electron/lib/renderer/content-scripts-injector.js

const {webFrame} = require('electron')

// Check whether pattern matches.
// https://developer.chrome.com/extensions/match_patterns
const matchesPattern = function (pattern) {
  if (pattern === '<all_urls>') return true

  const regexp = new RegExp('^' + pattern.replace(/\*/g, '.*') + '$')
  return location.href.match(regexp)
}

// Run injected scripts.
// https://developer.chrome.com/extensions/content_scripts
const injectContentScript = function (script) {
  for (const match of script.matches) {
    if (!matchesPattern(match)) return
  }

  for (const js of script.js) {
    const fire = () => webFrame.executeJavaScript(js)
    if (script.runAt === 'document_start') {
      process.once('document-start', fire)
    } else if (script.runAt === 'document_end') {
      process.once('document-end', fire)
    } else if (script.runAt === 'document_idle') {
      document.addEventListener('DOMContentLoaded', fire)
    }
  }
}

// Read the renderer process preferences.
const preferences = process.getRenderProcessPreferences()
if (preferences) {
  for (const pref of preferences) {
    if (pref.contentScripts) {
      pref.contentScripts.forEach(injectContentScript)
    }
  }
}