Resolve promises one after another (i.e. in sequence)?


Consider the following code that reads an array of files in a serial/sequential manner. readFiles returns a promise, which is resolved only once all files have been read in sequence.

var readFile = function(file) {
  ... // Returns a promise.
};

var readFiles = function(files) {
  return new Promise((resolve, reject) => 

    var readSequential = function(index) {
      if (index >= files.length) {
        resolve();
      } else {
        readFile(files[index]).then(function() {
          readSequential(index + 1);
        }).catch(reject);
      }
    };

   readSequential(0); // Start!

  });
};

The above code works, but I don't like having to do recursion for things to occur sequentially. Is there a simpler way that this code can be re-written so that I don't have to use my weird readSequential function?

Originally I tried to use Promise.all, but that caused all of the readFile calls to happen concurrently, which is not what I want:

var readFiles = function(files) {
  return Promise.all(files.map(function(file) {
    return readFile(file);
  }));
};

Update 2017: I would use an async function if the environment supports it:

async function readFiles(files) {
  for(const file of files) {
    await readFile(file);
  }
};

If you'd like, you can defer reading the files until you need them using an async generator (if your environment supports it):

async function* readFiles(files) {
  for(const file of files) {
    yield await readFile(file);
  }
};

Update: In second thought - I might use a for loop instead:

var readFiles = function(files) {
  var p = Promise.resolve(); // Q() in q

  files.forEach(file =>
      p = p.then(() => readFile(file)); 
  );
  return p;
};

Or more compactly, with reduce:

var readFiles = function(files) {
  return files.reduce((p, file) => {
     return p.then(() => readFile(file));
  }, Promise.resolve()); // initial
};

In other promise libraries (like when and Bluebird) you have utility methods for this.

For example, Bluebird would be:

var Promise = require("bluebird");
var fs = Promise.promisifyAll(require("fs"));

var readAll = Promise.resolve(files).map(fs.readFileAsync,{concurrency: 1 });
// if the order matters, you can use Promise.each instead and omit concurrency param

readAll.then(function(allFileContents){
    // do stuff to read files.
});

Although there is really no reason not to use async await today.


Here is how I prefer to run tasks in series.

function runSerial() {
    var that = this;
    // task1 is a function that returns a promise (and immediately starts executing)
    // task2 is a function that returns a promise (and immediately starts executing)
    return Promise.resolve()
        .then(function() {
            return that.task1();
        })
        .then(function() {
            return that.task2();
        })
        .then(function() {
            console.log(" ---- done ----");
        });
}

What about cases with more tasks? Like, 10?

function runSerial(tasks) {
  var result = Promise.resolve();
  tasks.forEach(task => {
    result = result.then(() => task());
  });
  return result;
}

This question is old, but we live in a world of ES6 and functional JavaScript, so let's see how we can improve.

Because promises execute immediately, we can't just create an array of promises, they would all fire off in parallel.

Instead, we need to create an array of functions that returns a promise. Each function will then be executed sequentially, which then starts the promise inside.

We can solve this a few ways, but my favorite way is to use reduce.

It gets a little tricky using reduce in combination with promises, so I have broken down the one liner into some smaller digestible bites below.

The essence of this function is to use reduce starting with an initial value of Promise.resolve([]), or a promise containing an empty array.

This promise will then be passed into the reduce method as promise. This is the key to chaining each promise together sequentially. The next promise to execute is func and when the then fires, the results are concatenated and that promise is then returned, executing the reduce cycle with the next promise function.

Once all promises have executed, the returned promise will contain an array of all the results of each promise.

ES6 Example (one liner)

/*
 * serial executes Promises sequentially.
 * @param {funcs} An array of funcs that return promises.
 * @example
 * const urls = ['/url1', '/url2', '/url3']
 * serial(urls.map(url => () => $.ajax(url)))
 *     .then(console.log.bind(console))
 */
const serial = funcs =>
    funcs.reduce((promise, func) =>
        promise.then(result => func().then(Array.prototype.concat.bind(result))), Promise.resolve([]))

ES6 Example (broken down)

