feat: describe conditional values

[deemp]

Why:

Closes: #23795

I used this hidden knowledge in freckle/stack-action#35 and decided to document it.

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[nguyenalex836]

@deemp Thanks so much for opening a PR! I'll get this triaged for review ✨

[ferdnyc]

This... isn't correct at all.

The workflow is parsed in a string context (except for numerals and booleans), so JavaScript truthiness is irrelevant. There's no way to define a true JavaScript NaN or undefined in YML, you end up with the strings 'NaN' or 'undefined'. Nearly any string will evaluate true.

In this workflow:

    strategy:
      matrix:
        runme:
          - NaN
          - undefined
          - yes
          - no
          - false
          - 0
          - 1
          - -0

    steps:
      - name: true-path
        if: matrix.runme
        run: echo "${{ matrix.runme }} evaluates true"
      - name: false-path
        if: '! matrix.runme'
        run: echo "${{ matrix.runme }} evaluates false"

...The only matrix.runme values for which false-path will be executed are false and 0. (And -0, technically, but that's only because it's interpreted as 0. There's no distinction in YAML.) All of the others will evaluate true.

[ferdnyc]

Oh, sorry, null is also a valid YAML primitive, and it will also evaluate false.

Honestly, I think this is pretty well explained in the Literals section of the Expressions docs.

[deemp]

1. I ran the suggested job (link). I can confirm that the falsy values are false, 0, -0, "", '', null. I can't see where these values are listed in the Literals section. What do you think about just extending the boolean docs?

   Data type	Literal value
   boolean	falsy (false, 0, -0, "", '', null) or truthy (true and other non-falsy values)

2. I believe it's necessary to mention in the if docs that it expects a boolean and will run a step if the boolean is truthy. BTW, the docs already mention a truthy value (link), but don't explain it.

[deemp]

Updated the PR

[SiaraMist]

Thank you so much for updating the docs, @deemp! I'll go ahead and get this pull request merged. ✨

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