Verifying that path strings are prepended a forward slash

[michaeljdeeb]

Some funny things can occur if the path variable is improperly set in the front matter of a page.

This will check that all path strings are prepended with a forward slash.

Some interesting things to note, possibly for documentation or to improve upon the check.

• No additional testing was added, but this fix passes npm test
• This checking mechanism only works for gatsby build or the initial launch of gatsby develop (not thrown during hot reloading)
• If your pages are structured similarly to the starter-blog, not having a forward slash at the end of your path string will cause files that aren't index to appear outside the folder with the folder name appended to them.
  • Ex. /pages/2015-05-01-hello-world/salty_egg.jpg would become /hello-worldsalty_egg.jpg if path were set to /hello-world
  • Opted not to check for this as someone may want a page such as 404.md to have the path /404.html for GitHub Pages.

[KyleAMathews]

Merged this, thanks! These sorts of invariant run-time checks are worth a 1000 Stackoverflow answers :-) Basically any time there's a question on something here or elsewhere that we can add an automated check for we should.

I tweaked the messaging a bit and added a link in the message here (eventually we'll have actual doc pages that we can link to) in 2856acc

The output now looks like:

Invariant Violation:
          Paths must be prepended with a forward slash. You set the path to
          "hello-world/" in "2015-05-01-hello-world/index.md" but instead it should be "/hello-world/"

          See http://bit.ly/1qeNpdy for more.

Let me know what you think.

[michaeljdeeb]

That looks great.

Linking to this PR is a great idea for the time being.

[jlengstorf]

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