// broken down to for easier understanding

const concat = list => Array.prototype.concat.bind(list)
const promiseConcat = f => x => f().then(concat(x))
const promiseReduce = (acc, x) => acc.then(promiseConcat(x))
/*
 * serial executes Promises sequentially.
 * @param {funcs} An array of funcs that return promises.
 * @example
 * const urls = ['/url1', '/url2', '/url3']
 * serial(urls.map(url => () => $.ajax(url)))
 *     .then(console.log.bind(console))
 */
const serial = funcs => funcs.reduce(promiseReduce, Promise.resolve([]))

Usage:

// first take your work
const urls = ['/url1', '/url2', '/url3', '/url4']

// next convert each item to a function that returns a promise
const funcs = urls.map(url => () => $.ajax(url))

// execute them serially
serial(funcs)
    .then(console.log.bind(console))

To do this simply in ES6:

function(files) {
  // Create a new empty promise (don't do that with real people ;)
  var sequence = Promise.resolve();

  // Loop over each file, and add on a promise to the
  // end of the 'sequence' promise.
  files.forEach(file => {

    // Chain one computation onto the sequence
    sequence = 
      sequence
        .then(() => performComputation(file))
        .then(result => doSomething(result)); 
        // Resolves for each file, one at a time.

  })

  // This will resolve after the entire chain is resolved
  return sequence;
}

Simple util for standard Node.js promise:

function sequence(tasks, fn) {
    return tasks.reduce((promise, task) => promise.then(() => fn(task)), Promise.resolve());
}

UPDATE

items-promise is a ready to use NPM package doing the same.


I've had to run a lot of sequential tasks and used these answers to forge a function that would take care of handling any sequential task...

function one_by_one(objects_array, iterator, callback) {
    var start_promise = objects_array.reduce(function (prom, object) {
        return prom.then(function () {
            return iterator(object);
        });
    }, Promise.resolve()); // initial
    if(callback){
        start_promise.then(callback);
    }else{
        return start_promise;
    }
}

The function takes 2 arguments + 1 optional. First argument is the array on which we will be working. The second argument is the task itself, a function that returns a promise, the next task will be started only when this promise resolves. The third argument is a callback to run when all tasks have been done. If no callback is passed, then the function returns the promise it created so we can handle the end.

Here's an example of usage:

var filenames = ['1.jpg','2.jpg','3.jpg'];
var resize_task = function(filename){
    //return promise of async resizing with filename
};
one_by_one(filenames,resize_task );

Hope it saves someone some time...


Nicest solution that I was able to figure out was with bluebird promises. You can just do Promise.resolve(files).each(fs.readFileAsync); which guarantees that promises are resolved sequentially in order.


This is a slight variation of another answer above. Using native Promises:

function inSequence(tasks) {
    return tasks.reduce((p, task) => p.then(task), Promise.resolve())
}

Explanation

If you have these tasks [t1, t2, t3], then the above is equivalent to Promise.resolve().then(t1).then(t2).then(t3). It's the behavior of reduce.

How to use

First You need to construct a list of tasks! A task is a function that accepts no argument. If you need to pass arguments to your function, then use bind or other methods to create a task. For example:

var tasks = files.map(file => processFile.bind(null, file))
inSequence(tasks).then(...)

My preferred solution:

function processArray(arr, fn) {
    return arr.reduce(
        (p, v) => p.then((a) => fn(v).then(r => a.concat([r]))),
        Promise.resolve([])
    );
}

It's not fundamentally different from others published here but:

  • Applies the function to items in series
  • Resolves to an array of results
  • Doesn't require async/await (support is still quite limited, circa 2017)
  • Uses arrow functions; nice and concise

Example usage:

const numbers = [0, 4, 20, 100];
const multiplyBy3 = (x) => new Promise(res => res(x * 3));

// Prints [ 0, 12, 60, 300 ]
processArray(numbers, multiplyBy3).then(console.log);

Tested on reasonable current Chrome (v59) and NodeJS (v8.1.2).


Use Array.prototype.reduce, and remember to wrap your promises in a function otherwise they will already be running!

// array of Promise providers

const providers = [
  function(){
     return Promise.resolve(1);
  },
  function(){
     return Promise.resolve(2);
  },
  function(){
     return Promise.resolve(3);
  }
]


const inSeries = function(providers){

  const seed = Promise.resolve(null); 

  return providers.reduce(function(a,b){
      return a.then(b);
  }, seed);
};

nice and easy... you should be able to re-use the same seed for performance, etc.

