Core concepts #

Validators #

Validators are core of Earl. They are used every time you want to assert something during testing.

A great example of a validator is toEqual which compares the provided value with the expected value using a deep equality algorithm.

const value = { foo: 'bar', baz: Math.random() }

expect(value).toEqual({
  foo: 'bar',
  // this is a matcher, continue reading to learn about them
  baz: expect.a(Number),
})

Another great example is toThrow which can be used to check for errors:

function safeDiv(a: number, b: number) {
  if (b === 0) {
    throw new Error('Division by zero')
  }
  return a / b
}

expect(() => safeDiv(1, 0)).toThrow('Division by zero')

Validators can also be negated using the not modifier:

expect(1).not.toEqual(2)

Matchers #

Matchers are used with validators to match more than a single value. They are attached directly to expect object. For example with expect.anything() you can match any value.

Combine it with validator like beEqual to create (not very useful) assertion that works with any value:

expect(something).toEqual(expect.anything())

Of course matchers can be nested.