The use of Handlebars.js is at the core of how Ghost themes operate. Handlebars is what is known as a "logic-less" templating language, which in a nutshell means its job is simply to display blog-specific content.

When creating themes for Ghost, it is important to understand that the platform keeps a very clear delineation between design, which is the area themes are responsible for, and functionality, which is the area Ghost itself and apps are responsible for. The logic-less nature of Handlebars helps to ensure that Ghost themes don't step outside the role for which they are intended, and that is controlling the presentation of a blog.

Unlike PHP-driven templating systems, Ghost's Handlebars-driven template files don't handle any manipulation of data, custom database queries, or any other processes driven by logic. In a Ghost theme, you can't write functions, create variables, or evaluate conditions outside of checking whether or not there is content available...