Rendering in backbone.js with underscore templates

I try to render a list of item using underscore's template. But it displays nothing.

I tried to create 2 views, one is a task and one is a task list, but I'm stuck at using template.

What does .render() do?
if I use template, do I still need to render?

I thought the data will map the template variable when you do this.$el.html(template_var)?

<!DOCTYPE HTML>
<html>
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>


 <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css">
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js"></script>
<script src="http://underscorejs.org/underscore-min.js"></script>
<script src="http://backbonejs.org/backbone-min.js"></script>

        <script type="text/template" id="task_list">
            <li><%=title%></li>
            <li><%=priority%></li>
        </script>

        </script>

<script type="text/javascript">
        (function(){

        window.App = {
            Models:{},
            Collections:{},
            Views:{}
        };

        window.template = function(id){
            return _.template( $('#' + id).html() );
        }
        

        App.Models.Task = Backbone.Model.extend({
                title: 'default',
                priority: 1
        });

        App.Collections.Task = Backbone.Collection.extend({
            model: App.Models.Task
        });

        App.Views.Task = Backbone.View.extend({
            template: template('task_list'),
            render(){
                var template = this.template( this.model.toJSON() );
                console.log(template)
                this.$el.html(template);
                return this;
            }
        })

        App.Views.Tasks = Backbone.View.extend({
            tagName: 'ul',
            render: function(){
                this.collection.each( this.addOne, this);
                return this;
            },
            addOne: function(task){
                var taskView = new App.Views.Task({ model: task})
                taskView.render();
                this.$el.append(taskView.render().el);
            }
        })

        var tasks = new App.Collections.Task([
            {
                    title: 'Go to store',
                    priority: 4
                },
                {
                    title: 'Eat',
                    priority: 3
                },
                {
                    title: 'Sleep',
                    priority: 4
                }
        ])

        var tasksView = new App.Views.Tasks({collection: tasks})
        $('body').html(tasksView.el)

        })()

</script>

</body>
</html>

You were really close, but there were small problems.

Backbone does a lot of things, but one thing it doesn't do for you is the rendering logic, which is left completely up to the developer. So, you need:

a template to separate the HTML from the the JS logic.
a render function which does the rendering using your favorite technique, jQuery in our case.

A Backbone view always has a root DOM element (el) even if it was not rendered yet, which is a div by default if tagName isn't specified.

So your task view looked something like this when rendered:

<div>
    <li>Go to store</li>
    <li>4</li>
</div>

I changed the template a little to work.

I moved the CSS back to the HEAD section. This is a standard, but was not really one of the problem.

Default attributes in a model should be specified in the defaults property.

Defining function with the shorthand syntax like the following is only available in ES6 (ECMAScript 2015).

render(){

Starting with ECMAScript 2015 (ES6), a shorter syntax for method definitions on objects initializers is introduced. It is a shorthand for a function assigned to the method's name.

And to make it compatible with most browser, you should use:

render: function() {

You also forgot to call render on the list view.

$('body').html(tasksView.el)

Should be:

$('body').html(tasksView.render().el);

Since your code is inside an IIFE, you don't need to make your app and functions global with the window, a local var is enough and is a better coding practice.

<!DOCTYPE HTML>
<html>

<head>
    <meta charset="UTF-8">
    <title>Title</title>

    <!-- CSS should be in the head -->
    <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css">
</head>

<body>



    <script src="https://ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.js"></script>
    <script src="http://underscorejs.org/underscore.js"></script>
    <script src="http://backbonejs.org/backbone.js"></script>

    <script type="text/template" id="task_list">
        <%=title%> (<%=priority%>)
    </script>

    <script type="text/javascript">
        (function() {

            var App = {
                Models: {},
                Collections: {},
                Views: {}
            };

            function template(id) {
                return _.template($('#' + id).html());
            }


            App.Models.Task = Backbone.Model.extend({
                defaults: {
                    title: 'default',
                    priority: 1
                }
            });

            App.Collections.Task = Backbone.Collection.extend({
                model: App.Models.Task
            });

            App.Views.Task = Backbone.View.extend({
                template: template('task_list'),
                tagName: 'li',
                render: function() {
                    var template = this.template(this.model.toJSON());
                    console.log(template)
                    this.$el.html(template);
                    return this;
                }
            });

            App.Views.Tasks = Backbone.View.extend({
                tagName: 'ul',
                render: function() {
                    this.collection.each(this.addOne, this);
                    return this;
                },
                addOne: function(task) {
                    var taskView = new App.Views.Task({
                        model: task
                    })
                    this.$el.append(taskView.render().el);
                }
            });

            var tasks = new App.Collections.Task([{
                title: 'Go to store',
                priority: 4
            }, {
                title: 'Eat',
                priority: 3
            }, {
                title: 'Sleep',
                priority: 4
            }]);

            var tasksView = new App.Views.Tasks({
                collection: tasks
            });
            $('body').html(tasksView.render().el);

        })()
    </script>

</body>

</html>