feat(babel-preset-gatsby-package): add conditional compilation plugin

[wardpeet]

Description

Added an extra feature to babel-preset-gatsby-package to enable Conditional compilation. It's a fancy word to add if cases that are evaluated at build time. In the long run, we'll have a real bundle that would also compile the code path away.

To enable the features you have to set availableCompilerFlags as an option in the babel-preset-gatsby-package.

{
  "presets": [["babel-preset-gatsby-package", { "availableCompilerFlags": ["GATSBY_MAJOR"] }]]
}

In code you can set it:

if (_CFLAGS_.GATSBY_MAJOR === '4') {
  // THIS IS ONLY HIT WHEN COMPILER OPTION GATSBY_MAJOR=4 is set
} else {
  // THIS IS ONLY HIT WHEN NO COMPILER_OPTION IS SET OR GATSBY_MAJOR != 4
}

These flags are parsed from process.env.COMPILER_OPTIONS, the syntax used is GATSBY_MAJOR=4. If you need multiple variables, you have to comma separate them GATSBY_MAJOR=4,ANOTHER=hello.

Why not use comments?
I thought of using comments instead which would allow us to remove code blocks using babel but these bits would not be caught by eslint or typescript and the babel transformer would become more complex. We can do this in a further iteration if we see more benefits around this.

Related Issues

[ch35493]

[wardpeet]

These are not necessary anymore, else I had to change a few things in babelrc

[wardpeet]

This is necessary to find the global CFLAGS types in the monorepo