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Custom Rules

info

You should be familiar with ESLint's developer guide and Development > Architecture before writing custom rules.

As long as you are using @typescript-eslint/parser as the parser in your ESLint configuration, custom ESLint rules generally work the same way for JavaScript and TypeScript code. The main three changes to custom rules writing are:

  • Utils Package: we recommend using @typescript-eslint/utils to create custom rules
  • AST Extensions: targeting TypeScript-specific syntax in your rule selectors
  • Typed Rules: using the TypeScript type checker to inform rule logic

Utils Package​

The @typescript-eslint/utils package acts as a replacement package for eslint that exports all the same objects and types, but with typescript-eslint support. It also exports common utility functions and constants most custom typescript-eslint rules tend to use.

caution

@types/eslint types are based on @types/estree and do not recognize typescript-eslint nodes and properties. You should generally not need to import from eslint when writing custom typescript-eslint rules in TypeScript.

RuleCreator​

The recommended way to create custom ESLint rules that make use of typescript-eslint features and/or syntax is with the ESLintUtils.RuleCreator function exported by @typescript-eslint/utils.

It takes in a function that transforms a rule name into its documentation URL, then returns a function that takes in a rule module object. RuleCreator will infer the allowed message IDs the rule is allowed to emit from the provided meta.messages object.

This rule bans function declarations that start with a lower-case letter:

import { ESLintUtils } from '@typescript-eslint/utils';

const createRule = ESLintUtils.RuleCreator(
  name => `https://example.com/rule/${name}`,
);

// Type: RuleModule<"uppercase", ...>
export const rule = createRule({
  create(context) {
    return {
      FunctionDeclaration(node) {
        if (node.id != null) {
          if (/^[a-z]/.test(node.id.name)) {
            context.report({
              messageId: 'uppercase',
              node: node.id,
            });
          }
        }
      },
    };
  },
  name: 'uppercase-first-declarations',
  meta: {
    docs: {
      description:
        'Function declaration names should start with an upper-case letter.',
      recommended: 'warn',
    },
    messages: {
      uppercase: 'Start this name with an upper-case letter.',
    },
    type: 'suggestion',
    schema: [],
  },
  defaultOptions: [],
});

RuleCreator rule creator functions return rules typed as the RuleModule interface exported by @typescript-eslint/utils. It allows specifying generics for:

  • MessageIds: a union of string literal message IDs that may be reported
  • Options: what options users may configure for the rule (by default, [])

If the rule is able to take in rule options, declare them as a tuple type containing a single object of rule options:

import { ESLintUtils } from '@typescript-eslint/utils';

type MessageIds = 'lowercase' | 'uppercase';

type Options = [
  {
    preferredCase?: 'lower' | 'upper';
  },
];

// Type: RuleModule<MessageIds, Options, ...>
export const rule = createRule<Options, MessageIds>({
  // ...
});

Undocumented Rules​

Although it is generally not recommended to create custom rules without documentation, if you are sure you want to do this you can use the ESLintUtils.RuleCreator.withoutDocs function to directly create a rule. It applies the same type inference as the createRules above without enforcing a documentation URL.

import { ESLintUtils } from '@typescript-eslint/utils';

export const rule = ESLintUtils.RuleCreator.withoutDocs({
  create(context) {
    // ...
  },
  meta: {
    // ...
  },
});
caution

We recommend any custom ESLint rule include a descriptive error message and link to informative documentation.

AST Extensions​

@typescript-eslint/estree creates AST nodes for TypeScript syntax with names that begin with TS, such as TSInterfaceDeclaration and TSTypeAnnotation. These nodes are treated just like any other AST node. You can query for them in your rule selectors.

This version of the above rule instead bans interface declaration names that start with a lower-case letter:

import { ESLintUtils } from '@typescript-eslint/utils';

export const rule = createRule({
  create(context) {
    return {
      TSInterfaceDeclaration(node) {
        if (/^[a-z]/.test(node.id.name)) {
          // ...
        }
      },
    };
  },
  // ...
});

Node Types​

TypeScript types for nodes exist in a TSESTree namespace exported by @typescript-eslint/utils. The above rule body could be better written in TypeScript with a type annotation on the node:

An AST_NODE_TYPES enum is exported as well to hold the values for AST node type properties. TSESTree.Node is available as union type that uses its type member as a discriminant.

