var cart = [{ productName: "Fish", quantity: 4, cost: 10.40},

            { productName: "Beef", quantity: 2, cost: 5.50},

            { productName: "Green stuff", quantity: 25, cost: 0.75}];

​

// Get extended line items => { productName: "XXX", totalCost: XX.XX}

​

var lineItems = [];

for ( var i = 0; i < cart.length; i++) {

  var line = { productName: cart[i].productName,

               totalCost: cart[i].quantity * cart[i].cost};

  lineItems.push( line);

}

​

console.log( lineItems);

​

var total = 0;

for ( var i = 0; i < lineItems.length; i++) {

  total += lineItems[i].totalCost;

}

​

console.log( total);

​

var easierLines = _.map(cart, function(f) {

  return { productName: f.productName, total: f.cost * f.quantity};

});

​

var easierTotal = _.reduce(easierLines, function(sum, current) {

  return sum + current.total;

}, 0);

​

console.log( easierTotal);