# ESLint Plugin Kit

## Description

A collection of utilities to help build ESLint plugins.

## Installation

For Node.js and compatible runtimes:

```shell
npm install @eslint/plugin-kit
# or
yarn add @eslint/plugin-kit
# or
pnpm install @eslint/plugin-kit
# or
bun add @eslint/plugin-kit
```

For Deno:

```shell
deno add @eslint/plugin-kit
```

## Usage

This package exports the following utilities:

- [`ConfigCommentParser`](#configcommentparser) - used to parse ESLint configuration comments (i.e., `/* eslint-disable rule */`)
- [`VisitNodeStep` and `CallMethodStep`](#visitnodestep-and-callmethodstep) - used to help implement `SourceCode#traverse()`
- [`Directive`](#directive) - used to help implement `SourceCode#getDisableDirectives()`
- [`TextSourceCodeBase`](#textsourcecodebase) - base class to help implement the `SourceCode` interface

### `ConfigCommentParser`

To use the `ConfigCommentParser` class, import it from the package and create a new instance, such as:

```js
import { ConfigCommentParser } from "@eslint/plugin-kit";

// create a new instance
const commentParser = new ConfigCommentParser();

// pass in a comment string without the comment delimiters
const directive = commentParser.parseDirective(
	"eslint-disable prefer-const, no-var -- I don't want to use these.",
);

// will be undefined when a directive can't be parsed
if (directive) {
	console.log(directive.label); // "eslint-disable"
	console.log(directive.value); // "prefer-const, no-var"
	console.log(directive.justification); // "I don't want to use these."
}
```

There are different styles of directive values that you'll need to parse separately to get the correct format:

```js
import { ConfigCommentParser } from "@eslint/plugin-kit";

// create a new instance
const commentParser = new ConfigCommentParser();

// list format
const list = commentParser.parseListConfig("prefer-const, no-var");
console.log(Object.entries(list)); // [["prefer-const", true], ["no-var", true]]

// string format
const strings = commentParser.parseStringConfig("foo:off, bar");
console.log(Object.entries(strings)); // [["foo", "off"], ["bar", null]]

// JSON-like config format
const jsonLike = commentParser.parseJSONLikeConfig(
	"radix:[error, always], prefer-const: warn",
);
console.log(Object.entries(jsonLike.config)); // [["radix", ["error", "always"]], ["prefer-const", "warn"]]
```

### `VisitNodeStep` and `CallMethodStep`

The `VisitNodeStep` and `CallMethodStep` classes represent steps in the traversal of source code. They implement the correct interfaces to return from the `SourceCode#traverse()` method.

The `VisitNodeStep` class is the more common of the two, where you are describing a visit to a particular node during the traversal. The constructor accepts three arguments:

- `target` - the node being visited. This is used to determine the method to call inside of a rule. For instance, if the node's type is `Literal` then ESLint will call a method named `Literal()` on the rule (if present).
- `phase` - either 1 for enter or 2 for exit.
- `args` - an array of arguments to pass into the visitor method of a rule.

For example:

```js
import { VisitNodeStep } from "@eslint/plugin-kit";

class MySourceCode {
	traverse() {
		const steps = [];

		for (const { node, parent, phase } of iterator(this.ast)) {
			steps.push(
				new VisitNodeStep({
					target: node,
					phase: phase === "enter" ? 1 : 2,
					args: [node, parent],
				}),
			);
		}

		return steps;
	}
}
```

The `CallMethodStep` class is less common and is used to tell ESLint to call a specific method on the rule. The constructor accepts two arguments:

- `target` - the name of the method to call, frequently beginning with `"on"` such as `"onCodePathStart"`.
- `args` - an array of arguments to pass to the method.

