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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.

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',
)