When mapping ko.mapping.fromJS values are null

I'm working with knockoutjs and I'm trying to populate ViewModel instance from JSON data. According to knockoutjs documentation I may use this statement:

ko.mapping.fromJS(data, viewModel);

Here is my code :

var pledgeVM=function(){            
      this.name=ko.observable();
      this.Assets=ko.observableArray([]);
      this.StartEdit=function(assetModel){
      };
 };
        pledge = {"name":"Moses","Assets":[{"CityId":13,"commetns":null},{"CityId":14,"commetns":null}]};
        var pledgeVMinstance=new pledgeVM();
        ko.mapping.fromJS(pledge,pledgeVMinstance);

For some reason data not populated (pledgeVMinstance.name() is undefined) unless I change the statement to:

ko.mapping.fromJS(pledge,{},pledgeVMinstance);

Maybe somebody can explain me why things going that way.

It happened because ko.mapping.fromJS has the following signature:

ko.mapping.fromJS(data, mappingOptions, viewModel);

Where data - is your json data, mappingOptions - is the instructions to mapping plug how to map your date, viewModel - is object to store mapped data.

ko.mapping.fromJS(data) - this syntax will create view model.

ko.mapping.fromJS(data, mappingOptions) - this will create view model with particular options.

ko.mapping.fromJS(data, {}, viewModel) - and this one convers your data without mapping options and put it to view model.

Read the documentation for better understanding: http://knockoutjs.com/documentation/plugins-mapping.html