Helical is a general purpose code generator. It takes a data model definition and a set of templates (called generators) and generates an output.

Usage no npm install needed!

<script type="module">
  import helical from 'https://cdn.skypack.dev/helical';



Helical is a general purpose code generator. It takes a data model definition and a set of templates (called generators) and generates an output.

Data model definition

It is just a JSON file with no given structure. The structure depends on the templates used. This file can be stored at any location.

Manifest and generators

You need a manifest file and a set of templates. You have to put all of them in the same directory. The templates can be in subdirectories, but the manifest file must always be in the root of the directory and it must be called helical.json.

The templating language used in all the templates is nunjucks

The manifest file is a JSON file with two sections:

  • generators
  • options

Here there is an example:

  "generators": [
      "source": "generators/entity.js",
      "path": "models/{{ object.name | lower }}.js",
      "foreach": "entities"
      "source": "generators/endpoint.js",
      "path": "controllers/{{ ancestors[0].name | lower }}-{{ object.action | lower }}.js",
      "foreach": "entities.endpoints"
      "source": "generators/app.js",
      "path": "index.js",
      "foreach": ""
  "options": [
      "name": "css",
      "describe": "CSS preprocessor",
      "type": "string",
      "choices": ["less", "sass"]

And this could be a data model suitable of being processed by these generators:

  "projectName": "project_name",
  "entities": [
      "name": "User",
      "fields": [
          "name": "email",
          "type": "string"
          "name": "name",
          "type": "string"
      "endpoints": [
          "action": "show",
          "method": "get"
          "action": "create",
          "method": "post"
          "action": "edit",
          "method": "put"
          "action": "delete",
          "method": "delete"


Every generator must have at least these attributes: source, path and foreach.

  • source: this is the relative path to the template.
  • path: this is an inline template to generate the output filename.
  • foreach: tells helical which objects should be processed by this generator. Leave it empty if you only want to execute this template once for the whole data model.

In the example above for the first generator helical will loop over all the entities in your data model. And for each entity it will run the entity.js template. If the data model file contains an entity with { "name": "User" } then after processing that entity the output will be stored in a file called models/user.js.

Every template, including the inlined templates to generate the filename (path attribute), will receive these objects:

  • object: the object being processed.
  • ancestors: an array of ancestor objects.
  • root: the root of the data model.
  • options: see section options below.

In the example above the second generator will be processed for each endpoint of each entity. Every time the endpoint.js template is processed it will receive these objects:

  • object: the endpoint being processed.
  • ancestors: in this example ancestors[0] will reference the parent entity of this endpoint.
  • root: the root of the data model.


Optionally you can define an array of options in your helical.json file. These are values that the user can pass in the command line interface to the generators. Each option has the following attributes:

  • name. The name of the option. If you have an option called css in your templates you can use options.css to access the value
  • describe (optional). A description of this option
  • type (optional). The type of this option. Can be string, array, boolean, count or string. See the yargs documentation about this.
  • choices (optional). You can specify an array of allowed values for this option.
  • default (optional). The default value for this option.

If you specify a not boolean option and there's not a default value and the user didn't specify the option in the command line then the application will exit with a message like this:

Usage: helical --model model.json --generator /path/to/some-generator

  --model, -m      The data model file                                                             [string] [required]
  --generator, -g  The directory that contains the helical generator                               [string] [required]
  --output, -o     Output directory                                                                [string] [required]
  --force, -f      Override files existing files                                                             [boolean]
  -h, --help       Show help                                                                                 [boolean]
  --css            CSS preprocessor                                      [string] [required] [choices: "less", "sass"]

Missing required argument: css

Static files

You can create a static directory and everything on it will be copied to the output directory. This is a good place to put files that are not plain text such as images, fonts, or simply plain text files that don't need to be processed.

Next steps message

You can render a message after all the files are generated creating a template next-steps.txt. This template will have access to the root and options objects.

Overriding files

By default helical will not override existing files. But you can use the --force option to change this behavior.

Watching for file changes

While developing helical generators or just to see interactively how the generators change when editing your data model, you can use the --watch option in the command line.

Note: files starting with a dot (.) are ignored.

Error notifications

If you want to get notified when an error occurs processing any file, parsing the model, etc. you can use the --notify option. Right now it only notifies about errors, but in a future it will notify if the whole process got fixed if you change any file and it was giving errors previously.


npm install helical -g


In this repository there is an example in the example directory. You can run it with:

helical \
  --model example/model.json \
  --generator example \
  --output output \
  --css less

This is the output:

Wrote output/models/user.js
Wrote output/controllers/user-show.js
Wrote output/controllers/user-create.js
Wrote output/controllers/user-edit.js
Wrote output/controllers/user-delete.js
Wrote output/index.js

Skipping objects

You can avoid some objects to be processed by returning an empty file name in the path attribute of a generator. Example:

  "source": "endpoint.js",
  "path": "{% if object.action != 'delete' %}controllers/{{ ancestors[0].name | lower }}-{{ object.action | lower }}.js{% endif %}",
  "foreach": "entities.endpoints"

In this case endpoints with "action": "delete" won't generate any output.