How to chain and share prior results with Promises

Question

I'm using the bluebird library and need to make a series of HTTP requests and need to some of the response data to the next HTTP request. I've built a function that handles my requests called callhttp(). This takes a url and the body of a POST.

I'm calling it like this:

var payload = '{"Username": "joe", "Password": "password"}';
var join = Promise.join;
join(
    callhttp("172.16.28.200", payload),
    callhttp("172.16.28.200", payload),
    callhttp("172.16.28.200", payload),
    function (first, second, third) {
    console.log([first, second, third]);
});

The first request gets an API key which needs to be passed to the second request and so on. How do get the response data from the first request?

UPDATE

This is the callhttp function:

var Promise = require("bluebird");
var Request = Promise.promisify(require('request'));

function callhttp(host, body) {

    var options = {
        url: 'https://' + host + '/api/authorize',
        method: "POST",
        headers: {
            'content-type': 'application/json'
        },
        body: body,
        strictSSL: false
    };

    return Request(options).spread(function (response) {
        if (response.statusCode == 200) {
           // console.log(body)
            console.log(response.connection.getPeerCertificate().subject.CN)
            return {
                data: response.body
            };
        } else {
            // Just an example, 200 is not the only successful code
            throw new Error("HTTP Error: " + response.statusCode );
        }
    });
}

Answer 1

Edit in 2023. Now that we have async/await in every Javascript environment you execute in these days, the real answer to this is to use async/await as it vastly simplifies the collection and use of intermediate results into normal Javscript variables that can be used in any part of the sequence. See the end of this answer for that conclusion. The other parts of this answer were all written in 2015 before async/await was widely available.

There are a few models for dependent promises and passing data from one to the next. Which one works best depends upon whether you only need the previous data in the next call or whether you need access to all prior data. Here are several models:

Feed Result of One to the Next

callhttp(url1, data1).then(function(result1) {
     // result1 is available here
     return callhttp(url2, data2);
}).then(function(result2) {
     // only result2 is available here
     return callhttp(url3, data3);
}).then(function(result3) {
     // all three are done now, final result is in result3
});

Assign Intermediate Results to Higher Scope

var r1, r2, r3;
callhttp(url1, data1).then(function(result1) {
     r1 = result1;
     return callhttp(url2, data2);
}).then(function(result2) {
     r2 = result2;
     // can access r1 or r2
     return callhttp(url3, data3);
}).then(function(result3) {
     r3 = result3;
     // can access r1 or r2 or r3
});

Accumulate Results in One Object

var results = {};
callhttp(url1, data1).then(function(result1) {
     results.result1 = result1;
     return callhttp(url2, data2);
}).then(function(result2) {
     results.result2 = result2;
     // can access results.result1 or results.result2
     return callhttp(url3, data3);
}).then(function(result3) {
     results.result3 = result3;
     // can access results.result1 or results.result2 or results.result3
});

Nest, so all Previous Results Can Be Accessed

callhttp(url1, data1).then(function(result1) {
     // result1 is available here
     return callhttp(url2, data2).then(function(result2) {
         // result1 and result2 available here
         return callhttp(url3, data3).then(function(result3) {
             // result1, result2 and result3 available here
         });
     });
})

Break the Chain into Independent Pieces, Collect Results

If some parts of the chain can proceed independently, rather than one after the other, then you can launch them separately and use Promise.all() to know when those multiple pieces are done and you then will have all the data from those independent pieces:

var p1 = callhttp(url1, data1);
var p2 = callhttp(url2, data2).then(function(result2) {
    return someAsync(result2);
}).then(function(result2a) {
    return someOtherAsync(result2a);
});
var p3 = callhttp(url3, data3).then(function(result3) {
    return someAsync(result3);
});
Promise.all([p1, p2, p3]).then(function(results) {
    // multiple results available in results array
    // that can be processed further here with
    // other promises
});

Sequence with await in ES7

Since the promise chain is just a mechanism for sequencing asynchronous operations, in ES7, you can also use await and then the intermediate results are all available in the same scope (perhaps simpler than the separate scopes of the chained .then() handlers):

async function someFunction(...) {

    const r1 = await callhttp(url1, data1);

    // can use r1 here to formulate second http call
    const r2 = await callhttp(url2, data2);

    // can use r1 and r2 here to formulate third http call
    const r3 = await callhttp(url3, data3);

    // do some computation that has access to r1, r2 and r3
    return someResult;
}

someFunction(...).then(result => {
    // process final result here
}).catch(err => {
    // handle error here
});