Exceptions

We all write tests that deal with exceptions and promise rejections. For example:

• Assert that a call to an invalid API endpoint rejects the promise with an HTTP 404 error.
• Assert that rendering a component without a required property throws an exception.

Usually, you might wrap these function calls inside a try/catch statement and write assertions for the error object.

import { test } from '@pawel-up/lupa/testing'

test('validate property', ({ assert }) => {
  try {
    validateProperty(undefined)
  } catch (error) {
    assert.equal(error.message, 'Property is required')
  }
})

There are two main problems with the try/catch statement in testing:

• You will get a "false positive" passing test if the code inside the try block never raises an exception!
• The try/catch statement adds visual noise to your tests, especially when writing nested statements.

To solve this, Lupa's bundled assertion library has dedicated methods to assert a function call throws an exception safely.

Using dedicated assertion methods

assert.throws

The assert.throws method accepts a function as the first parameter and the error message (or regex) you expect as the second parameter.

This method only works with synchronous function calls.

import { test } from '@pawel-up/lupa/testing'

test('validate property', ({ assert }) => {
  assert.throws(
    () => validateProperty(undefined),
    'Property is required'
  )
})

assert.rejects

Similar to assert.throws, the assert.rejects method also accepts a function and the error message for assertion. However, in this case, the callback function must return a Promise.

import { test } from '@pawel-up/lupa/testing'

test('do not fetch invalid user', async ({ assert }) => {
  
  await assert.rejects(
    async () => fetchUser(9999),
    /User not found/
  )
  
})

High order assertion

An elegant alternative API that is completely independent of the assertion library is to expect a test to throw an exception directly on the test instance using the test.throws method.

Let's re-write the above two examples with a high-order assertion:

import { test } from '@pawel-up/lupa/testing'

test('validate property', () => {
  // If this doesn't throw, the test fails
  validateProperty(undefined)
})
.throws('Property is required')

import { test } from '@pawel-up/lupa/testing'

test('do not fetch invalid user', async () => {
  
  /**
   * This call will throw an exception, and
   * there is no need to handle it within the test callback
   */
  await fetchUser(9999)
  
})
.throws(/User not found/)