prefer-expect-type-of.md

vitest/prefer-expect-type-of

📝 Enforce using expectTypeOf instead of expect(typeof ...).

⚠️ This rule warns in the 🌐 all config.

🔧 This rule is automatically fixable by the --fix CLI option.

This rule enforces using Vitest's expectTypeOf function instead of expect(typeof ...) for type checking in tests.

Rule Details

Vitest provides expectTypeOf as a more expressive and type-safe way to test types compared to using expect(typeof ...). This rule automatically converts expect(typeof value).toBe(type) patterns to use the appropriate expectTypeOf matcher.

Incorrect

import { test, expect } from 'vitest'

test('type checking', () => {
  expect(typeof 'hello').toBe('string')
  expect(typeof 42).toBe('number')
  expect(typeof true).toBe('boolean')
  expect(typeof {}).toBe('object')
  expect(typeof () => {}).toBe('function')
  expect(typeof Symbol()).toBe('symbol')
  expect(typeof 123n).toBe('bigint')
  expect(typeof undefined).toBe('undefined')
})

Correct

import { test, expectTypeOf } from 'vitest'

test('type checking', () => {
  expectTypeOf('hello').toBeString()
  expectTypeOf(42).toBeNumber()
  expectTypeOf(true).toBeBoolean()
  expectTypeOf({}).toBeObject()
  expectTypeOf(() => {}).toBeFunction()
  expectTypeOf(Symbol()).toBeSymbol()
  expectTypeOf(123n).toBeBigInt()
  expectTypeOf(undefined).toBeUndefined()
})