csp-util.mjs

#!/usr/bin/env node

import fs from 'fs'
import path from 'path'
import * as htmlparser2 from 'htmlparser2'
import { replaceInFileSync } from 'replace-in-file'

const TARGET_FOLDER = path.join(process.cwd(), 'public')

const INPUT_FILE = path.join(TARGET_FOLDER, 'index.html')
const OUTPUT_FILE = path.join(TARGET_FOLDER, '_headers')
const TOKEN_TO_REPLACE = '__REPLACE_ME__'

console.log('Copying the CSP for Netlify headers')

const cspContent = getCspContentFrom(INPUT_FILE)

updateNetlifyHeaderFile(cspContent, OUTPUT_FILE)

function getCspContentFrom(inputFile) {
  console.log(`Getting the CSP content from ${inputFile}`)
  try {
    const fileContents = fs.readFileSync(inputFile, { encoding: 'utf-8' })

    let found = false
    let retVal = ''

    const parser = new htmlparser2.Parser({
      onopentag: (name, attributes) => {
        if (name === 'meta') {
          if (
            attributes['http-equiv'] &&
            'Content-Security-Policy' === attributes['http-equiv']
          ) {
            console.log('Found the CSP content:', attributes['content'])
            found = true
            retVal = attributes['content']
          }
        }
      },
    })

    parser.write(fileContents)

    if (found === false) {
      throw new Error(`Could not find the CSP`)
    }

    return retVal
  } catch (err) {
    console.error(
      'Could not retrieve the CSP content. Did you build first? Is the gatsby-config.js still correct?'
    )
    throw err
  }
}

function updateNetlifyHeaderFile(cspText, outputFile) {
  console.log(`Updating the CSP in the output file [${outputFile}]`)
  const replacementOptions = {
    files: outputFile,
    from: new RegExp(TOKEN_TO_REPLACE, 'g'),
    to: cspText,
  }

  try {
    console.log(`Modifying Netlify's header file`)
    const changes = replaceInFileSync(replacementOptions)
    if (changes && changes.length && changes.length > 0) {
      console.log(`Modified Netlify's headers file successfully`)
    } else {
      throw new Error(
        `Failed to find the expected token to replace: ${TOKEN_TO_REPLACE}`
      )
    }
  } catch (error) {
    console.error(`Failed to modify Netlify's header file`)
    throw error
  }
}