Working with Plugins

Each plugin is an npm module with a name in the format of eslint-plugin-<plugin-name>, such as eslint-plugin-jquery. You can also use scoped packages in the format of @<scope>/eslint-plugin-<plugin-name> such as @jquery/eslint-plugin-jquery.

Create a Plugin

The easiest way to start creating a plugin is to use the Yeoman generator. The generator will guide you through setting up the skeleton of a plugin.

Rules in Plugins

Plugins can expose additional rules for use in ESLint. To do so, the plugin must export a rules object containing a key-value mapping of rule ID to rule. The rule ID does not have to follow any naming convention (so it can just be dollar-sign, for instance).

module.exports = {
    rules: {
        "dollar-sign": {
            create: function (context) {
                // rule implementation ...
            }
        }
    }
};

To use the rule in ESLint, you would use the unprefixed plugin name, followed by a slash, followed by the rule name. So if this plugin were named eslint-plugin-myplugin, then in your configuration you’d refer to the rule by the name myplugin/dollar-sign. Example: "rules": {"myplugin/dollar-sign": 2}.

Environments in Plugins

Plugins can expose additional environments for use in ESLint. To do so, the plugin must export an environments object. The keys of the environments object are the names of the different environments provided and the values are the environment settings. For example:

module.exports = {
    environments: {
        jquery: {
            globals: {
                $: false
            }
        }
    }
};

There’s a jquery environment defined in this plugin. To use the environment in ESLint, you would use the unprefixed plugin name, followed by a slash, followed by the environment name. So if this plugin were named eslint-plugin-myplugin, then you would set the environment in your configuration to be "myplugin/jquery".

Plugin environments can define the following objects:

  1. globals - acts the same globals in a configuration file. The keys are the names of the globals and the values are true to allow the global to be overwritten and false to disallow.
  2. parserOptions - acts the same as parserOptions in a configuration file.

Processors in Plugins

You can also create plugins that would tell ESLint how to process files other than JavaScript. In order to create a processor, the object that is exported from your module has to conform to the following interface:

module.exports = {
    processors: {
        "processor-name": {
            // takes text of the file and filename
            preprocess: function(text, filename) {
                // here, you can strip out any non-JS content
                // and split into multiple strings to lint

                return [ // return an array of code blocks to lint
                    { text: code1, filename: "0.js" },
                    { text: code2, filename: "1.js" },
                ];
            },

            // takes a Message[][] and filename
            postprocess: function(messages, filename) {
                // `messages` argument contains two-dimensional array of Message objects
                // where each top-level array item contains array of lint messages related
                // to the text that was returned in array from preprocess() method

                // you need to return a one-dimensional array of the messages you want to keep
                return [].concat(...messages);
            },

            supportsAutofix: true // (optional, defaults to false)
        }
    }
};

The preprocess method takes the file contents and filename as arguments, and returns an array of code blocks to lint. The code blocks will be linted separately but still be registered to the filename.

A code block has two properties text and filename; the text property is the content of the block and the filename property is the name of the block. Name of the block can be anything, but should include the file extension, that would tell the linter how to process the current block. The linter will check --ext CLI option to see if the current block should be linted, and resolve overrides configs to check how to process the current block.

It’s up to the plugin to decide if it needs to return just one part, or multiple pieces. For example in the case of processing .html files, you might want to return just one item in the array by combining all scripts, but for .md file where each JavaScript block might be independent, you can return multiple items.

The postprocess method takes a two-dimensional array of arrays of lint messages and the filename. Each item in the input array corresponds to the part that was returned from the preprocess method. The postprocess method must adjust the locations of all errors to correspond to locations in the original, unprocessed code, and aggregate them into a single flat array and return it.

Reported problems have the following location information:

{
    line: number,
    column: number,

    endLine?: number,
    endColumn?: number
}

By default, ESLint will not perform autofixes when a processor is used, even when the --fix flag is enabled on the command line. To allow ESLint to autofix code when using your processor, you should take the following additional steps:

  1. Update the postprocess method to additionally transform the fix property of reported problems. All autofixable problems will have a fix property, which is an object with the following schema:

     {
         range: [number, number],
         text: string
     }
    

    The range property contains two indexes in the code, referring to the start and end location of a contiguous section of text that will be replaced. The text property refers to the text that will replace the given range.

    In the initial list of problems, the fix property will refer to a fix in the processed JavaScript. The postprocess method should transform the object to refer to a fix in the original, unprocessed file.

  2. Add a supportsAutofix: true property to the processor.

You can have both rules and processors in a single plugin. You can also have multiple processors in one plugin. To support multiple extensions, add each one to the processors element and point them to the same object.

Specifying Processor in Config Files

To use a processor, add its ID to a processor section in the config file. Processor ID is a concatenated string of plugin name and processor name with a slash as a separator. This can also be added to a overrides section of the config, to specify which processors should handle which files.

For example:

plugins:
  - a-plugin
overrides:
  - files: "*.md"
    processor: a-plugin/markdown

See Specifying Processor for details.

File Extension-named Processor

If a processor name starts with ., ESLint handles the processor as a file extension-named processor especially and applies the processor to the kind of files automatically. People don’t need to specify the file extension-named processors in their config files.

For example:

module.exports = {
    processors: {
        // This processor will be applied to `*.md` files automatically.
        // Also, people can use this processor as "plugin-id/.md" explicitly.
        ".md": {
            preprocess(text, filename) { /* ... */ },
            postprocess(messageLists, filename) { /* ... */ }
        }
    }
}

Configs in Plugins

You can bundle configurations inside a plugin by specifying them under the configs key. This can be useful when you want to provide not just code style, but also some custom rules to support it. Multiple configurations are supported per plugin. Note that it is not possible to specify a default configuration for a given plugin and that users must specify in their configuration file when they want to use one.

// eslint-plugin-myPlugin

module.exports = {
    configs: {
        myConfig: {
            plugins: ["myPlugin"],
            env: ["browser"],
            rules: {
                semi: "error",
                "myPlugin/my-rule": "error",
                "eslint-plugin-myPlugin/another-rule": "error"
            }
        },
        myOtherConfig: {
            plugins: ["myPlugin"],
            env: ["node"],
            rules: {
                "myPlugin/my-rule": "off",
                "eslint-plugin-myPlugin/another-rule": "off"
                "eslint-plugin-myPlugin/yet-another-rule": "error"
            }
        }
    }
};

If the example plugin above were called eslint-plugin-myPlugin, the myConfig and myOtherConfig configurations would then be usable by extending off of "plugin:myPlugin/myConfig" and "plugin:myPlugin/myOtherConfig", respectively.

{
    "extends": ["plugin:myPlugin/myConfig"]
}

Note: Please note that configuration will not enable any of the plugin’s rules by default, and instead should be treated as a standalone config. This means that you must specify your plugin name in the plugins array as well as any rules you want to enable that are part of the plugin. Any plugin rules must be prefixed with the short or long plugin name. See Configuring Plugins for more information.

Peer Dependency

To make clear that the plugin requires ESLint to work correctly you have to declare ESLint as a peerDependency in your package.json. The plugin support was introduced in ESLint version 0.8.0. Ensure the peerDependency points to ESLint 0.8.0 or later.

{
    "peerDependencies": {
        "eslint": ">=0.8.0"
    }
}

Testing

ESLint provides the RuleTester utility to make it easy to test the rules of your plugin.

Share Plugins

In order to make your plugin available to the community you have to publish it on npm.

Recommended keywords:

Add these keywords into your package.json file to make it easy for others to find.

Further Reading