Controls incorrectly populating an undefined option

Describe the bug Since upgrading to v6.2.5 all optional component props using string literals are also outputting an “undefined” option. Whilst technically accurate, this is causing confusion with users, particularly where default prop values are set that would make an undefined value an impossible outcome.

Using TypeScript and React. We’ve upgraded directly from v6.1.2 so unclear exactly when the behaviour changed.

To Reproduce Steps to reproduce the behavior:

1. Create a component using a string literal prop type
2. View in Canvas and look in Controls panel
3. undefined value should be present

Expected behavior I’d expect undefined to be ignored even if it was technically present on the typing (due to being optional), as has historically been the case.

Screenshots Incorrect in v6.2.5:

Expected behaviour as of v6.1.2:

System

  System:
    OS: Windows 10 10.0.19042
    CPU: (8) x64 Intel(R) Core(TM) i7-8650U CPU @ 1.90GHz
  Binaries:
    Node: 12.18.2 - C:\Program Files\nodejs\node.EXE
    npm: 6.14.5 - C:\Program Files\nodejs\npm.CMD
  Browsers:
    Chrome: 89.0.4389.114
    Edge: Spartan (44.19041.423.0), Chromium (89.0.774.68)
  npmPackages:
    @storybook/addon-a11y: ^6.2.5 => 6.2.5
    @storybook/addon-actions: ^6.2.5 => 6.2.5
    @storybook/addon-essentials: ^6.2.5 => 6.2.5
    @storybook/addon-links: ^6.2.5 => 6.2.5
    @storybook/node-logger: ^6.2.5 => 6.2.5
    @storybook/preset-create-react-app: ^3.1.7 => 3.1.7
    @storybook/react: ^6.2.5 => 6.2.5
    @storybook/theming: ^6.2.5 => 6.2.5

[Yonom]

We are also explaining the same issue as the comments above

Storybook is passing "undefined" (a string) instead of undefined to the component (this might be another bug next to the fact that undefined appears at all)

[vinassefranche]

Hi!

I don’t know if I need to open another issue but it seems to be on the same subject. I have this ‘undefined’ option too and if I use the value of the prop in a object, the actual value passed is ‘undefined’ (as string) which can crash storybook. You can see below the gif of the behavior and you can find the repository to reproduce the issue here

Despite that, storybook controls are amazing so thanks for them!