Custom Parsers

ESLint custom parsers let you extend ESLint to support linting new non-standard JavaScript language features or custom syntax in your code. A parser is responsible for taking your code and transforming it into an abstract syntax tree (AST) that ESLint can then analyze and lint.

Creating a Custom Parser

Methods in Custom Parsers

A custom parser is a JavaScript object with either a parse or parseForESLint method. The parse method only returns the AST, whereas parseForESLint also returns additional values that let the parser customize the behavior of ESLint even more.

Both methods should take in the source code as the first argument, and an optional configuration object as the second argument, which is provided as parserOptions in a configuration file.

// customParser.js

const espree = require("espree");

// Logs the duration it takes to parse each file.
function parse(code, options) {
    const label = `Parsing file "${options.filePath}"`;
    console.time(label);
    const ast = espree.parse(code, options);
    console.timeEnd(label);
    return ast; // Only the AST is returned.
};

module.exports = { parse };
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`parse` Return Object

The parse method should simply return the AST object.

`parseForESLint` Return Object

The parseForESLint method should return an object that contains the required property ast and optional properties services, scopeManager, and visitorKeys.

ast should contain the AST object.
services can contain any parser-dependent services (such as type checkers for nodes). The value of the services property is available to rules as context.sourceCode.parserServices. Default is an empty object.
scopeManager can be a ScopeManager object. Custom parsers can use customized scope analysis for experimental/enhancement syntaxes. The default is the ScopeManager object which is created by eslint-scope.
- Support for scopeManager was added in ESLint v4.14.0. ESLint versions that support scopeManager will provide an eslintScopeManager: true property in parserOptions, which can be used for feature detection.
visitorKeys can be an object to customize AST traversal. The keys of the object are the type of AST nodes. Each value is an array of the property names which should be traversed. The default is KEYS of eslint-visitor-keys.
- Support for visitorKeys was added in ESLint v4.14.0. ESLint versions that support visitorKeys will provide an eslintVisitorKeys: true property in parserOptions, which can be used for feature detection.

Meta Data in Custom Parsers

For easier debugging and more effective caching of custom parsers, it’s recommended to provide a name and version in a meta object at the root of your custom parsers, like this:

// preferred location of name and version
module.exports = {
    meta: {
        name: "eslint-parser-custom",
        version: "1.2.3"
    }
};
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The meta.name property should match the npm package name for your custom parser and the meta.version property should match the npm package version for your custom parser. The easiest way to accomplish this is by reading this information from your package.json.

AST Specification

The AST that custom parsers should create is based on ESTree. The AST requires some additional properties about detail information of the source code.

All Nodes

All nodes must have range property.

range (number[]) is an array of two numbers. Both numbers are a 0-based index which is the position in the array of source code characters. The first is the start position of the node, the second is the end position of the node. code.slice(node.range[0], node.range[1]) must be the text of the node. This range does not include spaces/parentheses which are around the node.
loc (SourceLocation) must not be null. The loc property is defined as nullable by ESTree, but ESLint requires this property. The SourceLocation#source property can be undefined. ESLint does not use the SourceLocation#source property.

The parent property of all nodes must be rewritable. Before any rules have access to the AST, ESLint sets each node’s parent property to its parent node while traversing.

The `Program` Node

The Program node must have tokens and comments properties. Both properties are an array of the below Token interface.

interface Token {
    type: string;
    loc: SourceLocation;
    // See the "All Nodes" section for details of the `range` property.
    range: [number, number];
    value: string;
}
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tokens (Token[]) is the array of tokens which affect the behavior of programs. Arbitrary spaces can exist between tokens, so rules check the Token#range to detect spaces between tokens. This must be sorted by Token#range[0].
comments (Token[]) is the array of comment tokens. This must be sorted by Token#range[0].

The range indexes of all tokens and comments must not overlap with the range of other tokens and comments.

The `Literal` Node

The Literal node must have raw property.

raw (string) is the source code of this literal. This is the same as code.slice(node.range[0], node.range[1]).

Packaging a Custom Parser

To publish your custom parser to npm, perform the following:

Create a custom parser following the Creating a Custom Parser section above.
Create an npm package for the custom parser.
In your package.json file, set the main field as the file that exports your custom parser.
Publish the npm package.

For more information on publishing an npm package, refer to the npm documentation.

Once you’ve published the npm package, you can use it by adding the package to your project. For example:

npm install eslint-parser-myparser --save-dev
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Then add the custom parser to your ESLint configuration file with the parser property. For example:

// .eslintrc.js

module.exports = {
  parser: 'eslint-parser-myparser',
  // ... rest of configuration
};
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To learn more about using ESLint parsers in your project, refer to Configure a Parser.

Example

For a complex example of a custom parser, refer to the @typescript-eslint/parser source code.

A simple custom parser that provides a context.sourceCode.parserServices.foo() method to rules.

// awesome-custom-parser.js
var espree = require("espree");
function parseForESLint(code, options) {
    return {
        ast: espree.parse(code, options),
        services: {
            foo: function() {
                console.log("foo");
            }
        },
        scopeManager: null,
        visitorKeys: null
    };
};

module.exports = { parseForESLint };
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Include the custom parser in an ESLint configuration file:

// .eslintrc.json
{
    "parser": "./path/to/awesome-custom-parser.js"
}
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