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:

Define entry file patterns
Find dependencies in configuration files

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

This tutorial walks through example plugins so you’ll be ready to write your own!

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.

In summary: if tailwind is listed as a dependency then tailwind.config.* files are added as entry files.

With many tools, the dynamic configuration file import dependencies such as plugins or reporters with regular require or import statements. In this case, we have no extra work in the Knip plugin, as they’ll be treated as regular entry files. All internal and external dependencies of the tailwind.config.ts entry file will be marked as used.

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(toDeferResolve);
};

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

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

{
  "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. what resolves as the default export) into the resolveConfig function:

6. `resolveConfig`

This function receives the exported value of the config files, and executes the resolveConfig function with this object. The plugin should return the 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.

Example 3: custom entry paths

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. If that’s the case, than we can define resolveEntryPaths in our plugin to take this from the configuration object and return it to Knip:

7. resolveEntryPaths

Here’s an example from the Preconstruct plugin:

const resolveEntryPaths: ResolveConfig<PreconstructConfig> = async config => {
  return (config.entrypoints ?? []).map(toEntry);
};

With Preconstruct, you can configure entrypoints. If this function is implemented in a plugin, Knip will use its return value over the default entry patterns. The result is that you don’t need to duplicate this customization in both the tool (e.g. Preconstruct) and Knip.

Inputs

You may have noticed the toDeferResolve and toEntry functions. They’re a way for plugins to tell what they’ve found and how to handle it. The more precise a plugin can be, the better it is for results and performance. Here’s a list of all input type functions:

toEntry

An entry input is just like an entry in the configuration. It should either be an absolute or relative path, and glob patterns are allowed.

toProductionEntry

A production entry input is just like an production in the configuration. It should either be an absolute or relative path, and it can have glob patterns.

toDependency

The dependency indicates the entry is a dependency, belonging in either the "dependencies" or "devDependencies" section of package.json.

toProductionDependency

The production dependency indicates the entry is a production dependency, expected to be listed in "dependencies".

toDeferResolve

The deferResolve input type is used to defer the resolution of a specifier. This could be resolved to a dependency or an entry file. For instance, the specifier "input" could be resolved to "input.js", "input.tsx", "input/index.js" or the "input" package name. Local files are added as entry files, package names are external dependencies.

If this does not lead to a resolution, the specifier will be reported under “unresolved imports”.

toDeferResolveEntry

The deferResolveEntry input type is similar to deferResolve, but it’s used for entry files only (not dependencies) and unresolved inputs are ignored. It’s different from toEntry as glob patterns are not supported.

toConfig

The config input type is a way for plugins to reference a configuration file that should be handled by a different plugin. For instance, Angular configurations might contain references to tsConfig and karmaConfig files, so these config files can then be handled by the TypeScript and Karma plugins, respectively.

Requires the pluginName option.

toBinary

The binary input type isn’t used by plugins directly, but by the shell script parser (through the getInputsFromScripts helper). Think of GitHub Actions worfklow YAML files or husky scripts. Using this input type, a binary is “assigned” to the dependency that has it as a "bin" in their package.json.

Options

When creating inputs from specifiers, extra options can be provided.

dir

The optional dir argument assigns the input to a different workspace. For instance, GitHub Action workflows are always stored in the root workspace, and support working-directory in job steps. For example:

jobs:
  stylelint:
    runs-on: ubuntu-latest
    steps:
      - run: npx esbuild
        working-directory: packages/app

The GitHub Action plugin understands working-directory and adds this dir to the input:

toDependency('esbuild', { dir: 'packages/app' });

Knip now understands esbuild is a dependency of the workspace in the packages/app directory.

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 executables of the tool.

For now, there are two resources available to learn more:

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! Feel free to join the Knip Discord channel if you have any questions.