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Custom Rules

important

This page describes how to write your own custom ESLint rules using typescript-eslint. You should be familiar with ESLint's developer guide and ASTs 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 four 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
  • Testing: using @typescript-eslint/rule-tester's RuleTester instead of ESLint core's

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.',
    },
    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>({
  // ...
});

Extra Rule Docs Types​

By default, rule meta.docs is allowed to contain only description and url as described in ESLint's Custom Rules > Rule Structure docs. Additional docs properties may be added as a type argument to ESLintUtils.RuleCreator:

interface MyPluginDocs {
  recommended: boolean;
}

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

createRule({
  // ...
  meta: {
    docs: {
      description: '...',
      recommended: true,
    },
    // ...
  },
});

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.

Handling rule options​

ESLint rules can take options. When handling options, you will need to add information in at most three places:

  • The Options generic type argument to RuleCreator, where you declare the type of the options
  • The meta.schema property, where you add a JSON schema describing the options shape
  • The defaultOptions property, where you add the default options value
type MessageIds = 'lowercase' | 'uppercase';

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

export const rule = createRule<Options, MessageIds>({
  meta: {
    // ...
    schema: [
      {
        type: 'object',
        properties: {
          preferredCase: {
            type: 'string',
            enum: ['lower', 'upper'],
          },
        },
        additionalProperties: false,
      },
    ],
  },
  defaultOptions: [
    {
      preferredCase: 'lower',
    },
  ],
  create(context, options) {
    if (options[0].preferredCase === 'lower') {
      // ...
    }
  },
});
warning

When reading the options, use the second parameter of the create function, not context.options from the first parameter. The first is created by ESLint and does not have the default options applied.

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

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

Typed Rules​

tip

Read TypeScript's Compiler APIs > Type Checker APIs 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 services object.

That services object contains:

  • program: A full TypeScript ts.Program object if type checking is enabled, or null otherwise
  • 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

If type checking is enabled, that services object additionally contains:

  • getTypeAtLocation: Wraps the type checker function, with a TSESTree.Node parameter instead of a ts.Node
  • getSymbolAtLocation: Wraps the type checker function, with a TSESTree.Node parameter instead of a ts.Node

Those additional objects internally map from ESTree nodes to their TypeScript equivalents, then call to the TypeScript program. By using 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 TypeScript type checker via typescript-eslint's services:

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

export const rule = createRule({
  create(context) {
    return {
      ForOfStatement(node) {
        // 1. Grab the parser services for the rule
        const services = ESLintUtils.getParserServices(context);

        // 2. Find the TS type for the ES node
        const type = services.getTypeAtLocation(node.right);

        // 3. Check the TS type's backing symbol for being an enum
        if (type.symbol.flags & ts.SymbolFlags.Enum) {
          context.report({
            messageId: 'loopOverEnum',
            node: node.right,
          });
        }
      },
    };
  },
  meta: {
    docs: {
      description: 'Avoid looping over enums.',
    },
    messages: {
      loopOverEnum: 'Do not loop over enums.',
    },
    type: 'suggestion',
    schema: [],
  },
  name: 'no-loop-over-enum',
  defaultOptions: [],
});
note

Rules can retrieve their full backing TypeScript type checker with services.program.getTypeChecker(). This can be necessary for TypeScript APIs not wrapped by the parser services.

Conditional Type Information​

We recommend against changing rule logic based solely on whether services.program exists. In our experience, users are generally surprised when rules behave differently with or without type information. Additionally, if they misconfigure their ESLint config, they may not realize why the rule started behaving differently. Consider either gating type checking behind an explicit option for the rule or creating two versions of the rule instead.

tip

Documentation generators such as eslint-doc-generator can automatically indicate in a rule's docs whether it needs type information.

Testing​

@typescript-eslint/rule-tester 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.

Below is a quick-start guide. For more in-depth docs and examples see the @typescript-eslint/rule-tester package documentation.

Testing Untyped Rules​

For rules that don't need type information, no constructor parameters are necessary:

import { RuleTester } from '@typescript-eslint/rule-tester';
import rule from './my-rule';

const ruleTester = new RuleTester();

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

Testing Typed Rules​

For rules that do need type information, parserOptions must be passed in as well. We recommend using parserOptions.projectService with options to allow a default project for each test file.

import { RuleTester } from '@typescript-eslint/rule-tester';
import rule from './my-typed-rule';

const ruleTester = new RuleTester({
  languageOptions: {
    parserOptions: {
      projectService: {
        allowDefaultProject: ['*.ts*'],
      },
      tsconfigRootDir: __dirname,
    },
  },
});

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

See Rule Tester > Type-Aware Testing for more details.