#prefer-to-have-length

#Configuration

import { defineConfig, rstestPlugin } from '@rslint/core';

export default defineConfig([
  rstestPlugin.configs.recommended,
  {
    rules: {
      'rstest/prefer-to-have-length': 'error',
    },
  },
]);

#Rule Details

This rule requires toHaveLength() when toBe(), toEqual() or toStrictEqual() compares a value's .length. The dedicated matcher states the intended length check directly and produces a more useful failure message.

The rule recognizes Rstest globals, imports and aliases, namespace and require bindings, rstack/test, import.meta.rstest, expect.soft(), Playwright's ordinary value assertions, and the expect supplied by a test context. Dot, string-literal and template-literal accessors are recognized, including negated assertions. It does not require type information.

Optional .length access, promise modifiers, expect.poll(), browser-only expect.element() assertions, dynamic property or matcher names, and Chai-style matchers are left unchanged. When an assertion chain contains multiple matchers or continues into another member access, each toBe(), toEqual() and toStrictEqual() matcher is reported without a fix, unless an earlier matcher such as property(), toContain() or toThrow() has changed the assertion value.

#Incorrect

expect(files.length).toBe(3);
expect(usernames['length']).toEqual(2);
expect(messages.length).not.toStrictEqual(0);

#Correct

expect(files).toHaveLength(3);
expect(usernames).toHaveLength(2);
expect(messages).not.toHaveLength(0);

#Autofix

The autofix removes the .length accessor and replaces toBe with toHaveLength when the receiver is visibly an array, string or function expression and the expected length is a numeric expression other than negative zero. It preserves the Rstest expect source, negation, comments outside the removed accessor, and the matcher's dot, quote or template-literal style. Explicit type arguments on expect() and toBe() are removed because the rewritten calls receive a different value and toHaveLength is not generic.

Reading .length from another expression can invoke a getter. A receiver whose direct .length access cannot be proven inert, a custom assertion message, a non-literal expected value, extra arguments or using the assertion result would move that access across another evaluated expression, so those cases are reported without a fix. Comparisons through toEqual() or toStrictEqual() are also not fixed because custom equality testers can change their result, while toHaveLength() does not use those testers. Negative zero is not fixed because toBe() distinguishes it from zero while toHaveLength() does not. The fix is also withheld when a removed type argument list contains a comment or an accessor cannot be rewritten safely.