Adding properties to the view model created by using the Knockout JS mapping plugin

Question

I am working through the mapping plugin example on the Knockoutjs website.

This is the example data.

Knockout JS Mapping Plugin

var data = {
    name: 'Scott',
    children: [
        { id : 1, name : 'Alice' }
    ]
}

The example shows how to override the mapping for one of the children but how do I alter the mapping for the base object.

If for example I wanted to add a "FavouriteChild" property to Scott how would I go about it?

I assume I need to use the create function on the base mapping but I cannot find an example of the syntax anywhere.

var myChildModel = function(data) {
    ko.mapping.fromJS(data, {}, this);

    this.nameLength = ko.computed(function() {
        return this.name().length;
    }, this);
}

var mapping = {
    'children': {
        create: function(options) {
            return new myChildModel(options.data);
        }
    }
}

var viewModel = ko.mapping.fromJS(data, mapping);

EDIT : From the accepted answer below I found this to work

<span data-bind='text: AdditionalProperty'>

The knockout code

var mapping = {
    create: function (options) {
        //customize at the root level.  
        var innerModel = ko.mapping.fromJS(options.data);

        innerModel.AdditionalProperty = 'Hello World';

        return innerModel;
    }
}

var viewModel = ko.mapping.fromJS(data, mapping);

//use this as our model bindings
ko.applyBindings(viewModel);

Answer 1

You need to use a create method on the mapping object itself like:

var mapping = {
  //customize at the root level.  
  create: function(options) {
     //first map the vm like normal
     var vm = ko.mapping.fromJS(options.data);

     //now manipulate the returned vm in any way that you like
     vm.someProperty = "test";

     vm.someComputed = ko.computed(function() {
          return vm.first() + " " + vm.last();
     });

     //return our vm that has been mapped and tweaked
     return vm;
  }
};

Answer 2

Here is a continuation to this answer based on RP Niemeyer's solution

This answer here is based on the solution above and from his blog--Thanks for that! I thought I should add some details because it addresses when the array is not a first level object.

 var data = {
person: {
       children: [{ id: 1, firstName: 'Scot', lastName: 'Weise'}]
    }
};

var mapping = {
    'children': {
        create: function(options) {
            return (new (function() {
                // setup the computed binding for some reason I had
                //  to use ko.dependentObservable instead of ko.computed
                // or errors showed up.
                this.fullName = ko.dependentObservable(function() {
                    return this.firstName() + ' ' + this.lastName();
                    }, this);
                    ko.mapping.fromJS(options.data, { }, this);
                })(/* call the ctor here */));
            }
        }
    };

    // Create view model and map the data object to the model.
    var viewModel = ko.mapping.fromJS(data, {});

    // update the viewModel with the mapping object that contains 
    // some a computed property(s)
    viewModel.person = ko.mapping.fromJS(viewModel.person, mapping);
    ko.applyBindings(viewModel);

Note that the person is the first level object and children is a sub property of that person. The line viewModel.person = ko.mapping.fromJS(viewModel.person, mapping) wasn't intutive to me at first.

And here is a slight variation

The person object is an observable that is added or updated after it is originally created from the server json data.

var viewModel = {};
$(document).ready(function () {
    var person = getPerson();

    // selected person is added to the viewModel
    viewModel.selectedPerson = ko.observable(person);
    ko.applyBindings(viewModel);
});

function getPerson() { 
// you would probably pass this into the function as a parameter. 
var person =
    {
        name: 'jim',
        children: [{ id: 1, firstName: 'jane', lastName: 'bob'}]
    };

    var mapping = {
        'children': {
            create: function (options) {
                return (new (function () {
                    // setup the computed binding
                    this.fullName = ko.dependentObservable(function () {
                    return this.firstName() + ' ' + this.lastName();
                    }, this);
                    ko.mapping.fromJS(options.data, {}, this);
                })(/* call the ctor here */));
            }
        }
    };

    var result = ko.mapping.fromJS(person, mapping);
    return result;
}

Some binding code in the html

Ultimately, you'll need to put it to some use at some point like this:

<div data-bind="foreach:viewModel.selectedPerson().children">
    <span data-bind="text:fullName"></span>
</div>

Thank for your help! I couldn't have made it this far without your blog post.

Answer 3

Note that to define additional computed observables on a child you will need to pass another set of mapping options

var mapping = {
  create: function(options) {
    //customize at the root level.  

    var mapping2 = {
      'children': {
        create: function(options) {
          //customize children also

        }
      }
    }

    //call ko.mapping.fromJS on the options.data as well with further customization
    ko.mapping.fromJS(options.data, mapping2, this);
  }
};