Using theme options in gatsby-config.js

Gatsby themes allow you to pass in options in the same way plugins get options.

module.exports = {
  plugins: [
    {
      resolve: "gatsby-theme-awesome",
      options: {
        myThing: true,
        blogPostFormat: "mdx"
      }
    }
  ]
};

Since gatsby-config has historically exported an object, as seen above, we needed to add a way to grab the options. This is why the function gatsby-config exists, which we can use in our theme to control which plugins are added (or anything else).

module.exports = options => {
  const plugins = [];
  if (options.blogPostFormat === "mdx") {
    plugins.push({
      resolve: `gatsby-plugin-mdx`
    });
  }
  return {
    plugins
  };
};

Note that trying to use the function gatsby-config in the root of a site will fail because there's nowhere to get the options from.