It's important to guard against empty arrays or arrays with only 1 element when using reduce, so this technique is your best bet:

   const providers = [
      function(v){
         return Promise.resolve(v+1);
      },
      function(v){
         return Promise.resolve(v+2);
      },
      function(v){
         return Promise.resolve(v+3);
      }
    ]

    const inSeries = function(providers, initialVal){

        if(providers.length < 1){
            return Promise.resolve(null)
        }

        return providers.reduce((a,b) => a.then(b), providers.shift()(initialVal));
    };

and then call it like:

inSeries(providers, 1).then(v => {
   console.log(v);  // 7
});

I created this simple method on the Promise object:

Create and add a Promise.sequence method to the Promise object

Promise.sequence = function (chain) {
    var results = [];
    var entries = chain;
    if (entries.entries) entries = entries.entries();
    return new Promise(function (yes, no) {
        var next = function () {
            var entry = entries.next();
            if(entry.done) yes(results);
            else {
                results.push(entry.value[1]().then(next, function() { no(results); } ));
            }
        };
        next();
    });
};

Usage:

var todo = [];

todo.push(firstPromise);
if (someCriterium) todo.push(optionalPromise);
todo.push(lastPromise);

// Invoking them
Promise.sequence(todo)
    .then(function(results) {}, function(results) {});

The best thing about this extension to the Promise object, is that it is consistent with the style of promises. Promise.all and Promise.sequence is invoked the same way, but have different semantics.

Caution

Sequential running of promises is not usually a very good way to use promises. It's usually better to use Promise.all, and let the browser run the code as fast as possible. However, there are real use cases for it - for example when writing a mobile app using javascript.


You can use this function that gets promiseFactories List:

function executeSequentially(promiseFactories) {
    var result = Promise.resolve();
    promiseFactories.forEach(function (promiseFactory) {
        result = result.then(promiseFactory);
    });
    return result;
}

Promise Factory is just simple function that returns a Promise:

function myPromiseFactory() {
    return somethingThatCreatesAPromise();
}

It works because a promise factory doesn't create the promise until it's asked to. It works the same way as a then function – in fact, it's the same thing!

You don't want to operate over an array of promises at all. Per the Promise spec, as soon as a promise is created, it begins executing. So what you really want is an array of promise factories...

If you want to learn more on Promises, you should check this link: https://pouchdb.com/2015/05/18/we-have-a-problem-with-promises.html


My answer based on https://stackoverflow.com/a/31070150/7542429.

Promise.series = function series(arrayOfPromises) {
    var results = [];
    return arrayOfPromises.reduce(function(seriesPromise, promise) {
      return seriesPromise.then(function() {
        return promise
        .then(function(result) {
          results.push(result);
        });
      });
    }, Promise.resolve())
    .then(function() {
      return results;
    });
  };

This solution returns the results as an array like Promise.all().

Usage:

Promise.series([array of promises])
.then(function(results) { 
  // do stuff with results here
});

I really liked @joelnet's answer, but to me, that style of coding is a little bit tough to digest, so I spent a couple of days trying to figure out how I would express the same solution in a more readable manner and this is my take, just with a different syntax and some comments.

// first take your work
const urls = ['/url1', '/url2', '/url3', '/url4']

// next convert each item to a function that returns a promise
const functions = urls.map((url) => {
  // For every url we return a new function
  return () => {
    return new Promise((resolve) => {
      // random wait in milliseconds
      const randomWait = parseInt((Math.random() * 1000),10)
      console.log('waiting to resolve in ms', randomWait)
      setTimeout(()=>resolve({randomWait, url}),randomWait)
    })
  }
})


const promiseReduce = (acc, next) => {
  // we wait for the accumulator to resolve it's promise
  return acc.then((accResult) => {
    // and then we return a new promise that will become
    // the new value for the accumulator
    return next().then((nextResult) => {
      // that eventually will resolve to a new array containing
      // the value of the two promises
      return accResult.concat(nextResult)
    })
  })
};
// the accumulator will always be a promise that resolves to an array
const accumulator = Promise.resolve([])

// we call reduce with the reduce function and the accumulator initial value
functions.reduce(promiseReduce, accumulator)
  .then((result) => {
    // let's display the final value here
    console.log('=== The final result ===')
    console.log(result)
  })

