Q:

Knockout - groupable array

From the tutorial, I am trying to understand this promising framework. 
My question is how to extract some data from an observable array (filter)? 
I found a similar quesion here :
similar question
But I don't know how to implement it. 
here is the test :
jsfiddle
HTML code:
<h2>Your seat reservations (<span data-bind="text: seats().length"></span>)</h2>
<h4>
Distribution by meal type:
    <table>
        <thead>
            <tr data-bind="foreach: availableMeals">
                <th><span data-bind="text: mealName"></span></th>
            </tr>
        </thead>
        <tbody>
            <tr data-bind="foreach: availableMeals">
                <td>???</td>
            </tr>
        </tbody>
    </table>
</h4>
<table>
    <thead><tr>
        <th>Passenger name</th><th>Meal</th><th>Surcharge</th><th></th>
    </tr></thead>
    <tbody data-bind="foreach: seats">
        <tr>
            <td><input data-bind="value: name" /></td>
            <td><select data-bind="options: $root.availableMeals, value: meal, optionsText: 'mealName'"></select></td>
            <td data-bind="text: formattedPrice"></td>
            <td><a href="#" data-bind="click: $root.removeSeat">Remove</a></td>
        </tr>    
    </tbody>
</table>

<button data-bind="click: addSeat, enable: seats().length < 7">Reserve another seat</button>

<h3 data-bind="visible: totalSurcharge() > 0">
    Total surcharge: $<span data-bind="text: totalSurcharge().toFixed(2)"></span>
</h3>

JS code :
// extend from the link to the similar question from stackoverflow:
ko.observableArray.fn.distinct = function(prop) {
    var target = this;
    target.index = {};
    target.index[prop] = ko.observable({});    
    //Group by already set up, bail out. 
    if (target.index && target.index[prop]) return target;
    ko.computed(function() {
        //rebuild index
        var propIndex = {};

        ko.utils.arrayForEach(target(), function(item) {
            var key = ko.utils.unwrapObservable(item[prop]);
            if (key) {
                propIndex[key] = propIndex[key] || [];
                propIndex[key].push(item);            
            }
        });   

        target.index[prop](propIndex);
    });

    return target;
};

// Class to represent a row in the seat reservations grid
function SeatReservation(name, initialMeal) {
    var self = this;
    self.name = name;
    self.meal = ko.observable(initialMeal);

    self.formattedPrice = ko.computed(function() {
        var price = self.meal().price;
        return price ? " $" + price.toFixed(2) : "None";        
    });    
}

// Overall viewmodel for this screen, along with initial state
function ReservationsViewModel() {
    var self = this;

    // Non-editable catalog data - would come from the server
    self.availableMeals = [
        { mealName: "Standard (sandwich)", price: 0 },
        { mealName: "Premium (lobster)", price: 34.95 },
        { mealName: "Ultimate (whole zebra)", price: 290 }
    ];    

    // Editable data
    self.seats = ko.observableArray([
        new SeatReservation("Steve", self.availableMeals[0]),
        new SeatReservation("Bert", self.availableMeals[0]),
        new SeatReservation("Steve", self.availableMeals[1]),
        new SeatReservation("John", self.availableMeals[1]),
        new SeatReservation("Frank", self.availableMeals[1]),
        new SeatReservation("Evan", self.availableMeals[2])
    ]);

    // Computed data
    self.totalSurcharge = ko.computed(function() {
       var total = 0;
       for (var i = 0; i < self.seats().length; i++)
           total += self.seats()[i].meal().price;
       return total;
    });    

    // Operations
    self.addSeat = function() {
        self.seats.push(new SeatReservation("", self.availableMeals[0]));
    }
    self.removeSeat = function(seat) { self.seats.remove(seat) }
}

ko.applyBindings(new ReservationsViewModel());

I worked from the KO tutorial : the pb is I want to display a distribution of customers by meal types? 

A:

Out of the box Knockout doesn't provide any "groupable array". 
However Knockout comes with some utility functions like:

ko.utils.arrayForEach
ko.utils.arrayFilter

which can help you to build your own custom grouping by hand. 
So here is a how sample mealDistribution implementation could look like:
self.mealDistribution = ko.computed(function () {
        var seatCount = self.seats().length;
        var result = [];
        ko.utils.arrayForEach(self.availableMeals, function (meal) {
            var mealCount = ko.utils.arrayFilter(self.seats(), function (item) {
                return item.meal() == meal}).length;
            result.push(mealCount / seatCount);

        });
        return result;
    });

Demo JSFiddle. 
If you want to have some more sophisticated array handling functions you should checkout the Underscore.js library which has some really nice functions like groupBy, countBy

