knockout.js - 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;
  }
};