Alternative solution for EloquentJavaScript exercise 17.2

Promise-all.js

function all(promises) {
  return new Promise(function(success, fail) {
    // Your code here.
    function promiseThen(i) {
      return promises[i].then(function(res) {
        //we should return the res here.
        //push into results here won't work, since
        //promises here settles at different time
        //and elements in results will be out of order.
        return res;
      }, fail);
    }
    
    function createPtFunc(pt) {
      return function() {
        //Instead of just return pt, push into results
        //here.
        return pt.then(function(res) {
          results.push(res);
        });
      };
    }
    
    var results = [];
    var sequence = Promise.resolve();
    for (var i = 0; i < promises.length; i++) {
      var pt = promiseThen(i);
      sequence = sequence.then(createPtFunc(pt));
    }
    sequence.then(function() {
      success(results);
    });
    
    /*
    var results = [];
    var pro = Promise.resolve();
    promises.push(pro);
    promises.reduce(function(sequence, promise) {
      return sequence.then(function() {
        return promise;
      }).then(function(res) {
        if (promise == pro)
          success(results);
        else
          results.push(res);
      }, fail);
    }, Promise.resolve());
    */
  });
}

// Test code.
all([]).then(function(array) {
  console.log("This should be []:", array);
});

function soon(val) {
  return new Promise(function(success) {
    setTimeout(function() { success(val); },
               Math.random() * 500);
  });
}

all([soon(1), soon(2), soon(3)]).then(function(array) {
  console.log("This should be [1, 2, 3]:", array);
});


function fail() {
  return new Promise(function(success, fail) {
    fail(new Error("boom"));
  });
}
all([soon(1), fail(), soon(3)]).then(function(array) {
  console.log("We should not get here");
}, function(error) {
  if (error.message != "boom")
    console.log("Unexpected failure:", error);
});