catch implicit mutable arrays

README.md

no-mutable-arrays

Prevent usage of implicit mutable arrays. As input parameters we almost always want readonly arrays.

This has a probably too many false positives in implementation code.

no-mutable-arrays.js

module.exports = {
  meta: {
    messages: {
      implicitMutable:
        "Don't use implicit mutable arrays. Either use readonly arrays with `{{suggestion}}` or make the mutability explicit with `MutableArray<T>`.",
    },
  },
  create(context) {
    return {
      TSTypeReference(node) {
        if (node.typeName.name === 'Array') {
          context.report({
            node,
            messageId: 'implicitMutable',
            data: { suggestion: 'ReadonlyArray<T>' },
          });
        }
      },
      TSArrayType(node) {
        const isReadonly =
          node.parent.type === 'TSTypeOperator' && node.parent.operator === 'readonly';
        if (!isReadonly) {
          context.report({
            node,
            messageId: 'implicitMutable',
            data: { suggestion: 'readonly T[]' },
          });
        }
      },
    };
  },
};

no-mutable-arrays.test.js

const eslint = require('eslint');
const rule = require('./no-mutable-arrays');

const ruleTester = new eslint.RuleTester({
  parser: require.resolve('@typescript-eslint/parser'),
  parserOptions: {
    ecmaFeatures: { jsx: true },
  },
});
ruleTester.run('no-mutable-arrays', rule, {
  valid: [
    'type Pass = readonly T[];',
    'type Pass = MutableArray<T>;',
    'type Pass = ReadonlyArray<T>;',
  ],
  invalid: [
    {
      code: 'type Fail = T[];',
      errors: [
        {
          message:
            "Don't use implicit mutable arrays. Either use readonly arrays with `readonly T[]` or make the mutability explicit with `MutableArray<T>`.",
        },
      ],
    },
    {
      code: 'type Fail = Array<T>;',
      errors: [
        {
          message:
            "Don't use implicit mutable arrays. Either use readonly arrays with `ReadonlyArray<T>` or make the mutability explicit with `MutableArray<T>`.",
        },
      ],
    },
  ],
});