Run a promise-based operation on each item in an array, in sequence

array_promises_in_sequence.js

// An array of data to operate on
var dat = ['a', 'b', 'c', 'd', null, 'e'];

/**
 * A long running task to run for each element of the array.
 * It must return a promise
 */
function doThing(arg) {
  return new Promise(
    function (resolve, reject) {

      setTimeout(function () {
        console.log(arg);
        if (arg) {
          resolve('resolved - ' + arg);
        } else {
          reject('Error: falsy value');
        }
      }, 1000);
    }
  );
}

/**
 * Get a handle on the "resolve" method
 * of an initial promise, so we can start
 * the work after our chain of promises is
 * assembled
 */
var go;
var lastPromise = new Promise(
  function (resolve, reject) {
    go = resolve;
  }
);

/**
 * Generate an executor for a given index in the
 * array. The outer function will be evaluated
 * at promise-chain-assembly time, the inner function
 * that's returned will be run at promise-chain-run time
 */
function makeExecutor(arg) {
  return function (result /*result of previous task*/ ) {
    return doThing(arg);
  };
}

/**
 * Loop over the array of data to create a promise
 * chain
 */
for (var i = 0; i < dat.length; i += 1) {
  lastPromise = lastPromise
    .then(makeExecutor(dat[i]));
}

/**
 * Attach an error handler at the end of the chain
 */
lastPromise.catch(function (err) {
  console.err('Error: ', err);
});

// Wait a moment, then start running the tasks
setTimeout(function () {
  console.log('and we\'re off!');
  go();
}, 500);

If you run this in a browser that supports ES6 promises (i.e,. Chrome), you should see the following output

and we're off!
a
b
c
d
null
Error:  Error: falsy value