Packlets provide a lightweight alternative to NPM packages for organizing source files within a single project. The formalism is validated using ESLint rules.

When building a large application, it's a good idea to organize source files into modules, so that their dependencies can be managed. For example, suppose an application's source files can be grouped as follows:

• src/logging/*.ts - the logging system
• src/data-model/*.ts - the data model
• src/reports/*.ts - the report engine
• src/*.ts - other arbitrary files such as startup code and the main application

Using file folders is helpful, but it's not very strict. Files undersrc/logging can easily import files from/src/reports, creating a confusing circular import. They can also import arbitrary application files. Also, there is no clear distinction between which files are the "public API" forsrc/logging versus its private implementation details.

All these problems can be solved by reorganizing the project into NPM packages (orRush projects). Something like this:

• @my-app/logging - the logging system
• @my-app/data-model - the data model
• @my-app/reports - the report engine
• @my-app/application - other arbitrary files such as startup code and the main application

However, separating code in this way has some downsides. The projects need to build separately, which has some tooling costs (for example, "watch mode" now needs to consider multiple projects). In a large monorepo, the library may attract other consumers, before the API has been fully worked out.

Packlets provide a lightweight alternative that offers many of the same benefits of packages, but without thepackage.json file. It's a great way to prototype your project organization before later graduating your packlets into proper NPM packages.

With packlets, our folders would be reorganized as follows:

• src/packlets/logging/*.ts - the logging system
• src/packlets/data-model/*.ts - the data model
• src/packlets/reports/*.ts - the report engine
• src/*.ts - other arbitrary files such as startup code and the main application

Thepacklets-tutorial sample project illustrates this layout in full detail.

The basic design can be summarized in 5 rules:

1. A "packlet" is defined to be a folder path./src/packlets/<packlet-name>/index.ts. Theindex.ts file will have the exported APIs. The<packlet-name> name must consist of lower case words separated by hyphens, similar to an NPM package name.

   Example file paths:

   src/packlets/controlssrc/packlets/loggersrc/packlets/my-long-name

   NOTE: Thepacklets cannot be nested deeper in the tree. Like with NPM packages,src/packlets is a flat namespace.

2. Files outside the packlet folder can only import the packlet rootindex.ts:

   src/app/App.ts

   // Okayimport{MainReport}from'../packlets/reports';// Error: The import statement does not use the packlet's entry point (@rushstack/packlets/mechanics)import{MainReport}from'../packlets/reports/index';// Error: The import statement does not use the packlet's entry point (@rushstack/packlets/mechanics)import{MainReport}from'../packlets/reports/MainReport';

3. Files inside a packlet folder should import their siblings directly, not via their ownindex.ts (which might create a circular reference):

   src/packlets/logging/Logger.ts

   // Okayimport{MessageType}from"./MessageType";// Error: Files under a packlet folder must not import from their own index.ts file (@rushstack/packlets/mechanics)import{MessageType}from".";// Error: Files under a packlet folder must not import from their own index.ts file (@rushstack/packlets/mechanics)import{MessageType}from"./index";

4. Packlets may reference other packlets, but not in a way that would introduce a circular dependency:

   src/packlets/data-model/DataModel.ts

   // Okayimport{Logger}from'../../packlets/logging';

   src/packlets/logging/Logger.ts

   // Error: Packlet imports create a circular reference:  (@rushstack/packlets/circular-deps)//   "logging" is referenced by src/packlets/data-model/DataModel.ts//   "data-model" is referenced by src/packlets/logging/Logger.tsimport{DataModel}from'../../packlets/data-model';

5. Other source files are allowed outside thesrc/packlets folder. They may import a packlet, but packlets must only import from other packlets or NPM packages.

   src/app/App.ts

   // Okayimport{MainReport}from'../packlets/reports';

   src/packlets/data-model/ExampleModel.ts

   // Error: A local project file cannot be imported. A packlet's dependencies must be// NPM packages and/or other packlets. (@rushstack/packlets/mechanics)import{App}from'../../app/App';

To enable packlet validation for a simpletypescript-eslint setup, reference the@rushstack/eslint-plugin-packlets project like this:

<my-project>/.eslintrc.js

module.exports={root:true,parser:'@typescript-eslint/parser',plugins:['@typescript-eslint'],extends:['eslint:recommended','plugin:@typescript-eslint/recommended','plugin:@rushstack/eslint-plugin-packlets/recommended'// <--- ADD THIS],parserOptions:{project:'./tsconfig.json',sourceType:'module',tsconfigRootDir:__dirname}};

Or, if you are using the@rushstack/eslint-config ruleset, add the"packlets" mixin like this:

<my-project>/.eslintrc.js

// This is a workaround for https://github.com/eslint/eslint/issues/3458require('@rushstack/eslint-config/patch/modern-module-resolution');module.exports={extends:["@rushstack/eslint-config/profile/node","@rushstack/eslint-config/mixins/packlets"// <--- ADD THIS],parserOptions:{tsconfigRootDir:__dirname}};

The@rushstack/eslint-plugin-packlets plugin implements three separate rules:

• @rushstack/packlets/mechanics - validates most of the import path rules outlined above.
• @rushstack/packlets/circular-deps - detects circular dependencies between packlets. This rule requires an ESLint configuration that enables full type information from the TypeScript compiler.
• @rushstack/packlets/readme - requires each packlet to have a README.md file. This rule is disabled by default.

If you'd like to require a README.md file in each packlet folder, enable the optional@rushstack/packlets/readme rule.

TheminimumReadmeWords option allows you to specify a minimum number of words of documentation in the README.md file. The default value is10 words.

Example configuration with the@rushstack/packlets/readme rule enabled:

<my-project>/.eslintrc.js

// This is a workaround for https://github.com/eslint/eslint/issues/3458require('@rushstack/eslint-config/patch/modern-module-resolution');module.exports={extends:["@rushstack/eslint-config/profile/node","@rushstack/eslint-config/mixins/packlets"],parserOptions:{tsconfigRootDir:__dirname},overrides:[{files:['*.ts','*.tsx'],rules:{'@rushstack/packlets/readme':[// <--- ADD THIS'warn',{minimumReadmeWords:10}]}}]};

• CHANGELOG.md - Findout what's new in the latest version
• @rushstack/eslint-config documentation

@rushstack/eslint-plugin-packlets is part of theRush Stack family of projects.The idea for packlets was originally proposed by@bartvandenende-wmand@victor-wm.