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Created Nov 2, 2017
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// Copyright (c) 2014 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
* Get the current URL.
* @param {function(string)} callback called when the URL of the current tab
* is found.
function getCurrentTabUrl(callback) {
// Query filter to be passed to chrome.tabs.query - see
var queryInfo = {
active: true,
currentWindow: true
chrome.tabs.query(queryInfo, (tabs) => {
// chrome.tabs.query invokes the callback with a list of tabs that match the
// query. When the popup is opened, there is certainly a window and at least
// one tab, so we can safely assume that |tabs| is a non-empty array.
// A window can only have one active tab at a time, so the array consists of
// exactly one tab.
var tab = tabs[0];
// A tab is a plain object that provides information about the tab.
// See
var url = tab.url;
// tab.url is only available if the "activeTab" permission is declared.
// If you want to see the URL of other tabs (e.g. after removing active:true
// from |queryInfo|), then the "tabs" permission is required to see their
// "url" properties.
console.assert(typeof url == 'string', 'tab.url should be a string');
// Most methods of the Chrome extension APIs are asynchronous. This means that
// you CANNOT do something like this:
// var url;
// chrome.tabs.query(queryInfo, (tabs) => {
// url = tabs[0].url;
// });
// alert(url); // Shows "undefined", because chrome.tabs.query is async.
* Change the background color of the current page.
* @param {string} color The new background color.
function changeBackgroundColor(color) {
var script = '"' + color + '";';
// See
// chrome.tabs.executeScript allows us to programmatically inject JavaScript
// into a page. Since we omit the optional first argument "tabId", the script
// is inserted into the active tab of the current window, which serves as the
// default.
code: script
* Gets the saved background color for url.
* @param {string} url URL whose background color is to be retrieved.
* @param {function(string)} callback called with the saved background color for
* the given url on success, or a falsy value if no color is retrieved.
function getSavedBackgroundColor(url, callback) {
// See We check
// for chrome.runtime.lastError to ensure correctness even when the API call
// fails., (items) => {
callback(chrome.runtime.lastError ? null : items[url]);
* Sets the given background color for url.
* @param {string} url URL for which background color is to be saved.
* @param {string} color The background color to be saved.
function saveBackgroundColor(url, color) {
var items = {};
items[url] = color;
// See We omit the
// optional callback since we don't need to perform any action once the
// background color is saved.;
// This extension loads the saved background color for the current tab if one
// exists. The user can select a new background color from the dropdown for the
// current page, and it will be saved as part of the extension's isolated
// storage. The API is used for this purpose. This is different
// from the window.localStorage API, which is synchronous and stores data bound
// to a document's origin. Also, using instead of
// allows the extension data to be synced across multiple
// user devices.
document.addEventListener('DOMContentLoaded', () => {
getCurrentTabUrl((url) => {
var dropdown = document.getElementById('dropdown');
// Load the saved background color for this page and modify the dropdown
// value, if needed.
getSavedBackgroundColor(url, (savedColor) => {
if (savedColor) {
dropdown.value = savedColor;
// Ensure the background color is changed and saved when the dropdown
// selection changes.
dropdown.addEventListener('change', () => {
saveBackgroundColor(url, dropdown.value);
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