background.js

const browserAppData = this.browser || this.chrome;
const tabs = {};
const inspectFile = 'inspect.js';
const activeIcon = 'active-64.png';
const defaultIcon = 'default-64.png';

const inspect = {
  toggleActivate: (id, type, icon) => {
    this.id = id;
    browserAppData.tabs.executeScript(id, { file: inspectFile }, () => { browserAppData.tabs.sendMessage(id, { action: type }); });
    browserAppData.browserAction.setIcon({ tabId: id, path: { 19: 'icons/' + icon } });
  }
};

function isSupportedProtocolAndFileType(urlString) {
  if (!urlString) { return false; }
  const supportedProtocols = ['https:', 'http:', 'file:'];
  const notSupportedFiles = ['xml', 'pdf', 'rss'];
  const extension = urlString.split('.').pop().split(/\#|\?/)[0];
  const url = document.createElement('a');
  url.href = urlString;
  return supportedProtocols.indexOf(url.protocol) !== -1 && notSupportedFiles.indexOf(extension) === -1;
}

function toggle(tab) {
  if (isSupportedProtocolAndFileType(tab.url)) {
    if (!tabs[tab.id]) {
      tabs[tab.id] = Object.create(inspect);
      inspect.toggleActivate(tab.id, 'activate', activeIcon);
    } else {
      inspect.toggleActivate(tab.id, 'deactivate', defaultIcon);
      for (const tabId in tabs) {
        if (tabId == tab.id) delete tabs[tabId];
      }
    }
  }
}

function deactivateItem(tab) {
  if (tab[0]) {
    if (isSupportedProtocolAndFileType(tab[0].url)) {
      for (const tabId in tabs) {
        if (tabId == tab[0].id) {
          delete tabs[tabId];
          inspect.toggleActivate(tab[0].id, 'deactivate', defaultIcon);
        }
      }
    }
  }
}

function getActiveTab() {
  browserAppData.tabs.query({ active: true, currentWindow: true }, tab => { deactivateItem(tab); });
}

browserAppData.commands.onCommand.addListener(command => {
  if (command === 'toggle-xpath') {
    browserAppData.tabs.query({ active: true, currentWindow: true }, tab => {
      toggle(tab[0]);
    });
  }
});

browserAppData.tabs.onUpdated.addListener(getActiveTab);
browserAppData.browserAction.onClicked.addListener(toggle);