Using matchers #

Matchers allow matching whole ranges of values rather than just one literal, they can be combined with other validators or used with mocks. Let's take a look at few different examples.

String related matchers #

Imagine that we want to match any string in our test case. We can use a generic a(Class) matcher, matching any instance of a given class.

import { expect } from 'earljs'

expect('abc').toEqual(expect.a(String))

expect.a(Class) works with builtin types like strings, numbers etc as well as custom classes. It's smart enough to leverage typeof check for builtins - so you don't have to worry about that 😃

What if you don't want to match any string but rather a string containing another string or better, matching some pattern?

import { expect } from 'earljs'

// match any string containing "Doe"
expect('John Doe').toEqual(expect.stringMatching('Doe'))

// match any string containing Doe or doe
expect('John Doe').toEqual(expect.stringMatching(/[Dd]oe/))

Composing matchers #

The real power of matchers comes from the fact that they be part of the bigger pattern.

import { expect } from 'earljs'

// match any John
expect({ name: 'John', surname: 'Doe' }).toEqual({
  name: 'John',
  surname: expect.a(String),
})

If you're familiar with pattern matching from languages like Scala or OCaml, earl's matchers are designed in a similar way.

Few other examples:

// use error matcher combined with string matcher to only match
// errors containing "unexpected" word in their message
expect(() => {
  throw new Error('Totally unexpected error! :(')
}).toThrow(expect.stringMatching('unexpected'))