Testing throwing errors with mocha.

error.test.mjs

import { expect } from 'chai'

// usage:
//   npm install mocha chai
//   npx mocha error.test.mjs

describe('catching Error in test', () => {
  it('this test must pass', () => {
    const fn = () => {
      throw new Error('expected error')
    }

    try {
      fn()

      // if we are testing if the correct error is thrown,
      // must make sure that the fn will thrown an error an,
      // otherwise, fail the test
      throw new Error('unexpected error')
    } catch (err) {
      expect(err).to.have.property('message', 'expected error')
    }
  })

  it('this test must fail', () => {
    const fn = () => 'no error'

    try {
      fn()

      // the function will not thrown an error, which is unexpected,
      // so throwing one afterward will fail the test
      throw new Error('unexpected error')
    } catch (err) {
      expect(err).to.have.property('message', 'expected error')
    }
  })

  it('this test should have failed', () => {
    const fn = () => 'no error'

    try {
      fn()
      // if no error is thrown, the test will pass and 
      // the expectations below will never be reached
    } catch (err) {
      expect(err).to.have.property('message', 'expected error')
    }
  })

  // to understand better how tests works, we can simply
  // throw or not errors, when an error is thrown, the test fails
  it('throws error and fails test', () => {
    throw new Error('some error')
  })

  // expectations are not required to pass a test
  it('does not thrown an error and pass test', () => {
    'no error'
  })
});

error.test.mjs.output

  catching Error in test
    ✔ this test must pass
    1) this test must fail
    ✔ this test should have failed
    2) throws error and fails test
    ✔ does not thrown an error and pass test


  3 passing (7ms)
  2 failing

  1) catching Error in test
       this test must fail:

      AssertionError: expected [Error: unexpected error] to have property 'message' of 'expected error', but got 'unexpected error'
      + expected - actual

      -unexpected error
      +expected error
      
      at Context.<anonymous> (file:///home/diovani/Development/Node/mocha-error-false-positive/error.test.mjs:35:27)
      at processImmediate (internal/timers.js:461:21)

  2) catching Error in test
       throws error and fails test:
     Error: some error
      at Context.<anonymous> (file:///home/diovani/Development/Node/mocha-error-false-positive/error.test.mjs:54:11)
      at processImmediate (internal/timers.js:461:21)

In case someone wonders,

• yes we can just use expect(fn).to.throw, but that will not work for async functions
• and yes, we can use chai-as-promised for async functions

This is just the simplest way to check for errors without depending on chai or other assertion library.