Introduction to eslint-plugin-classname-arg-last

Prerequisite Knowledge#

When using Tailwind CSS, combining class names is a common requirement. We often use a method called cn to efficiently and elegantly manage classes, which typically encapsulates the following two commonly used tools:

1. `clsx`#

A lighter and more performant alternative to classnames.
Supports combining classes in the form of strings, objects, arrays, etc.

Example:

clsx('btn', { 'btn-primary': isPrimary }) // => 'btn btn-primary'

2. `tailwind-merge` (abbreviated as `twMerge`)#

Used to merge conflicting Tailwind CSS class names.
Handles conflicts like text-sm text-lg, retaining the last class with higher priority.

Example:

twMerge('text-sm text-lg') // => 'text-lg'

3. `cn` Method Encapsulation#

Combining the above two libraries into a unified cn method:

import { clsx, type ClassValue } from 'clsx'
import { twMerge } from 'tailwind-merge'

export function cn(...inputs: ClassValue[]) {
  return twMerge(clsx(...inputs))
}

4. Why should `className` be the last parameter?#

To ensure that user-provided styles can override default styles, it is recommended to place className last:

cn('default-class', className)

Because twMerge merges from left to right, the later classes will take precedence. If the order is reversed, the component's default styles may override user intentions.

What Did I Do?#

To maintain consistency in the practice of placing className at the end of the parameters, I developed an ESLint plugin for static detection and automatic fixing with the help of Grok3, compatible with Eslint v7-v9.

Source:#

eslint-plugin-classname-arg-last

Example:#

// Error example
cn(className, 'text-sm')

// Correct usage
cn('text-sm', className)

Configuration:#

Add to .eslintrc.js:

module.exports = {
  // ...
  plugins: ['classname-arg-last'],
  rules: {
    'classname-arg-last/classname-arg-last': 'error',
  },
}

With this plugin, you can standardize practices during the development phase and reduce style conflicts caused by order issues.