As Bergi noticed, I think the best and clear solution is use BlueBird.each, code below:

const BlueBird = require('bluebird');
BlueBird.each(files, fs.readFileAsync);

First, you need to understand that a promise is executed at the time of creation.
So for example if you have a code:

["a","b","c"].map(x => returnsPromise(x))

You need to change it to:

["a","b","c"].map(x => () => returnsPromise(x))

Then we need to sequentially chain promises:

["a", "b", "c"].map(x => () => returnsPromise(x))
    .reduce(
        (before, after) => before.then(_ => after()),
        Promise.resolve()
    )

executing after(), will make sure that promise is created (and executed) only when its time comes.


I use the following code to extend the Promise object. It handles rejection of the promises and returns an array of results

Code

/*
    Runs tasks in sequence and resolves a promise upon finish

    tasks: an array of functions that return a promise upon call.
    parameters: an array of arrays corresponding to the parameters to be passed on each function call.
    context: Object to use as context to call each function. (The 'this' keyword that may be used inside the function definition)
*/
Promise.sequence = function(tasks, parameters = [], context = null) {
    return new Promise((resolve, reject)=>{

        var nextTask = tasks.splice(0,1)[0].apply(context, parameters[0]); //Dequeue and call the first task
        var output = new Array(tasks.length + 1);
        var errorFlag = false;

        tasks.forEach((task, index) => {
            nextTask = nextTask.then(r => {
                output[index] = r;
                return task.apply(context, parameters[index+1]);
            }, e=>{
                output[index] = e;
                errorFlag = true;
                return task.apply(context, parameters[index+1]);
            });
        });

        // Last task
        nextTask.then(r=>{
            output[output.length - 1] = r;
            if (errorFlag) reject(output); else resolve(output);
        })
        .catch(e=>{
            output[output.length - 1] = e;
            reject(output);
        });
    });
};

Example

function functionThatReturnsAPromise(n) {
    return new Promise((resolve, reject)=>{
        //Emulating real life delays, like a web request
        setTimeout(()=>{
            resolve(n);
        }, 1000);
    });
}

var arrayOfArguments = [['a'],['b'],['c'],['d']];
var arrayOfFunctions = (new Array(4)).fill(functionThatReturnsAPromise);


Promise.sequence(arrayOfFunctions, arrayOfArguments)
.then(console.log)
.catch(console.error);

If you want you can use reduce to make a sequential promise, for example:

[2,3,4,5,6,7,8,9].reduce((promises, page) => {
    return promises.then((page) => {
        console.log(page);
        return Promise.resolve(page+1);
    });
  }, Promise.resolve(1));

it'll always works in sequential.


Using modern ES:

const series = async (tasks) => {
  const results = [];

  for (const task of tasks) {
    const result = await task;

    results.push(result);
  }

  return results;
};

//...

const readFiles = await series(files.map(readFile));

With Async/Await (if you have the support of ES7)

function downloadFile(fileUrl) { ... } // This function return a Promise

async function main()
{
  var filesList = [...];

  for (const file of filesList) {
    await downloadFile(file);
  }
}

(you must use for loop, and not forEach because async/await has problems running in forEach loop)

Without Async/Await (using Promise)

function downloadFile(fileUrl) { ... } // This function return a Promise

function downloadRecursion(filesList, index)
{
  index = index || 0;
  if (index < filesList.length)
  {
    downloadFile(filesList[index]).then(function()
    {
      index++;
      downloadRecursion(filesList, index); // self invocation - recursion!
    });
  }
  else
  {
    return Promise.resolve();
  }
}

function main()
{
  var filesList = [...];
  downloadRecursion(filesList);
}

On the basis of the question's title, "Resolve promises one after another (i.e. in sequence)?", we might understand that the OP is more interested in the sequential handling of promises on settlement than sequential calls per se.

This answer is offered :

  • to demonstrate that sequential calls are not necessary for sequential handling of responses.
  • to expose viable alternative patterns to this page's visitors - including the OP if he is still interested over a year later.
  • despite the OP's assertion that he does not want to make calls concurrently, which may genuinely be the case but equally may be an assumption based on the desire for sequential handling of responses as the title implies.

