Writing A Plugin

Plugins provide Knip with entry files and dependencies it would be unable to find otherwise. Plugins always do at least one of the following:

1. Define entry file patterns
2. Find dependencies in configuration files

Knip v5.1.0 introduced a new plugin API, which makes them a breeze to write and maintain.

The new plugin API

Easy things should be easy, and complex things possible.

This tutorial walks through example plugins so you’ll be ready to write your own! The following examples demonstrate the elements a plugin can implement.

There’s a handy command available to easily create a new plugin and get started right away.

Example 1: entry

Let’s dive right in. Here’s the entire source code of the Tailwind plugin:

import type { IsPluginEnabled, Plugin } from '../../types/config.js';
import { hasDependency } from '../../util/plugin.js';

const title = 'Tailwind';

const enablers = ['tailwindcss'];

const isEnabled: IsPluginEnabled = ({ dependencies }) =>
  hasDependency(dependencies, enablers);

const entry = ['tailwind.config.{js,cjs,mjs,ts}'];

export default {
  title,
  enablers,
  isEnabled,
  entry,
} satisfies Plugin;

Yes, that’s the entire plugin! Let’s go over each item one by one:

1. title

The title of the plugin displayed in the list of plugins and in debug output.

2. enablers

An array of strings to match one or more dependencies in package.json so the isEnabled function can determine whether the plugin should be enabled or not. Regular expressions are allowed as well.

3. isEnabled

This function checks whether a match is found in the dependencies or devDependencies in package.json. The plugin is be enabled if the dependency is listed in package.json.

This function can be kept straightforward with the hasDependency helper.

4. entry

This plugin exports entry file patterns. This means that if the Tailwind plugin is enabled, then tailwind.config.* files are added as entry files. A Tailwind configuration file does not contain anything particular, so adding it as an entry to treat it as a regular source file is enough.

The next example shows how to handle a tool that has its own particular configuration object.

Example 2: config

Here’s the full source code of the nyc plugin:

import { toDeferResolve } from '../../util/input.js';
import { hasDependency } from '../../util/plugin.js';
import type { NycConfig } from './types.js';
import type {
  IsPluginEnabled,
  Plugin,
  ResolveConfig,
} from '../../types/config.js';

const title = 'nyc';

const enablers = ['nyc'];

const isEnabled: IsPluginEnabled = ({ dependencies }) =>
  hasDependency(dependencies, enablers);

const config = [
  '.nycrc',
  '.nycrc.{json,yml,yaml}',
  'nyc.config.js',
  'package.json',
];

const resolveConfig: ResolveConfig<NycConfig> = config => {
  const extend = config?.extends ?? [];
  const requires = config?.require ?? [];
  return [extend, requires].flat().map(id => toDeferResolve(id));
};

export default {
  title,
  enablers,
  isEnabled,
  config,
  resolveConfig,
} satisfies Plugin;

Here’s an example config file that will be handled by this plugin:

.nycrc.json

{
  "extends": "@istanbuljs/nyc-config-typescript",
  "check-coverage": true
}

Compared to the first example, this plugin has two new variables:

5. config

The config array contains all possible locations of the config file for the tool. Knip loads matching files and passes the results (i.e. its default export) into the resolveConfig function:

6. resolveConfig

This function receives the exported value of the config file, and executes the resolveConfig function with this object. The plugin should return the entry paths and dependencies referenced in this object.

Knip supports JSON, YAML, TOML, JavaScript and TypeScript config files. Files without an extension are provided as plain text strings.

Should I implement resolveConfig?

You should implement resolveConfig if any of these are true:

• The config file contains one or more options that represent entry points
• The config file references dependencies by strings (not import statements)

Example 3: entry paths

7. entry and production

Some tools operate mostly on entry files, some examples:

• Mocha looks for test files at test/*.{js,cjs,mjs}
• Storybook looks for stories at *.stories.@(mdx|js|jsx|tsx)

And some of those tools allow to configure those locations and patterns in configuration files, such as next.config.js or vite.config.ts. If that’s the case we can define resolveConfig in our plugin to take this from the configuration object and return it to Knip:

Here’s an example from the Mocha plugin:

const entry = ['**/test/*.{js,cjs,mjs}'];

const resolveConfig: ResolveConfig<MochaConfig> = localConfig => {
  const entryPatterns = localConfig.spec ? [localConfig.spec].flat() : entry;
  return entryPatterns.map(id => toEntry(id));
};

export default {
  entry,
  resolveConfig,
};

With Mocha, you can configure spec file patterns. The result of implementing resolveConfig is that users don’t need to duplicate this configuration in both the tool (e.g. Mocha) and Knip.

