JavaScript Promise를 순차적으로 해결하는 방법은 무엇입니까?

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!
  });
};

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

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

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

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

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

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.
});