If concurrent calls are genuinely not wanted then see Benjamin Gruenbaum's answer which covers sequential calls (etc) comprehensively.

If however, you are interested (for improved performance) in patterns which allow concurrent calls followed by sequential handling of responses, then please read on.

It's tempting to think you have to use Promise.all(arr.map(fn)).then(fn) (as I have done many times) or a Promise lib's fancy sugar (notably Bluebird's), however (with credit to this article) an arr.map(fn).reduce(fn) pattern will do the job, with the advantages that it :

  • works with any promise lib - even pre-compliant versions of jQuery - only .then() is used.
  • affords the flexibility to skip-over-error or stop-on-error, whichever you want with a one line mod.

Here it is, written for Q.

var readFiles = function(files) {
    return files.map(readFile) //Make calls in parallel.
    .reduce(function(sequence, filePromise) {
        return sequence.then(function() {
            return filePromise;
        }).then(function(file) {
            //Do stuff with file ... in the correct sequence!
        }, function(error) {
            console.log(error); //optional
            return sequence;//skip-over-error. To stop-on-error, `return error` (jQuery), or `throw  error` (Promises/A+).
        });
    }, Q()).then(function() {
        // all done.
    });
};

Note: only that one fragment, Q(), is specific to Q. For jQuery you need to ensure that readFile() returns a jQuery promise. With A+ libs, foreign promises will be assimilated.

The key here is the reduction's sequence promise, which sequences the handling of the readFile promises but not their creation.

And once you have absorbed that, it's maybe slightly mind-blowing when you realise that the .map() stage isn't actually necessary! The whole job, parallel calls plus serial handling in the correct order, can be achieved with reduce() alone, plus the added advantage of further flexibility to :

  • convert from parallel async calls to serial async calls by simply moving one line - potentially useful during development.

Here it is, for Q again.

var readFiles = function(files) {
    return files.reduce(function(sequence, f) {
        var filePromise = readFile(f);//Make calls in parallel. To call sequentially, move this line down one.
        return sequence.then(function() {
            return filePromise;
        }).then(function(file) {
            //Do stuff with file ... in the correct sequence!
        }, function(error) {
            console.log(error); //optional
            return sequence;//Skip over any errors. To stop-on-error, `return error` (jQuery), or `throw  error` (Promises/A+).
        });
    }, Q()).then(function() {
        // all done.
    });
};

That's the basic pattern. If you wanted also to deliver data (eg the files or some transform of them) to the caller, you would need a mild variant.


Your approach is not bad, but it does have two issues: it swallows errors and it employs the Explicit Promise Construction Antipattern.

You can solve both of these issues, and make the code cleaner, while still employing the same general strategy:

var Q = require("q");

var readFile = function(file) {
  ... // Returns a promise.
};

var readFiles = function(files) {
  var readSequential = function(index) {
    if (index < files.length) {
      return readFile(files[index]).then(function() {
        return readSequential(index + 1);
      });
    }
  };

  // using Promise.resolve() here in case files.length is 0
  return Promise.resolve(readSequential(0)); // Start!
};

If someone else needs a guaranteed way of STRICTLY sequential way of resolving Promises when performing CRUD operations you also can use the following code as a basis.

As long as you add 'return' before calling each function, describing a Promise, and use this example as a basis the next .then() function call will CONSISTENTLY start after the completion of the previous one:

getRidOfOlderShoutsPromise = () => {
    return readShoutsPromise('BEFORE')
    .then(() => {
        return deleteOlderShoutsPromise();
    })
    .then(() => {
        return readShoutsPromise('AFTER')
    })
    .catch(err => console.log(err.message));
}

deleteOlderShoutsPromise = () => {
    return new Promise ( (resolve, reject) => {
        console.log("in deleteOlderShouts");
        let d = new Date();
        let TwoMinuteAgo = d - 1000 * 90 ;
        All_Shouts.deleteMany({ dateTime: {$lt: TwoMinuteAgo}}, function(err) {
            if (err) reject();
            console.log("DELETED OLDs at "+d);
            resolve();        
        });
    });
}

readShoutsPromise = (tex) => {
    return new Promise( (resolve, reject) => {
        console.log("in readShoutsPromise -"+tex);
        All_Shouts
        .find({})
        .sort([['dateTime', 'ascending']])
        .exec(function (err, data){
            if (err) reject();
            let d = new Date();
            console.log("shouts "+tex+" delete PROMISE = "+data.length +"; date ="+d);
            resolve(data);
        });    
    });
}

