loadProducts.js

/**
 * Load a collection of products from a data store.
 * @param {array} productIds - Array of product IDs.
 * @param {function} load - Function to load a product by ID, e.g. fetch from API, etc.
 * @param {function} done - Done callback, to be fired when all products are loaded.
 */
function loadProducts(productIds, load, done) {
  
  // Set up vars that we will need in this function.
  var numberOfProductIds = productIds.length;
  var counter = 0;
  var products = [];
  
  // Loop through each product ID. Call the load function for each product ID.
  // Load will return a product via a callback.
  return productIds.forEach(function (productId, index) {
    return load(productId, function (product) {

      // Assign the product to the array. Increment the counter. If the counter
      // is equal to the number of products, then everything has been loaded.
      // Invoke the done callback.
      products[index] = product;
      counter += 1;
      if (counter === numberOfProductIds) {
        done(products);
      }
    }); 
  });
}