JSFiddle - React, Tailwind, and code Playground

HTML

<script src="http://cdnjs.cloudflare.com/ajax/libs/knockout.mapping/2.3.5/knockout.mapping.js"></script>

JavaScript

MyViewModel = function(data) {
    this.items = ko.mapping.fromJS(data);
}

MyViewModel.prototype = {
    foo: function() {
        console.log("foo");
        console.log(this.items().length);
    }
};

var data = [{}, {}, {}];
vm = new MyViewModel(data);

vm.foo();