How do I access previous promise results in a .the

Question

I have restructured my code to promises, and built a wonderful long flat promise chain, consisting of multiple .then() callbacks. In the end I want to return some composite value, and need to access multiple intermediate promise results. However the resolution values from the middle of the sequence are not in scope in the last callback, how do I access them?

function getExample() {
    return promiseA(…).then(function(resultA) {
        // Some processing
        return promiseB(…);
    }).then(function(resultB) {
        // More processing
        return // How do I gain access to resultA here?
    });
}

Answer 1

This days, I also hava meet some questions like you. At last, I find a good solution with the quesition, it's simple and good to read. I hope this can help you.

According to how-to-chain-javascript-promises

ok, let's look at the code:

const firstPromise = () => {
    return new Promise((resolve, reject) => {
        setTimeout(() => {
            console.log('first promise is completed');
            resolve({data: '123'});
        }, 2000);
    });
};

const secondPromise = (someStuff) => {
    return new Promise((resolve, reject) => {
        setTimeout(() => {
            console.log('second promise is completed');
            resolve({newData: `${someStuff.data} some more data`});
        }, 2000);
    });
};

const thirdPromise = (someStuff) => {
    return new Promise((resolve, reject) => {
        setTimeout(() => {
            console.log('third promise is completed');
            resolve({result: someStuff});
        }, 2000);
    });
};

firstPromise()
    .then(secondPromise)
    .then(thirdPromise)
    .then(data => {
        console.log(data);
    });

Answer 2

A less harsh spin on "Mutable contextual state"

Using a locally scoped object to collect the intermediate results in a promise chain is a reasonable approach to the question you posed. Consider the following snippet:

function getExample(){
    //locally scoped
    const results = {};
    return promiseA(...).then(function(resultA){
        results.a = resultA;
        return promiseB(...);
    }).then(function(resultB){
        results.b = resultB;
        return promiseC(...);
    }).then(function(resultC){
        //Resolve with composite of all promises
        return Promise.resolve(results.a + results.b + resultC);
    }).catch(function(error){
        return Promise.reject(error);
    });
}

• Global variables are bad, so this solution uses a locally scoped variable which causes no harm. It is only accessible within the function.
• Mutable state is ugly, but this does not mutate state in an ugly manner. The ugly mutable state traditionally refers to modifying the state of function arguments or global variables, but this approach simply modifies the state of a locally scoped variable that exists for the sole purpose of aggregating promise results...a variable that will die a simple death once the promise resolves.
• Intermediate promises are not prevented from accessing the state of the results object, but this does not introduce some scary scenario where one of the promises in the chain will go rogue and sabotage your results. The responsibility of setting the values in each step of the promise is confined to this function and the overall result will either be correct or incorrect...it will not be some bug that will crop up years later in production (unless you intend it to!)
• This does not introduce a race condition scenario that would arise from parallel invocation because a new instance of the results variable is created for every invocation of the getExample function.

Answer 3

Node 7.4 now supports async/await calls with the harmony flag.

Try this:

async function getExample(){

  let response = await returnPromise();

  let response2 = await returnPromise2();

  console.log(response, response2)

}

getExample()

and run the file with:

node --harmony-async-await getExample.js

Simple as can be!

Answer 4

Synchronous inspection

Assigning promises-for-later-needed-values to variables and then getting their value via synchronous inspection. The example uses bluebird's .value() method but many libraries provide similar method.

function getExample() {
    var a = promiseA(…);

    return a.then(function() {
        // some processing
        return promiseB(…);
    }).then(function(resultB) {
        // a is guaranteed to be fulfilled here so we can just retrieve its
        // value synchronously
        var aValue = a.value();
    });
}

This can be used for as many values as you like:

function getExample() {
    var a = promiseA(…);

    var b = a.then(function() {
        return promiseB(…)
    });

    var c = b.then(function() {
        return promiseC(…);
    });

    var d = c.then(function() {
        return promiseD(…);
    });

    return d.then(function() {
        return a.value() + b.value() + c.value() + d.value();
    });
}

Answer 5

I am not going to use this pattern in my own code since I'm not a big fan of using global variables. However, in a pinch it will work.

User is a promisified Mongoose model.

var globalVar = '';

User.findAsync({}).then(function(users){
  globalVar = users;
}).then(function(){
  console.log(globalVar);
});

Answer 6

I think you can use hash of RSVP.

Something like as below :

    const mainPromise = () => {
        const promise1 = new Promise((resolve, reject) => {
            setTimeout(() => {
                console.log('first promise is completed');
                resolve({data: '123'});
            }, 2000);
        });

        const promise2 = new Promise((resolve, reject) => {
            setTimeout(() => {
                console.log('second promise is completed');
                resolve({data: '456'});
            }, 2000);
        });

        return new RSVP.hash({
              prom1: promise1,
              prom2: promise2
          });

    };


   mainPromise()
    .then(data => {
        console.log(data.prom1);
        console.log(data.prom2);
    });