Edit in JSFiddle

var mappingFilter = {
    'ID': {
        create: function(data) {
            return ko.utils.unwrapObservable(data.data);
        }
    }
}
var info = {
    Name: "JohnH",
    ID: "1"
};
var viewModel = ko.mapping.fromJS(info, mappingFilter);
var isFunc = typeof viewModel.ID == "function";
if (isFunc == false) $("#result").append("ID property is not a function");
else $("#result").append("ID property is a function");
ko.applyBindings(viewModel);

<div id="result"></div>
</br>

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