[index.js] Preact-friendly class component check

[petermikitsh]

Taking the same approach as acdlite/recompose@1c9cf7b, this PR augments the class checking logic to support Preact.

In my particular use case, I am using react-final-form-arrays (which depends on this project, though it is inlined in the distribution) (see line 59).

When using Preact, I get this exception:

[start:server] webpack-internal:///./node_modules/react-final-form-arrays/dist/react-final-form-arrays.es.js:68
[start:server]     throw new Error('Can only polyfill class components');
[start:server]     ^
[start:server] 
[start:server] Error: Can only polyfill class components
[start:server]     at polyfill (webpack-internal:///./node_modules/react-final-form-arrays/dist/react-final-form-arrays.es.js:68:11)
[start:server]     at eval (webpack-internal:///./node_modules/react-final-form-arrays/dist/react-final-form-arrays.es.js:512:1)

This PR fixes that thrown exception.

[bvaughn]

prototype && prototype.isReactComponent is the standard way (within the React codebase) that we check for React components. I think preact-compat supports this too by adding that property to its components. So I'm not sure why this change would be needed?

[bvaughn]

Why this change? I think Node 8.3+ supports the spread operator, and our devEngines block specifies 8.5+ or 9+

[petermikitsh]

@bvaughn Reverted this. When I first began work on this PR, I was using Node 8.0 on my machine. Switching to 8.5 fixed the issue.

[bvaughn]

Why this change?

[petermikitsh]

@bvaughn: I was getting this runtime error (running the test suite without this change) =>

> jest test.js

 FAIL  ./test.js
  ● Test suite failed to run

    SecurityError: localStorage is not available for opaque origins
      
      at Window.get localStorage [as localStorage] (node_modules/jsdom/lib/jsdom/browser/Window.js:257:15)
          at Array.forEach (<anonymous>)

Test Suites: 1 failed, 1 total
Tests:       0 total
Snapshots:   0 total
Time:        0.824s, estimated 4s

I've actually seen this error when using Jest in other projects. The solution I've come across (fixing the exception) is discussed in jestjs/jest#6766 (comment).

[petermikitsh]

I'm not using preact-compat. I'm using preact (core). They don't define isReactComponent (see https://github.com/developit/preact/blob/master/src/preact.js).

In preact core, isReactComponent always evaluates to falsey and the error unnecessarily trips the exception. I agree that using isReactComponent is the conventional approach, but checking for a render function is functionally equivalent (unit test suites pass), with the added bonus of supporting preact. The latter is the primary reason for this PR.

[bvaughn]

I see.

I think there's value in keeping this logic consistent in all places (here and React core). It's been around for a long time and changing it might have unforeseen effects (false positives or negatives). For example, this is a valid React component but would fail the proposed check:

class Example extends React.Component {
  render = () => <div />; 
}

I also think preact-compat is a reasonable requirement in order for a Preact app to be able to use a package like react-lifecycles-compat.

Hope you understand. Thanks for taking the time to submit a PR and talk it through though!

[petermikitsh]

Some interesting discussion (that I was not aware of prior to submitting to this PR):

• Simpler classful component check (fixes #458) acdlite/recompose#463 (comment)
• Remove createEagerFactory and createEagerElement acdlite/recompose#538 (comment)