var _ =  require('underscore');

var sequence = function(products) {
  _.reduce(products, function(promise, product) {
    return promise.then(function() {
      return queryFn(product);
    })
  }, Promise.resolve());
};

var queryFn = function(product) {
  return new Promise(function(resolve, reject) {
    console.log("querying " + product);
    setTimeout(function() {
      var result = product + " checked.";
      console.log(result);
      resolve(product + " checked.");
    }, 1000);   
  });
  
};

var products = ["a", "b", "c"];

sequence(products);