How to Create Custom TypeScript Linters and Formatters

Creating custom linters and formatters for TypeScript can help enforce coding standards and maintain code quality in your projects. In this article, we'll walk through the process of building custom TypeScript linters and formatters, using tools like ESLint and Prettier, and extending them with your own rules and configurations.

Step 1: Setting Up Your Development Environment

Before creating custom linters and formatters, ensure you have a suitable development environment. You'll need Node.js and npm or Yarn installed on your machine.

# Install Node.js and npm from https://nodejs.org# Initialize a new project
npm init -y

Step 2: Creating a Custom ESLint Rule

To create a custom ESLint rule, start by installing ESLint and setting up a basic configuration.

# Install ESLint
npm install eslint --save-dev

# Initialize ESLint configuration
npx eslint --init

Now, create a custom rule by defining it in a separate file. Here's an example of a custom rule that enforces a specific coding style:

import { Rule } from 'eslint';

const rule: Rule.RuleModule = {
  create(context) {
    return {
      Identifier(node) {
        if (node.name === 'foo') {
          context.report({
            node,
            message: 'Avoid using the identifier "foo".',
          });
        }
      },
    };
  },
};

export default rule;

Register the custom rule in your ESLint configuration file.

module.exports = {
  rules: {
    'no-foo': require('./path/to/custom-rule').default,
  },
};

Step 3: Creating a Custom Prettier Formatter

To create a custom Prettier formatter, start by installing Prettier and its associated tools.

# Install Prettier
npm install prettier --save-dev

Create a custom formatter by extending Prettier's functionality. Here’s a basic example:

import { FormatterOptions } from 'prettier';

const customFormatter = (text: string, options: FormatterOptions) => {
  // Implement custom formatting logic here
  return text; // Return formatted text
};

export default customFormatter;

Integrate your custom formatter with Prettier by using the Prettier API:

import { format } from 'prettier';
import customFormatter from './path/to/custom-formatter';

const formattedCode = format('const x = 1;', {
  parser: 'typescript',
  plugins: [customFormatter],
});
console.log(formattedCode);

Step 4: Testing Your Custom Tools

Testing is crucial to ensure your custom linters and formatters work as expected. Write test cases using tools like Jest or Mocha.

# Install Jest
npm install jest --save-dev

# Create a test file for your custom rule
import rule from './path/to/custom-rule';
import { RuleTester } from 'eslint';

const ruleTester = new RuleTester();

ruleTester.run('no-foo', rule, {
  valid: [
    // Valid test cases
  ],
  invalid: [
    // Invalid test cases
  ],
});

Conclusion

Creating custom TypeScript linters and formatters involves setting up your development environment, defining custom rules or formatters, and testing your implementations. By integrating these tools into your workflow, you can enforce coding standards and maintain code quality across your projects.