README.md

Other parsers

Your team may use some transpilers such as TypeScript to write project source codes. In this case, ESLint rules you create should be tested using the same transpiler.

Add other parser

We'll get be able to treat TypeScript / React JSX through this chapter.

type Props = {
  onClick: () => void,
};
const MyComponent = ({ onClick }: Props) => (
  <button onClick={() => onClick()} />
);

So we assume that we want to ban to use <button /> React component.

First of all, let's know AST structure for this. Do you remember? That's right, we have https://astexplorer.net

To turn on TypeScript/JSX parsing, switch the parser type to "@typescript-eslint/parser".

FYI, ESLint uses esprima as default.

Can you get the query to find <button />? Yes, it's JSXIdentifier[name='button'].

So our rule is implemented as the following:

/* src/rules/no-jsx-button.ts */

import { Rule } from "eslint";
import { Node } from "estree";

const rule: Rule.RuleModule = {
  create: (context) => {
    return {
      "JSXIdentifier[name='button']": (node: Node) => {
        context.report({
          message: "Don't use <button>",
          node,
        });
      },
    };
  },
};

export = rule;

Configure rule tester

Next, we need to test the above rule so add parser to our project.

$ npm i @typescript-eslint/parser --dev

And tell parser configuration to ESLint RuleTester.

/* src/rules/no-jsx-button.test.ts */

import { RuleTester } from "eslint";

import rule from "./no-jsx-button";

// Specify parser
const tester = new RuleTester({
  parser: "@typescript-eslint/parser",
  parserOptions: {
    ecmaFeatures: {
      jsx: true,
    },
  },
});

tester.run("no-jsx-button", rule, {
  valid: [
    {
      code: `(props: Props) => <div />`,
    },
  ],
  invalid: [
    {
      code: `(props: Props) => <button />`,
      errors: [{ message: "Don't use <button>" }],
    }
  ],
});

What value the parserOptions accepts is defined by each parser. For example, @typescript-eslint/parser 's configurable values are listed up in @typescript-eslint/parser configuration. The parser / parserOptions values also should exist at your team project's .eslintrc.

If you forget to configure RuleTester's parser, npm test outputs parsing errors such as:

Message:
  Should have no errors but had 1: [ { ruleId: null,
    fatal: true,
    severity: 2,
    message: "Parsing error: '>' expected.",
    line: 1,
    column: 23 } ]

Summary

• Test rules with parser same to one the project uses

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