Fix default args aggregation in StoryStore.

[vskh]

Issue: #12540

TL;DR

I faced a problem causing storyshots to fail with 0 tests found which is caused by undefined/null argument passed into one of the stories.

Symptoms

Snapshot test run fails with message:

 FAIL  src/storyshots.test.ts
  ● Test suite failed to run

    storyshots found 0 stories

      1 | import testStorySnapshots from "@storybook/addon-storyshots";
      2 | 
    > 3 | testStorySnapshots();
        | ^

      at testStorySnapshots (node_modules/@storybook/addon-storyshots/dist/api/index.js:108:15)
      at Object.<anonymous> (src/stoshots.test.ts:3:1)

The error is confusing and does not give any hints as the root cause. Storybook works fine and is not affected.

Sample repo that demonstrates an issue can be found here: https://github.com/vskh/storybook-storyshots-found-0-stories

Root cause

Passing in explicitly undefined argument to the story will break storyshots stories detection, e.g.:

export const Default: Story = args => ...;
Default.args = {
    prop: null
};

In StoryStore, ArgTypes inference in this case is returning undefined entry which then fails to decompose the way that default arguments aggregation expects (see the actual fix).

Retrospectively, now that I know the cause it seems to be not a big deal and rather me doing weird thing instead of omitting an argument (omitting makes it work fine) but I spent few hard hours trying to figure out how two packages with seemingly same configuration exhibit different behaviour so I thought to submit a fix if someone ever hits the same.

What I did

• fixed default arguments aggregation to handle this case
• added unit test

How to test

jest lib/client-api/src/story_store.ts

[tmeasday]

So is this issue here that argType inference on arg: undefined is itself argType: undefined?

Perhaps that itself is the issue here? WDYT @shilman? What should the argType be?

Thanks for tracking it down @vskh. I'm not sure you are wrong to set the arg to undefined.

[shilman]

I wonder if this fix will remove the need for argType?.defaultValue: #13029

[vskh]

Yes, that is what happening per my understanding. You can have a look at this line:

storybook/lib/client-api/src/inferArgTypes.ts

Line 32 in f286ca2

if (arg !== null && typeof arg !== 'undefined') {

[vskh]

Oops, did not see the last comment. Yeah, it should help. I guess my PR is not needed then?

[stale]

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