Why I By no means Use Shallow Rendering

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I take into account a couple of years in the past once I were given began with React I determined I had to
work out easy methods to check React elements. I attempted
shallow from enzyme and
right away determined that I might by no means use it to check my React elements. I have
expressed this sense on many events and get requested regularly why I
really feel the way in which I do about shallow rendering and why
React Checking out Library
won’t ever beef up shallow rendering.

So in the end I am popping out with it and explaining why I by no means use shallow
rendering and why I believe no person else will have to both. Here is my primary statement:

With shallow rendering, I will be able to refactor my element’s implementation and my
exams spoil. With shallow rendering, I will be able to spoil my software and my exams
say the whole lot’s nonetheless operating.

That is extremely regarding to me as a result of no longer simplest does it make trying out
irritating, however it additionally lulls you right into a false sense of safety. The rationale
I write exams is to be assured that my software works and there are some distance
higher techniques to try this than shallow rendering.

What even is shallow rendering?

For the needs of this text, let’s use this case as our topic below
check:

import * as React from 'react'
import {CSSTransition} from 'react-transition-group'

serve as Fade({youngsters, ...props}) {
  go back (
    <CSSTransition {...props} timeout={1000} className="fade">
      {youngsters}
    </CSSTransition>
  )
}

magnificence HiddenMessage extends React.Part {
  static defaultProps = {initialShow: false}
  state = {display: this.props.initialShow}
  toggle = () => {
    this.setState(({display}) => ({display: !display}))
  }
  render() {
    go back (
      <div>
        <button onClick={this.toggle}>Toggle</button>
        <Fade in={this.state.display}>
          <div>Hi global</div>
        </Fade>
      </div>
    )
  }
}

export {HiddenMessage}

Here is an instance of a check that makes use of shallow rendering with enzyme:

import * as React from 'react'
import Enzyme, {shallow} from 'enzyme'
import Adapter from 'enzyme-adapter-react-16'
import {HiddenMessage} from '../hidden-message'

Enzyme.configure({adapter: new Adapter()})

check('shallow', () => {
  const wrapper = shallow(<HiddenMessage initialShow={true} />)
  be expecting(wrapper.in finding('Fade').props()).toEqual({
    in: true,
    youngsters: <div>Hi global</div>,
  })
  wrapper.in finding('button').simulate('click on')
  be expecting(wrapper.in finding('Fade').props()).toEqual({
    in: false,
    youngsters: <div>Hi global</div>,
  })
})

To know shallow rendering, let’s upload a console.log(wrapper.debug())
which is able to sign off the construction of what enzyme has rendered for us:

<div>
  <button onClick={[Function]}>Toggle</button>
  <Fade in={true}>
    <div>Hi global</div>
  </Fade>
</div>

You’ll be able to understand that it is not in truth appearing the CSSTransition which is what
Fade is rendering. It is because as a substitute of in truth rendering elements
and calling into the Fade element, shallow simply appears to be like on the props that
can be carried out to the React components created by means of the element you are shallowly
rendering. In reality, if I have been to take the render means of the HiddenMessage
element and console.log what it is returning, I might get one thing that appears a
bit like this:

{
  "kind": "div",
  "props": {
    "youngsters": [
      {
        "type": "button",
        "props": {
          "onClick": [Function],
          "youngsters": "Toggle"
        }
      },
      {
        "kind": [Function: Fade],
        "props": {
          "in": true,
          "youngsters": {
            "kind": "div",
            "props": {
              "youngsters": "Hi global"
            }
          }
        }
      }
    ]
  }
}

Glance acquainted? So all shallow rendering is doing is taking the results of the
given element’s render means (which can be a React component (learn
What’s JSX?)) and giving us a wrapper object with some
utilities for traversing this JavaScript object. This implies it does not run
lifecycle strategies (as a result of we simply have the React components to take care of), it
does not will let you in truth engage with DOM components (as a result of not anything’s
in truth rendered), and it does not in truth try to get the react components
which are returned by means of your customized elements (like our Fade element).

