close
Rslint

    prefer-expect-type-of

    Unreleased

    Configuration

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

export default defineConfig([
  rstestPlugin.configs.recommended,
  {
    rules: {
      'rstest/prefer-expect-type-of': 'error',
    },
  },
]);

    Rule Details

    Prefer expect(value).toBeTypeOf(type) over expect(typeof value).toBe(type). The dedicated matcher states the intent directly and, when it fails, reports the value's actual type instead of a plain string comparison.

    Examples of incorrect code:

    expect(typeof value).toBe('string');
expect(typeof value).toEqual('number');
expect(typeof value).not.toBe('function');
expect.soft(typeof value).toBe('object');

    Examples of correct code:

    expect(value).toBeTypeOf('string');
expect(value).not.toBeTypeOf('function');
expect.soft(value).toBeTypeOf('object');
expect(typeof value === 'string').toBe(true);

    Recognized assertions

    The rule reports only a matcher that is actually called with exactly one argument. A broken chain such as expect(typeof value).toBe, a spread argument such as expect(typeof value).toBe(...types), and a Chai property assertion such as expect(typeof value).to.be.ok are all left alone. So is a matcher named by a computed key, expect(typeof value)[matcherName]('string'), because which assertion runs is only known at runtime.

    Every expect source is covered: globals, @rstest/core named imports and renamed imports, require('@rstest/core') destructuring, namespace imports, whole-module require, import.meta.rstest, @rstest/playwright, and the expect a test callback receives through its context (({ expect }) and ctx.expect). An expect from another assertion library, and a local variable that shadows expect, are not reported.

    expect.poll(fn) and expect.element(locator) are excluded: the first takes a callback and the second a locator, so neither carries the value being type checked.

    Fix

    The fix edits two places and leaves the rest of the call exactly as written: it removes the typeof operator from the assertion's argument, and renames the matcher accessor to toBeTypeOf. Everything else survives — the expect root as spelled at the call site, expect.soft, the optional second message argument of expect(actual, message), the modifier chain, the matcher argument, the accessor's own quoting, and the surrounding line breaks and comments:

    expect.soft(typeof value, 'should be a string')['toBe']('string');
// becomes
expect.soft(value, 'should be a string')['toBeTypeOf']('string');

    A comment written between typeof and its operand sits inside the removed span and is removed with it. Every comment outside those two spans is preserved.