Use production entries to target source files that represent production code.

Tip

Regardless of the presence of resolveConfig, add entry and production to the default export so they will be displayed in the plugin’s documentation as default values.

Example 4: Use the AST directly

If the resolveFromConfig function is impemented, Knip loads the configuration file and passes the default-exported object to this plugin function. However, that object might then not contain the information we need.

Here’s an example astro.config.ts configuration file with a Starlight integration:

import starlight from '@astrojs/starlight';
import { defineConfig } from 'astro/config';

export default defineConfig({
  integrations: [
    starlight({
      components: {
        Head: './src/components/Head.astro',
        Footer: './src/components/Footer.astro',
      },
    }),
  ],
});

With Starlight, components can be defined to override the default internal ones. They’re not otherwise referenced in your source code, so you’d have to manually add them as entry files (Knip itself did this).

In the Astro plugin, there’s no way to access this object containing components to add the component files as entry files if we were to try:

const resolveConfig: ResolveConfig<AstroConfig> = async config => {
  console.log(config); //  ¯\_(ツ)_/¯
};

This is why plugins can implement the resolveFromAST function.

7. resolveFromAST

Let’s take a look at the Astro plugin implementation. This example assumes some familiarity with Abstract Syntax Trees (AST) and the TypeScript compiler API. Knip will provide more and more AST helpers to make implementing plugins more fun and a little less tedious.

Anyway, let’s dive in. Here’s how we’re adding the Starlight components paths to the default production file patterns:

import ts from 'typescript';
import {
  getDefaultImportName,
  getImportMap,
  getPropertyValues,
} from '../../typescript/ast-helpers.js';

const title = 'Astro';

const production = [
  'src/pages/**/*.{astro,mdx,js,ts}',
  'src/content/**/*.mdx',
  'src/middleware.{js,ts}',
  'src/actions/index.{js,ts}',
];

const getComponentPathsFromSourceFile = (sourceFile: ts.SourceFile) => {
  const componentPaths: Set<string> = new Set();
  const importMap = getImportMap(sourceFile);
  const importName = getDefaultImportName(importMap, '@astrojs/starlight');

  function visit(node: ts.Node) {
    if (
      ts.isCallExpression(node) &&
      ts.isIdentifier(node.expression) &&
      node.expression.text === importName // match the starlight() function call
    ) {
      const starlightConfig = node.arguments[0];
      if (ts.isObjectLiteralExpression(starlightConfig)) {
        const values = getPropertyValues(starlightConfig, 'components');
        for (const value of values) componentPaths.add(value);
      }
    }

    ts.forEachChild(node, visit);
  }

  visit(sourceFile);

  return componentPaths;
};

const resolveFromAST: ResolveFromAST = (sourceFile: ts.SourceFile) => {
  // Include './src/components/Head.astro' and './src/components/Footer.astro'
  // as production entry files so they're also part of the analysis
  const componentPaths = getComponentPathsFromSourceFile(sourceFile);
  return [...production, ...componentPaths].map(id => toProductionEntry(id));
};

export default {
  title,
  production,
  resolveFromAST,
} satisfies Plugin;

Inputs

You may have noticed functions like toDeferResolve and toEntry. They’re a way for plugins to tell what they’ve found and how Knip should handle those. The more precision a plugin can provide here, the better results and performance will be.

Find all the details over at Writing A Plugin → Inputs.

Argument Parsing

As part of the script parser, Knip parses command-line arguments. Plugins can implement the arg object to add custom argument parsing tailored to the tool.

Read more in Writing A Plugin → Argument Parsing.

Create a new plugin

The easiest way to create a new plugin is to use the create-plugin script:

cd packages/knip
bun create-plugin --name tool

This adds source and test files and fixtures to get you started. It also adds the plugin to the JSON Schema and TypeScript types.

Run the test for your new plugin:

bun test test/plugins/tool.test.ts

You’re ready to implement and submit a new Knip plugin! 🆕 🎉

Wrapping Up

Feel free to check out the implementation of other similar plugins, and borrow ideas and code from those!

The documentation website takes care of generating the plugin list and the individual plugin pages from the exported plugin values.

Thanks for reading. If you have been following this guide to create a new plugin, this might be the right time to open a pull request!