Why other people use shallow rendering

Once I made up our minds early directly to by no means use shallow rendering, it was once as a result of I knew
that there have been higher techniques to get on the issues that shallow rendering makes
simple with out making the trade-offs shallow rendering forces you to make.
I latterly requested people
to inform me why they use shallow rendering. Listed here are some of the issues that
shallow rendering makes more uncomplicated:

There have been extra responses, however those sum up the principle arguments for the usage of shallow
rendering. Let’s deal with every of those:

Calling strategies in react elements

Have you ever ever observed or written a check that appears like this?

check('toggle toggles the state of display', () => {
  const wrapper = shallow(<HiddenMessage initialShow={true} />)
  be expecting(wrapper.state().display).toBe(true) // initialized correctly
  wrapper.example().toggle()
  wrapper.replace()
  be expecting(wrapper.state().display).toBe(false) // toggled
})

It is a nice reason why to make use of shallow rendering, however it is a in point of fact deficient trying out
apply. There are two in point of fact necessary issues that I attempt to believe when
trying out:

Will this check spoil when there is a mistake that may spoil the element
in manufacturing?
Will this check proceed to paintings when there is a absolutely backward suitable
refactor of the element?

This type of check fails either one of the ones concerns:

I may just mistakenly set onClick of the button to this.tgogle as a substitute of
this.toggle. My check continues to paintings, however my element is damaged.
I may just rename toggle to handleButtonClick (and replace the corresponding
onClick reference). My check breaks in spite of this being a refactor.

The rationale this sort of check fails the ones concerns is as a result of it is trying out
beside the point implementation main points. The person does not care one bit what issues are
known as. In reality, that check does not even test that the message is hidden
correctly when the display state is false or proven when the display state is
true. So no longer simplest does the check no longer do an excellent task preserving us secure from
breakages, it is also flakey and does not in truth check the rationale the element
exists within the first position.

In abstract, in case your check makes use of example() or state(), know that you are
trying out issues that the person could not perhaps find out about and even care about,
which is able to take your exams farther from providing you with self belief that issues will
paintings when your person makes use of them.

… it sort of feels like a waste …

There is no getting round the truth that shallow rendering is quicker than any
different type of trying out react elements. It is without a doubt method sooner than mounting
a react element. However we are speaking a handful of milliseconds right here. Sure, it
will upload up, however I might gladly wait an additional few seconds or mins for my exams to
end in trade for my exams in truth giving me self belief that my
software will paintings once I send it to customers.

Along with this, you will have to most definitely use Jest’s features for simplest
operating exams related for your adjustments whilst growing your exams so the
distinction wont be perceivable when operating the check suite in the community.

For exact unit trying out

It is a quite common false impression: “To unit check a react element you will have to
use shallow rendering so different elements don’t seem to be rendered.” It is true that
shallow rendering does not render different elements (as demonstrated above),
what is incorrect with that is that it is method too heavy passed as it does not
render any different elements. You do not get a decision.

Now not simplest does shallow rendering no longer render 3rd birthday party elements, it does not
even render in-file elements. As an example, the <Fade /> element we have now
above is an implementation element of the <HiddenMessage /> element, however
as a result of we are shallow rendering <Fade /> is not rendered so adjustments to that
element may just spoil our software however no longer our check. That is a big factor in
my thoughts and is proof to me that we are trying out implementation main points.

As well as, you’ll undoubtedly unit check react elements with out shallow
rendering. Checkout the phase close to the tip for an instance of this kind of check (makes use of
React Checking out Library, however it is advisable do that with enzyme as neatly) that makes use of Jest
mocking to mock out the <CSSTransition /> element.

I will have to upload that I typically am in opposition to mocking even 3rd birthday party elements
100% of the time. The argument for mocking 3rd birthday party elements I frequently pay attention
is
Checking out composed elements introduces new dependencies that would possibly cause an error whilst the unit itself would possibly nonetheless paintings as supposed..
However is not the purpose of trying out to be assured the applying works? Who cares
in case your unit works if the app is damaged? I undoubtedly wish to know if the
3rd birthday party element I am the usage of breaks my use case. I imply, I am not going to
rewrite their whole check base, but when I will be able to simply check my use case by means of no longer
mocking out their element then why don’t you do this and get the additional self belief?

