When we are writing testing code we sometimes need to test that our code is handling errors in a correct way - test the error handling in other words.

Imagine that you have the following simple function in a index.js-file

const checkIt = (value) => {
  return value > 1
}

module.exports.checkIt = checkIt

This can then be tested with these tests in tests.js

/* global describe, it */

const assert = require('assert')
const systemUnderTest = require('./index')

describe('how to test that somethings throws', () => {
  it('returns true if the value is above 1', (done) => {
    assert.equal(systemUnderTest.checkIt(2), true)
    done()
  })

  it('returns false if the value is below 1 but above 0', (done) => {
    assert.equal(systemUnderTest.checkIt(1), false)
    done()
  })
})

Sidebar: How to run these tests

1. Create a new empty directory (‘mkdir testingErrors && cd testingErrors’)
2. npm init -y to create a package.json
3. npm install mocha -D to install and save Mocha, that is our test runner
4. Update package.json script test to this mocha --watch *.js -R list
5. npm test to run the tests

Testing errors

Ok that is fine, but that code is a bit instabile. What if someone sends us less than 0… That is obviously (joking here, but play along) wrong and should throw an error telling the user that.

I want the code to be like this:

const checkIt = (value) => {
  if (value < 0) {
    throw new Error('Cannot be below 0')
  }
  return value > 1
}

module.exports.checkIt = checkIt

Here’s is how I would write a test to verify that it happens when I expect it

it('does throw an Error if the value is below 0', (done) => {
    assert.throws(
      () => {
        systemUnderTest.checkIt(-1)
      },
      {
        name: 'Error',
        message: 'Cannot be below 0'
      }
    )
    done()
  })

Let’s walk through the code:

• I’m using assert.throws (see this) that has a bit of a particular syntax. It takes two parameters:
  • A callback function that it will execute to get the exception
  • The second parameter is the expected Error
• In the callback we call the checkIt function as before, with a parameter that creates the error
• The second parameter { name: 'Error', message: 'Cannot be below 0'} describes the errors type and message.

When we run this Mocha will expect an error to thrown, by the callback function and will pass if the error is a Error with the message Cannot be below 0

Another one

Let’s do one more. Let’s say that we want to check that the sent in value is a number and not a stupid string, since we are going to check if it is greater than one.

const checkIt = (value) => {
  if (isNaN(value)) {
    // throw new TypeError('Not the error you expected')
    throw new TypeError('Please, only numbers to this function')
  }
  if (value < 0) {
    throw new Error('Cannot be below 0')
  }
  return value > 1
}

module.exports.checkIt = checkIt

We now know how to test that:

it('does throw a TypeError if the value is not a number', (done) => {
    const expectedError = new TypeError('Please, only numbers to this function')

    assert.throws(
      () => {
        systemUnderTest.checkIt('Ho ho Marcus doing crazy stuff')
      },
      expectedError
    )
    done()
  })

The only difference here is that I set up the expected error, as a separate value before I call the assert.throws

But how do I check that it doesn’t throw?

Well there’s actually a function for that too, in the standard assert library.

it('not throw an error if the value is above 1', (done) => {
    assert.doesNotThrow(
      () => {
        systemUnderTest.checkIt(1)
      },
      SyntaxError
    )
    done()
  })

This code simply verifies that a SyntaxError is not thrown when 1 is pass to the checkIt-function

All the code

Here’s all the code. First the index.js-file

const checkIt = (value) => {
  if (isNaN(value)) {
    throw new TypeError('Please, only numbers to this function')
  }
  if (value < 0) {
    throw new Error('Cannot be below 0')
  }
  return value > 1
}

module.exports.checkIt = checkIt

And here are the tests, that checks for errors:

const assert = require('assert')
const systemUnderTest = require('./index')

describe('how to test that somethings throws', () => {
  it('returns true if the value is above 1', (done) => {
    assert.equal(systemUnderTest.checkIt(2), true)
    done()
  })

  it('returns false if the value is below 1 but above 0', (done) => {
    assert.equal(systemUnderTest.checkIt(1), false)
    done()
  })

  it('not throw an error if the value is above 1', (done) => {
    assert.doesNotThrow(
      () => {
        systemUnderTest.checkIt(1)
      },
      SyntaxError
    )
    done()
  })

  it('does throw an Error if the value is below 0', (done) => {
    assert.throws(
      () => {
        systemUnderTest.checkIt(-1)
      },
      {
        name: 'Error',
        message: 'Cannot be below 0'
      }
    )
    done()
  })

  it('does throw a TypeError if the value is not a number', (done) => {
    const err = new TypeError('Please, only numbers to this function')

    assert.throws(
      () => {
        systemUnderTest.checkIt('Ho ho Marcus doing crazy stuff')
      },
      err
    )
    done()
  })
})

Conclusion

I hope that this made testing for errors a bit simpler to grasp.