"cannot read property ref of null" error when module.exports === undefined

[jeffmo]

It only happens every so often, but it's super confusing and hard to debug when it does:

If ever a module were to somehow clobber it's exports object with something that isn't an object, you get a nasty error like this:

TypeError: Cannot read property 'ref' of null

at mocks.js:285:11
at visit (mocks.js:275:5)
at removeUnusedRefs (mocks.js:284:3)
at Object.module.exports.getMetadata (mocks.js:364:5)
at mock-modules.js:153:16
at mock-modules.js:128:22
at _generateMock (mock-modules.js:162:20)
at _System._loadAutoMock (mock-modules.js:199:51)
at _System._moduleRequire (mock-modules.js:313:17)

Let's add an assertion earlier on to ensure that the error message is at least helpful.

A common scenario where people hit this is when they're trying to test a react component that look something like this:

var React = require('React');

module.exports = React.createClass({
  // ...
});

In this case, if you don't explicitly mark the React module as unmocked, module.exports is assigned the return value of React.createClass() -- which is a mock function, and thus returns undefined.

[ccheever]

I've also run into this problem with a different module (not React). It's really confusing and unclear how to fix it.

ccheever@charless-mbp:~/projects/crayon$npm test

> crayon@2.1.0 test /Users/ccheever/projects/crayon
> jest

Found 1 matching tests...
 FAIL  __tests__/crayon-test.coffee (0.022s)
● crayon › it Can be called like `crayon.red`
  - TypeError: /Users/ccheever/projects/crayon/crayon.js: Cannot read property 'ref' of null

[jeffmo]

I just committed 3e5ddee (and published v0.1.15) which should make the module mocker more accurately pass through undefined and null exports values to the mocked version of the exports.

I also considered making jest throw with an error that suggests that, when undefined is encountered as an exports object, you should refer to this issue as it's likely that you're doing something like:

var createClass = require('createClass');
module.exports = createClass({ /*...*/ });

But I suppose this could false positive in the extremely rare case where someone, for some reason, intentionally (or non-erroneously) set their exports object to undefined...in which case the error message could be even more misleading.

I'm not necessarily unwilling to do this -- but I do think errors of this type should surface a more debuggable stack trace now with error messages more along the lines of TypeError: undefined is not a function. I do wonder if this will lead to people finding the source of the problem in a more reasonable way.

If this continues to be a source of pain and we keep getting questions about this, I'm willing to re-consider short-circuiting intentional module.exports = undefined scenarios in favor of a detailed explanation of why you might need to unmock something.

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