For example:

```js
import { VisitNodeStep, CallMethodStep } from "@eslint/plugin-kit";

class MySourceCode {
    traverse() {
        const steps = [];

        for (const { node, parent, phase } of iterator(this.ast)) {
            steps.push(
                new VisitNodeStep({
                    target: node,
                    phase: phase === "enter" ? 1 : 2,
                    args: [node, parent],
                }),
            );

            // call a method indicating how many times we've been through the loop
            steps.push(
                new CallMethodStep({
                    target: "onIteration",
                    args: [steps.length]
                });
            )
        }

        return steps;
    }
}
```

### `Directive`

The `Directive` class represents a disable directive in the source code and implements the `Directive` interface from `@eslint/core`. You can tell ESLint about disable directives using the `SourceCode#getDisableDirectives()` method, where part of the return value is an array of `Directive` objects. Here's an example:

```js
import { Directive, ConfigCommentParser } from "@eslint/plugin-kit";

class MySourceCode {
	getDisableDirectives() {
		const directives = [];
		const problems = [];
		const commentParser = new ConfigCommentParser();

		// read in the inline config nodes to check each one
		this.getInlineConfigNodes().forEach(comment => {
			// Step 1: Parse the directive
			const { label, value, justification } =
				commentParser.parseDirective(comment.value);

			// Step 2: Extract the directive value and create the `Directive` object
			switch (label) {
				case "eslint-disable":
				case "eslint-enable":
				case "eslint-disable-next-line":
				case "eslint-disable-line": {
					const directiveType = label.slice("eslint-".length);

					directives.push(
						new Directive({
							type: directiveType,
							node: comment,
							value,
							justification,
						}),
					);
				}

				// ignore any comments that don't begin with known labels
			}
		});

		return {
			directives,
			problems,
		};
	}
}
```

### `TextSourceCodeBase`

The `TextSourceCodeBase` class is intended to be a base class that has several of the common members found in `SourceCode` objects already implemented. Those members are:

- `lines` - an array of text lines that is created automatically when the constructor is called.
- `getLoc(nodeOrToken)` - gets the location of a node or token. Works for nodes that have the ESLint-style `loc` property and nodes that have the Unist-style [`position` property](https://github.com/syntax-tree/unist?tab=readme-ov-file#position). If you're using an AST with a different location format, you'll still need to implement this method yourself.
- `getLocFromIndex(index)` - Converts a source text index into a `{ line: number, column: number }` pair. (For this method to work, the root node should always cover the entire source code text, and the `getLoc()` method needs to be implemented correctly.)
- `getIndexFromLoc(loc)` - Converts a `{ line: number, column: number }` pair into a source text index. (For this method to work, the root node should always cover the entire source code text, and the `getLoc()` method needs to be implemented correctly.)
- `getRange(nodeOrToken)` - gets the range of a node or token within the source text. Works for nodes that have the ESLint-style `range` property and nodes that have the Unist-style [`position` property](https://github.com/syntax-tree/unist?tab=readme-ov-file#position). If you're using an AST with a different range format, you'll still need to implement this method yourself.
- `getText(node, beforeCount, afterCount)` - gets the source text for the given node that has range information attached. Optionally, can return additional characters before and after the given node. As long as `getRange()` is properly implemented, this method will just work.
- `getAncestors(node)` - returns the ancestry of the node. In order for this to work, you must implement the `getParent()` method yourself.

Here's an example:

```js
import { TextSourceCodeBase } from "@eslint/plugin-kit";

export class MySourceCode extends TextSourceCodeBase {
	#parents = new Map();

	constructor({ ast, text }) {
		super({ ast, text });
	}

	getParent(node) {
		return this.#parents.get(node);
	}

	traverse() {
		const steps = [];

		for (const { node, parent, phase } of iterator(this.ast)) {
			//save the parent information
			this.#parent.set(node, parent);

			steps.push(
				new VisitNodeStep({
					target: node,
					phase: phase === "enter" ? 1 : 2,
					args: [node, parent],
				}),
			);
		}

		return steps;
	}
}
```

In general, it's safe to collect the parent information during the `traverse()` method as `getParent()` and `getAncestor()` will only be called from rules once the AST has been traversed at least once.

## License

Apache 2.0

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