Extending Matchers

Since Vitest is compatible with both Chai and Jest, you can use either the chai.use API or expect.extend, whichever you prefer.

This guide will explore extending matchers with expect.extend. If you are interested in Chai's API, check their guide.

To extend default matchers, call expect.extend with an object containing your matchers.

expect.extend({
  toBeFoo(received, expected) {
    const { isNot } = this
    return {
      // do not alter your "pass" based on isNot. Vitest does it for you
      pass: received === 'foo',
      message: () => `${received} is${isNot ? ' not' : ''} foo`
    }
  }
})

If you are using TypeScript, you can extend default Assertion interface in an ambient declaration file (e.g: vitest.d.ts) with the code below:

import 'vitest'

interface CustomMatchers<R = unknown> {
  toBeFoo: () => R
}

declare module 'vitest' {
  interface Assertion<T = any> extends CustomMatchers<T> {}
  interface AsymmetricMatchersContaining extends CustomMatchers {}
}

WARNING

Don't forget to include the ambient declaration file in your tsconfig.json.

The return value of a matcher should be compatible with the following interface:

interface ExpectationResult {
  pass: boolean
  message: () => string
  // If you pass these, they will automatically appear inside a diff when
  // the matcher does not pass, so you don't need to print the diff yourself
  actual?: unknown
  expected?: unknown
}

WARNING

If you create an asynchronous matcher, don't forget to await the result (await expect('foo').toBeFoo()) in the test itself.

The first argument inside a matcher's function is the received value (the one inside expect(received)). The rest are arguments passed directly to the matcher.

Matcher function have access to this context with the following properties:

• isNot

  Returns true, if matcher was called on not (expect(received).not.toBeFoo()).

• promise

  If matcher was called on resolved/rejected, this value will contain the name of modifier. Otherwise, it will be an empty string.

• equals

  This is a utility function that allows you to compare two values. It will return true if values are equal, false otherwise. This function is used internally for almost every matcher. It supports objects with asymmetric matchers by default.

• utils

  This contains a set of utility functions that you can use to display messages.

this context also contains information about the current test. You can also get it by calling expect.getState(). The most useful properties are:

• currentTestName

  Full name of the current test (including describe block).

• testPath

  Path to the current test.