A simple wrapper for nunjucks for use with Home Assistant frontend custom components to render templates instanteneously at HTML render time. This repository offers an easy way for developers to add templating support to Home Assistant custom cards.

Nunjucks is a templating engine for JavaScript that is heavily inspired by jinja2. Home Assistant uses jinja2 to process templates in card configurations on the backend, so the syntax of jinja2 and Nunjucks is virtually the same. This makes it an excellent alternative for Home Assistant templating for custom cards.

While some Home Assistant native cards support templating for certain fields, implementing proper Home Assistant jinja2 template support in custom cards can be difficult. Additionally Home Assistant jinja2 templates are processed by the Python backend, and can take several seconds to render. Nunjucks templates are processed by the frontend using the frontend hass object before your custom card's HTML is rendered, making nunjucks templating instanteous and much faster than traditional jinja2 templates.

Install using npm:

npm install ha-nunjucks

Then import renderTemplate from ha-nunjucks and provide it with the hass object and a template string you want to process.

import { renderTemplate } from 'ha-nunjucks';

const renderedString = renderTemplate(this.hass, templateString);

Rather than rendering templates on the backend, nunjucks renders templates on the frontend. This repository uses the Home Assistant object present in all custom cards to read entity state data.

You can also provide additional context to the renderTemplate function to pass to nunjucks if you want to make additional variables or project specific functions available to your users for use in templates.

import { renderTemplate } from 'ha-nunjucks';

const context = {
  foo: 'bar',
  doThing(thing: string) {
    return `doing ${thing}!`;
  },
};

const renderedString = renderTemplate(this.hass, templateString, context);

renderTemplate will return a string unless the result is true or false (not case sensitive), in which case it will return a boolean.

When the return type is expected to be a number, end users should cast these values using the nunjucks int or float filters to prevent undesired behavior caused by JavaScript forcing operations between disparate variable types. Numbers are not returned by default to prevent leading and trailing zeroes from being truncated from numerical strings.

renderTemplate will return an empty string for strings that may have been cast from nullish non-numerical values, such as undefined, null, and None (case sensitive).

The catch to this approach of rendering jinja2/nunjucks templates is that we have to reimplement all of the Home Assistant template extension functions and filters. If there are functions or filters that you use that are not currently supported, please make a feature request or try adding it to the project yourself and create a pull request.

So far a subset of the Home Assistant template extension functions have been implemented as documented below.

These variables just remap Python built-in constants to JavaScript ones.

The frontend data hass object has been exposed to users to call upon.

Because entity IDs contain periods in them, it's better to access it using bracket notation like so:

{{ hass["states"]["light.sunroom_ceiling"]["state"] }}

You can also use dot notation for everything but the entity ID.

{{ hass.states["light.sunroom_ceiling"].state }}

Functions used to determine an entity's state or an attribute.

A shorthand for an if else statement.

Functions that are not from the Home Assistant templating documentation