I will have to additionally upload that
I am in choose of depending extra closely on integration trying out.
While you do that, you wish to have to unit check fewer of your easy elements and wind
up simplest having to unit check edge circumstances for elements (which will mock all they
need). However even in those scenarios, I nonetheless suppose it ends up in extra self belief
and a extra maintainable testbase if you end up specific about which elements are
being mocked and that are being rendered by means of doing complete mounting and specific
mocks.

With out shallow rendering

I am an enormous believer of the tenet of
React Checking out Library:

The extra your exams resemble the way in which your device is used, the extra self belief they may be able to come up with.
— Kent C. Dodds 👋

That is why I wrote the library within the first position. As a side-note to this
shallow rendering submit, I wish to point out there are fewer techniques so that you can do
issues which are not possible for the person to do. Here is the record of items that
React Checking out Library can’t do (out of the field):

shallow rendering
Static rendering (like enzyme’s
render serve as).
Just about maximum of enzyme’s the right way to question components (like
in finding) which
come with the power to seek out by means of an element magnificence and even its displayName
(once more, the person does no longer care what your element is known as and neither
will have to your check). Observe: React Checking out Library helps querying for
components in ways in which inspire accessibility on your elements and extra
maintainable exams.
Getting an element example (like enzyme’s
example)
Getting and atmosphere an element’s props (props())
Getting and atmosphere an element’s state (state())

All of this stuff are issues which customers of your element can’t do, so your
exams mustn’t do them both. Under is a check of the <HiddenMessage />
element which resembles the way in which a person would use your element a lot more
carefully. As well as, it will probably test that you are the usage of <CSSTransition />
correctly (one thing the shallow rendering instance was once incapable of doing).

import {render, display} from '@testing-library/react'
import userEvent from '@testing-library/user-event'
import * as React from 'react'
import {CSSTransition} from 'react-transition-group'

import {HiddenMessage} from '../hidden-message'

// NOTE: you do NOT have to do that in each check.
// Be informed extra about Jest's __mocks__ listing:
// https://jestjs.io/doctors/en/manual-mocks
jest.mock('react-transition-group', () => {
  go back {
    CSSTransition: jest.fn(({youngsters, in: display}) => (display ? youngsters : null)),
  }
})

check('you'll mock issues with jest.mock', () => {
  render(<HiddenMessage initialShow={true} />)
  const toggleButton = display.getByText(/toggle/i)

  const context = be expecting.any(Object)
  const youngsters = be expecting.any(Object)
  const defaultProps = {youngsters, timeout: 1000, className: 'fade'}

  be expecting(CSSTransition).toHaveBeenCalledWith(
    {in: true, ...defaultProps},
    context,
  )
  be expecting(display.getByText(/hi global/i)).no longer.toBeNull()

  CSSTransition.mockClear()

  userEvent.click on(toggleButton)

  be expecting(display.queryByText(/hi global/i)).toBeNull()
  be expecting(CSSTransition).toHaveBeenCalledWith(
    {in: false, ...defaultProps},
    context,
  )
})

Conclusion

A couple of weeks in the past, my DevTipsWithKent (my weekdaily
livestream on YouTube) I livestreamed
“Migrating from shallow rendering react elements to specific element mocks“.
In that I display one of the vital pitfalls of shallow rendering I describe above
in addition to easy methods to use jest mocking as a substitute.

I’m hoping that is useful! We are all simply making an attempt our absolute best to ship a fantastic
revel in to customers. I want you good fortune in that undertaking!

P.S.

Any person introduced this up:

Shallow wrapper is just right to check small unbiased elements. With right kind
serializer it lets in to have transparent and comprehensible snapshots.

I very hardly ever use snapshot trying out with react and I without a doubt would not use it
with shallow. That is a recipe for implementation main points. The entire snapshot is
not anything however implementation main points (it is stuffed with element and prop names that
trade at all times on refactors). It is going to fail any time you contact the element
and the git diff for the snapshot will glance virtually similar to the only to your
adjustments to the element.

This may occasionally make other people careless about adjustments to the snapshot updates as a result of
they alter at all times. So it is principally nugatory (virtually worse than no
exams as it makes you suppose you are coated if you end up no longer and you will not
write right kind exams as a result of they are in position).

I do suppose that snapshots may also be helpful regardless that. For extra about this from me,
checkout every other weblog submit:

Efficient Snapshot Checking out

I’m hoping that is helping!

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Why I By no means Use Shallow Rendering

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