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 => () => fetch(url)) // execute them serially serial(funcs) .then(console.log.bind(console)) 

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; } 

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; } 

Addition example

const addTwo = async () => 2; const addThree = async (inValue) => new Promise((resolve) => setTimeout(resolve(inValue + 3), 2000)); const addFour = (inValue) => new Promise((res) => setTimeout(res(inValue + 4), 1000)); const addFive = async (inValue) => inValue + 5; // Function which handles promises from above async function sequenceAddition() { let sum = await [addTwo, addThree, addFour, addFive].reduce( (promise, currPromise) => promise.then((val) => currPromise(val)), Promise.resolve() ); console.log('sum:', sum); // 2 + 3 + 4 + 5 = 14 } // Run function. See console for result. sequenceAddition();

General syntax to use reduce()

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...

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); } 

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.

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).

With async/await of ES2016 (and maybe some features of ES2018), this can be reduced to this form:

function readFile(file) { ... // Returns a promise. } async function readFiles(files) { for (file in files) { await readFile(file) } } 

I haven't seen another answer express that simplicity. The OP said parallel execution of readFile was not desired. However, with IO like this it really makes sense to not be blocking on a single file read, while keeping the loop execution synchronous (you don't want to do the next step until all files have been read). Since I just learned about this and am a bit excited about it, I'll share that approach of parallel asynchronous execution of readFile with overall synchronous execution of readFiles.

async function readFiles(files) { await Promise.all(files.map(readFile)) } 

Isn't that a thing of beauty?

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(...) 

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.

I find myself coming back to this question many times and the answers aren't exactly giving me what I need, so putting this here for anyone that needs this too.

The code below does sequential promises execution (one after another), and each round consists of multiple callings:

async function sequence(list, cb) { const result = []; await list.reduce(async (promise, item) => promise .then(() => cb(item)) .then((res) => result.push(res) ), Promise.resolve()); return result; } 

Showcase:

<script src="https://cdnjs.cloudflare.com/ajax/libs/axios/0.15.3/axios.min.js"></script> <script src="https://unpkg.com/@babel/standalone@7/babel.min.js"></script> <script type="text/babel"> function sleep(ms) { return new Promise(resolve => setTimeout(resolve, ms)); } async function readFile(url, index) { console.log('Running index: ', index); // First action const firstTime = await axios.get(url); console.log('First API response: ', firstTime.data.activity); // Second action await sleep(1000); // Third action const secondTime = await axios.get(url); console.log('Second API response: ', secondTime.data.activity); // Fourth action await sleep(1000); return secondTime.data; } async function sequence(urls, fn) { const result = []; await urls.reduce(async (promise, url, index) => promise.then(() => fn(url, index)).then((res) => result.push(res)), Promise.resolve()); return result; } const urls = [ 'https://www.boredapi.com/api/activity', 'https://www.boredapi.com/api/activity', 'https://www.boredapi.com/api/activity', ]; (async function init() { const result = await sequence(urls, readFile); console.log('result', result); })() </script>

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.

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 }); 

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 }); 

Using modern ES:

const series = async (tasks) => { const results = []; for (const task of tasks) { const result = await task(); results.push(result); } return results; }; //... const tasks = files.map(file => { return () => readFile(file); }); const readFiles = await series(tasks); 

Note that promises are "hot" in JS so to defer each task from starting until the previous task completes, we wrap it in a lambda.

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

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.

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); 

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); 

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! }; 

This is my sequentially implementation that I use in various projects:

const file = [file1, file2, file3]; const fileContents = sequentially(readFile, files); // somewhere else in the code: export const sequentially = async <T, P>( toPromise: (element: T) => Promise<P>, elements: T[] ): Promise<P[]> => { const results: P[] = []; await elements.reduce(async (sequence, element) => { await sequence; results.push(await toPromise(element)); }, Promise.resolve()); return results; }; 

Here is my Angular/TypeScript approach, using RxJS:

1. Given an array of URL strings, convert it into an Observable using the from function.
2. Use pipe to wrap the Ajax request, immediate response logic, any desired delay, and error handling.
3. Inside of the pipe, use concatMap to serialize the requests. Otherwise, using Javascript forEach or map would make the requests at the same time.
4. Use RxJS ajax to make the call, and also to add any desired delay after each call returns.

Working example: https://stackblitz.com/edit/rxjs-bnrkix?file=index.ts

The code looks like this (I left in some extras so you can choose what to keep or discard):

import { ajax } from 'rxjs/ajax'; import { catchError, concatMap, delay, from, of, map, Observable } from 'rxjs'; const urls = [ 'https://randomuser.me/api/', 'https://randomuser.me/api/', 'https://randomuser.me/api/', ]; const delayAfterCall = 500; from(urls) .pipe( concatMap((url: string) => { return ajax.getJSON(url).pipe( map((response) => { console.log('Done! Received:', response); return response; }), catchError((error) => { console.error('Error: ', error); return of(error); }), delay(delayAfterCall) ); }) ) .subscribe((response) => { console.log('received email:', response.results[0].email); }); 

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.

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);

There's promise-sequence in nodejs.

const promiseSequence = require('promise-sequence'); return promiseSequence(arr.map(el => () => doPromise(el)));