add javascript code to tsx file in gatsby code example

Example: how to install gatsby with typescript

# Install this on the current gatsby project
npm install gatsby-plugin-typescript & npm install typescript --save-dev
# Yarn
yarn add gatsby-plugin-typescript & yarn add typescript --dev

# gatsby-config.js
module.exports = {
  plugins: [
    `gatsby-plugin-typescript`,
  ],
}

# https://medium.com/@stojanpeshov/how-to-setup-gatsbyjs-starter-with-typescript-eslint-prettier-269b1a0a812f