ESLint Prefer De Morgan's Law Rule

An ESLint plugin that suggests applying De Morgan's law for improved code readability.

Installation

npm install --save-dev @vitalyostanin/eslint-prefer-de-morgan-law

Usage

Flat Config (ESLint 9+)

import preferDeMorganLaw from '@vitalyostanin/eslint-prefer-de-morgan-law';

export default [
  {
    plugins: {
      'prefer-de-morgan-law': {
        rules: {
          'prefer-de-morgan-law': preferDeMorganLaw,
        },
      },
    },
    rules: {
      'prefer-de-morgan-law/prefer-de-morgan-law': 'error',
    },
  },
];

Legacy Config

{
  "plugins": ["@vitalyostanin/eslint-prefer-de-morgan-law"],
  "rules": {
    "@vitalyostanin/eslint-prefer-de-morgan-law/prefer-de-morgan-law": "error"
  }
}

Examples

The rule checks for negated logical expressions and suggests De Morgan's law transformations:

// Example 1: Simple case
// Before
if (!isAdmin || !isEnabled) { ... }
// After
if (!(isAdmin && isEnabled)) { ... }

// Example 2: More complex conditions
// Before
if (!user.isLoggedIn || !user.hasPermission('edit')) {
  showError();
}
// After
if (!(user.isLoggedIn && user.hasPermission('edit'))) {
  showError();
}

// Example 3: Nested expressions
// Before
if (!a || !b || !c) { ... }
// After
if (!(a && b && c)) { ... }

// Example 4: Comparisons
// Before
if (x !== 0 || y !== 10) { ... }
// After
if (!(x === 0 && y === 10)) { ... }

Rationale

De Morgan's law states that !(a || b) is equivalent to !a && !b, and vice versa. This rule helps improve code readability by:

• Reducing cognitive load
• Making logical conditions more explicit
• Following a consistent logical transformation pattern

Contributing

Contributions are welcome! Please feel free to submit a Pull Request.

License

MIT