How to Resolve JavaScript Promises Sequentially?

​​​​​​​Resolve Promises Sequentially

Promises are a common way to handle asynchronous operations in JavaScript. However, it can be challenging to control the order in which promises are resolved.

The Problem:

Consider the following code that reads a series of files serially:

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 with the first file!
  });
};

This code uses recursion to read the files one after another, but it can be challenging to work with.

Avoid Recursion with Async/Await:

In modern JavaScript, using async functions and await is a more elegant solution:

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

Alternatively, Use Iterators:

Async generators provide an alternative approach to reading files sequentially:

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

Consider a Simple Loop:

If you prefer a simpler approach, a basic for loop can suffice:

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

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

Or, Utilize Promise Reduction:

A more compact solution using reduce:

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

Library Utility Methods

Certain promise libraries (e.g., Bluebird) provide utility methods tailored for this purpose:

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.
});

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