#prefer-to-be-falsy

#Configuration

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

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

#Rule Details

Requires an assertion that a value is false to be spelled expect(value).toBeFalsy() rather than compared against the literal with toBe, toEqual or toStrictEqual. Most values checked this way are flags a function returns, and the named matcher says what the test is about — the value is falsy — instead of asserting an exact identity the test rarely depends on.

The literal has to be written out as the matcher's only argument. expect(value).toBe(0), expect(value).toBe(!1) and expect(value).toBe(flag) are all left alone, and so is a matcher that takes no argument at all, such as expect(value).toBe(). A type assertion around the literal is transparent, so expect(value).toBe(false as boolean) is reported. The matcher also has to be called: expect(value).toBe asserts nothing, so it is left alone, and so is expect.toBe(false), which never received a value to assert on. A matcher named by a computed key, expect(value)[matcherName](false), is not reported either, because which assertion runs is only known at runtime. Chai's falsy assertions are properties rather than matcher calls, and its equality form is named equal, so expect(value).to.be.false and expect(value).to.equal(false) are both outside this rule.

Modifiers are kept as written, so expect(value).not.toBe(false) becomes expect(value).not.toBeFalsy(). That asserts something wider than before — every truthy value passes, not just "anything other than false" — which is the assertion this rule is asking for.

Every expect source is recognized: globals, named and renamed imports, require destructuring, namespace imports, whole-module require, import.meta.rstest, Playwright integrations, and the expect a test callback receives through its TestContext. An expect from another assertion library, or a local variable that shadows expect, is not reported.

#Incorrect

test('reports an empty configuration as invalid', () => {
  expect(isValidConfig({})).toBe(false);
});

#Correct

test('reports an empty configuration as invalid', () => {
  expect(isValidConfig({})).toBeFalsy();
});

#Autofix

Renames the matcher to toBeFalsy and empties its argument list. Nothing else in the assertion changes: the expect root as written at the call site, expect.soft, the second message argument of expect(actual, message), the modifier chain, the accessor's own quoting, and the surrounding line breaks and comments are all left as they are. The assertion is reported without a fix when a comment sits between the parentheses, as in expect(value).toBe(/* the flag */ false), since emptying the argument list would delete it.