These variables need to be referenced in a separate HTML template file in the `templates` directory, which you render using the call to `JST.template_name({username: "John"});`. Everything in the `html` directory is a static file which is built at compile-time. If you need dynamic values from the server, like here, use a template.

Alternatively, if it's just a couple of values like this you can use jQuery directly without a template like `$('.profile_pic').text('John');` to manually set the text content of an element, but this isn't scalable, modular or reusable, so for anything more complex than the most trivial examples you should be using the template system.