Array push and pop method can be used for sequence of promises. You can also push new promises when you need additional data. This is the code, I will use in React Infinite loader to load sequence of pages.

var promises = [Promise.resolve()];

function methodThatReturnsAPromise(page) {
	return new Promise((resolve, reject) => {
		setTimeout(() => {
			console.log(`Resolve-${page}! ${new Date()} `);
			resolve();
		}, 1000);
	});
}

function pushPromise(page) {
	promises.push(promises.pop().then(function () {
		return methodThatReturnsAPromise(page)
	}));
}

pushPromise(1);
pushPromise(2);
pushPromise(3);


Most of the answers dont include the results of ALL promises individually, so in case someone is looking for this particular behaviour, this is a possible solution using recursion.

It follows the style of Promise.all:

  • Returns the array of results in the .then() callback.

  • If some promise fails, its returned immediately in the .catch() callback.

const promiseEach = (arrayOfTasks) => {
  let results = []
  return new Promise((resolve, reject) => {
    const resolveNext = (arrayOfTasks) => {
      // If all tasks are already resolved, return the final array of results
      if (arrayOfTasks.length === 0) return resolve(results)

      // Extract first promise and solve it
      const first = arrayOfTasks.shift()

      first().then((res) => {
        results.push(res)
        resolveNext(arrayOfTasks)
      }).catch((err) => {
        reject(err)
      })
    }
    resolveNext(arrayOfTasks)
  })
}

// Lets try it ????

const promise = (time, shouldThrowError) => new Promise((resolve, reject) => {
  const timeInMs = time * 1000
  setTimeout(()=>{
    console.log(`Waited ${time} secs`)
    if (shouldThrowError) reject(new Error('Promise failed'))
    resolve(time)
  }, timeInMs)
})

const tasks = [() => promise(1), () => promise(2)]

promiseEach(tasks)
  .then((res) => {
    console.log(res) // [1, 2]
  })
  // Oops some promise failed
  .catch((error) => {
    console.log(error)
  })

Note about the tasks array declaration:

In this case is not possible to use the following notation like Promise.all would use:

const tasks = [promise(1), promise(2)]

And we have to use:

const tasks = [() => promise(1), () => promise(2)]

The reason is that JavaScript starts executing the promise immediatelly after its declared. If we use methods like Promise.all, it just checks that the state of all of them is fulfilled or rejected, but doesnt start the exection itself. Using () => promise() we stop the execution until its called.


(function() {
  function sleep(ms) {
    return new Promise(function(resolve) {
      setTimeout(function() {
        return resolve();
      }, ms);
    });
  }

  function serial(arr, index, results) {
    if (index == arr.length) {
      return Promise.resolve(results);
    }
    return new Promise(function(resolve, reject) {
      if (!index) {
        index = 0;
        results = [];
      }
      return arr[index]()
        .then(function(d) {
          return resolve(d);
        })
        .catch(function(err) {
          return reject(err);
        });
    })
      .then(function(result) {
        console.log("here");
        results.push(result);
        return serial(arr, index + 1, results);
      })
      .catch(function(err) {
        throw err;
      });
  }

  const a = [5000, 5000, 5000];

  serial(a.map(x => () => sleep(x)));
})();

Here the key is how you call the sleep function. You need to pass an array of functions which itself returns a promise instead of an array of promises.


This is to extend on how to process a sequence of promises in a more generic way, supporting dynamic / infinite sequences, based on spex.sequence implementation:

var $q = require("q");
var spex = require('spex')($q);

var files = []; // any dynamic source of files;

var readFile = function (file) {
    // returns a promise;
};

function source(index) {
    if (index < files.length) {
        return readFile(files[index]);
    }
}

function dest(index, data) {
    // data = resolved data from readFile;
}

spex.sequence(source, dest)
    .then(function (data) {
        // finished the sequence;
    })
    .catch(function (error) {
        // error;
    });

Not only this solution will work with sequences of any size, but you can easily add data throttling and load balancing to it.