Testing errors #

Often you want to assert that a given operation results in an error being thrown. That's what toThrow validator is useful in earl:

import { expect } from 'earljs'

function somethingThatCanThrow() {
  throw new Error('Unexpected error!')
}

expect(() => somethingThatCanThrow()).toThrow('Unexpected error!')

When using toThrow we require you to pass parameterless function () => ... that will be executed and error will be caught. It's one of the limitation of JavaScript but don't worry! As long as you use TypeScript you will get a compile time error if you forget to do so.

If you're interested in testing async functions, take a look at Async functions guide.

Matching only part of an error message #

You can combine toThrow with string matcher to for example to check if error message matches another string:

expect(() => somethingThatCanThrow()).toThrow(expect.stringMatching('Unexpected')

Expecting any error #

expect(() => somethingThatCanThrow()).toThrow()

Expect a custom error #

import { expect } from 'earljs'

class HttpError extends Error {
  constructor(private readonly code: number) {
    super(`Http error with code: ${code}`)
  }
}

function somethingThatCanThrow() {
  throw new HttpError(500)
}

expect(() => somethingThatCanThrow()).toThrow(
  HttpError,
  'Http error with code: 500',
)