Working with Custom Parsers

If you want to use your own parser and provide additional capabilities for your rules, you can specify your own custom parser. If a parseForESLint method is exposed on the parser, this method will be used to parse the code. Otherwise, the parse method will be used. Both methods should take in the source code as the first argument, and an optional configuration object as the second argument (provided as parserOptions in a config file). The parse method should simply return the AST. 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.
  • 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.parserServices. Default is an empty object.
  • scopeManager can be a ScopeManager object. Custom parsers can use customized scope analysis for experimental/enhancement syntaxes. Default is the ScopeManager object which is created by eslint-scope.
    • Support for scopeManager was added in ESLint v4.14.0. ESLint versions which 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. Default is KEYS of eslint-visitor-keys.
    • Support for visitorKeys was added in ESLint v4.14.0. ESLint versions which support visitorKeys will provide an eslintVisitorKeys: true property in parserOptions, which can be used for feature detection.

You can find an ESLint parser project here.

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

The 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. On the other hand, SourceLocation#source property can be undefined. ESLint does not use the SourceLocation#source property.

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

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;
    range: [number, number]; // See "All nodes:" section for details of `range` property.
    value: string;
  • 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]).