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Make React PropType warnings throw errors with mocha.js, enzyme.js and sinon.js

Make React PropType warnings throw errors with enzyme.js + sinon.js + mocha.js

A simple stateless functional component that we want to test that it renders without propType warnings.

import React, { PropTypes } from 'react'

let VersionListItem = function ({ active, version }) {
  return (
    <a href='#' className={`list-group-item ${active}`}>
      <strong
        className='list-group-item-heading'>
        Version {version}
      </strong>
    </a>
  )
}
VersionListItem.propTypes = {
  active: PropTypes.string,
  version: PropTypes.number
}
export default VersionListItem
import { describe, it, before, after } from 'mocha'
import sinon from 'sinon'
import React from 'react'
import { shallow } from 'enzyme'
import VersionListItem from './version-list-item'

// Since react will console.error propType warnings, that which we'd rather have
// as errors, we use sinon.js to stub it into throwing these warning as errors
// instead.
before(() => {
  sinon.stub(console, 'error', (warning) => { throw new Error(warning) })
})
// While not forgetting to restore it afterwards
after(() => { console.error.restore() })

describe('<VersionListItem />', () => {
  const props = {
    active: 'active',
    version: 1
  }

  it('renders without error', () => {
    shallow(<VersionListItem {...props} />)
    // In this case there is no need for an assertion since we're only
    // interested in not getting any errors. And mocha will mark the test as a
    // failure if an error is thrown. :)
  })
})

Related reading:

ascoppa commented Aug 20, 2016

If you're only interested on testing PropTypes shouldn't you omit calling shallow and just write <VersionListItem {...props} /> ?

Works for me and keeps the test case simple (with just one concern).

AleksOrl commented Dec 1, 2016

I use this arcticle as an example for testing components in my project. I have found strange behaviour. I wrote about it on StackOverflow. How to avoid this problem?

How do i unit test the warnings returned by PropType , i wanted to unit test i.e, on setting a specific value the PropType produces a warning which i wanted to Unit test for the code coverage.

Owner

scmx commented Feb 3, 2017

Wow, I had no idea there were comments, forks and lots of stars here. Glad you all found it useful. I barely remember writing this.
@ascoppa good point.
@NaveenThally: Take a look at the sinon documentation http://sinonjs.org/ You might want to use a spy instead. Related reading https://www.sitepoint.com/sinon-tutorial-javascript-testing-mocks-spies-stubs/

hulkish commented Mar 5, 2017

I found this to be a bit less polluting:

in my helpers.unit.js:

import sinon from 'sinon';
sinon.stub(console, 'error', (warning) => {
  if (warning && warning.indexOf('Warning: Failed prop type:') > -1) {
    process.nextTick(() => {
      throw new Error(warning);
    });
  }
});

then i run: mocha --require helper.unit.js --compilers babel-register mytest.test.js

Owner

scmx commented Mar 8, 2017

@hulkish: Cool.
Protip, see https://mochajs.org/#mochaopts (if you're not already aware of it)

Mocha will attempt to load ./test/mocha.opts as a configuration file of sorts. The lines in this file are combined with any command-line arguments.

janzenz commented Mar 9, 2017

@scmx is the console.error.restore() part really necessary? I tried omitting it and there were no issues. I'm not also sure what that part actually does, I tried using it on my Chrome's console and it doesn't recognize restore() as a function. I also looked for it in the documentation but couldn't find any. Am I missing something? Thank you for sharing this!

In my mocha env, this only throws on first run. If I am watching the tests, subsequent runs pass (because prop warnings don't fire.) Has anyone else experienced that?

Owner

scmx commented Mar 28, 2017

@janzenz: Sinon stubs console.error to be different and provides the restore function to restore console.error to the original it was before. See http://sinonjs.org/releases/v2.1.0/stubs/

@evandavis: Try changing to beforeEach and afterEach instead of before and after. Perhaps it's being restored too soon.

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