For example, checking node.type can narrow down the type of the node:

import { AST_NODE_TYPES, TSESTree } from '@typescript-eslint/utils';

export function describeNode(node: TSESTree.Node): string {
  switch (node.type) {
    case AST_NODE_TYPES.ArrayExpression:
      return `Array containing ${node.elements.map(describeNode).join(', ')}`;

    case AST_NODE_TYPES.Literal:
      return `Literal value ${node.raw}`;

    default:
      return '🤷';
  }
}

Explicit Node Types​

Rule queries that use more features of esquery such as targeting multiple node types may not be able to infer the type of the node. In that case, it is best to add an explicit type declaration.

This rule snippet targets name nodes of both function and interface declarations:

import { AST_NODE_TYPES, ESLintUtils } from '@typescript-eslint/utils';

export const rule = createRule({
  create(context) {
    return {
      'FunctionDeclaration, TSInterfaceDeclaration'(
        node:
          | AST_NODE_TYPES.FunctionDeclaration
          | AST_NODE_TYPES.TSInterfaceDeclaration,
      ) {
        if (/^[a-z]/.test(node.id.name)) {
          // ...
        }
      },
    };
  },
  // ...
});

Type Checking​

tip

Read TypeScript's Compiler APIs > Using the Type Checker section for how to use a program's type checker.

The biggest addition typescript-eslint brings to ESLint rules is the ability to use TypeScript's type checker APIs.

@typescript-eslint/utils exports an ESLintUtils namespace containing a getParserServices function that takes in an ESLint context and returns a parserServices object.

That parserServices object contains:

  • program: A full TypeScript ts.Program object
  • esTreeNodeToTSNodeMap: Map of @typescript-eslint/estree TSESTree.Node nodes to their TypeScript ts.Node equivalents
  • tsNodeToESTreeNodeMap: Map of TypeScript ts.Node nodes to their @typescript-eslint/estree TSESTree.Node equivalents

By mapping from ESTree nodes to TypeScript nodes and retrieving the TypeScript program from the parser services, rules are able to ask TypeScript for full type information on those nodes.

This rule bans for-of looping over an enum by using the type-checker via typescript-eslint and TypeScript APIs:

import { ESLintUtils } from '@typescript-eslint/utils';
import * as ts from 'typescript';
import * as tsutils from 'tsutils';

export const rule = createRule({
  create(context) {
    return {
      ForOfStatement(node) {
        // 1. Grab the TypeScript program from parser services
        const parserServices = ESLintUtils.getParserServices(context);
        const checker = parserServices.program.getTypeChecker();

        // 2. Find the backing TS node for the ES node, then that TS type
        const originalNode = parserServices.esTreeNodeToTSNodeMap.get(
          node.right,
        );
        const nodeType = checker.getTypeAtLocation(originalNode);

        // 3. Check the TS node type using the TypeScript APIs
        if (tsutils.isTypeFlagSet(nodeType, ts.TypeFlags.EnumLike)) {
          context.report({
            messageId: 'loopOverEnum',
            node: node.right,
          });
        }
      },
    };
  },
  meta: {
    docs: {
      description: 'Avoid looping over enums.',
      recommended: 'error',
    },
    messages: {
      loopOverEnum: 'Do not loop over enums.',
    },
    type: 'suggestion',
    schema: [],
  },
  name: 'no-loop-over-enum',
  defaultOptions: [],
});

Testing​

@typescript-eslint/utils exports a RuleTester with a similar API to the built-in ESLint RuleTester. It should be provided with the same parser and parserOptions you would use in your ESLint configuration.

Testing Untyped Rules​

For rules that don't need type information, passing just the parser will do:

import { ESLintUtils } from '@typescript-eslint/utils';
import rule from './my-rule';

const ruleTester = new ESLintUtils.RuleTester({
  parser: '@typescript-eslint/parser',
});

ruleTester.run('my-rule', rule, {
  valid: [
    /* ... */
  ],
  invalid: [
    /* ... */
  ],
});

Testing Typed Rules​

For rules that do need type information, parserOptions must be passed in as well. Tests must have at least an absolute tsconfigRootDir path provided as well as a relative project path from that directory:

import { ESLintUtils } from '@typescript-eslint/utils';
import rule from './my-typed-rule';

const ruleTester = new ESLintUtils.RuleTester({
  parser: '@typescript-eslint/parser',
  parserOptions: {
    project: './tsconfig.json',
    tsconfigRootDir: __dirname,
  },
});

ruleTester.run('my-typed-rule', rule, {
  valid: [
    /* ... */
  ],
  invalid: [
    /* ... */
  ],
});
note

For now, ESLintUtils.RuleTester requires the following physical files be present on disk for typed rules:

  • tsconfig.json: tsconfig used as the test "project"
  • One of the following two files:
    • file.ts: blank test file used for normal TS tests
    • file.tsx: blank test file used for tests with parserOptions: { ecmaFeatures: { jsx: true } }