't pass promises to `then`, you must pass callback *functions*. Your function should look like this: `function createCollectionCms() { return cms.login().then(token => zebedee.createCollection(token).then(id => zebedee.deleteCollection(token,id))); }` ([or similar](https://stackoverflow.com/q/28250680/1048572)). — Bergi, Jan 09 '18 at 17:22

feihcsim · Accepted Answer · 2019-01-22T19:05:57.590

here's a readable and synchronous-looking alternative to using Promises using async/await (which actually still uses Promises under the hood)

// returns a Promise whose response is the token
const requestAccessToken = cmsUrl => request(/* request options with cmsUrl */)

// returns a Promise whose response is the collectionId
const createCollection = token => request(/* request options with token */)

// returns a Promise whose response is given by deleting the respective collection
const deleteCollection = collectionId => request(/* request options with collectionId */)

// create and execute an async function that puts all these functions together in a synchronous manner
const createThenDeleteCollection = async cmsUrl => {
  try { // if at any time an error is thrown, the error is caught and logged
    const token = await requestAccessToken(cmsUrl)
    const collectionId = await createCollection(token)
    const responseFromDeletingCollection = await deleteCollection(collectionId)
    console.log(responseFromDeletingCollection)
  } catch (err) {
    console.error(err)
  }
}
createThenDeleteCollection(process.env.CMS_URL)

a caveat that comes with using this method is that it isn't supported by IE if you're executing it in the browser, and if you're executing in Node, you'll need to use at least version 7.6

score 0 · Answer 2 · answered Jan 09 '18 at 17:23

Here you have a simple example showing how to pass values through the promise chain.

const one = () => new Promise((resolve) => {
    return resolve(1)
});

const two = (val) => new Promise((resolve) => {
    return resolve(val * 5);
})

const three = (val, val2) => new Promise((resolve) => {
    return resolve(val + val2);
}) 

one().then((r) => {
    //r is 1
    console.log('r is 1:', r)
  return r
})
//we pass the return value from the previous to the two function
.then(two)
.then((r) => {
    //r is 1 * 5
  console.log('r is 1 * 5:', r)
  return three(r, 10)
})
.then((r) => {
  //r is now 5 + 10
    console.log('r is now 5 + 10:', r)
})

If you need to pass more than one value use an array or an object. https://jsfiddle.net/rfk2Lxku/

Pass variable down through promise chain

2 Answers2