Simplest view model with a third-part template engine

ViewModel.js

function ViewModel(target_selector) {
    var ctx = this;
    var target = document.querySelector(target_selector);
    var template = target.innerHTML;

    this.render = function() {
        target.innerHTML = render(template, ctx); // render() should be the third-part template engine
    };
}

function BodyViewModel() {
    ViewModel.call(this, "body");

    this.title = "Hello World";
    this.user = { name: "Henrique", age: 31 };
    this.items = ['Daiane', 'Henrique', 'Letícia', 'Pedro Lucas'];

    this.render();
}

self.bodyViewModel = new BodyViewModel();