background.js

/*
 * Copyright © 2016–2017 Daniel Aleksandersen
 * SPDX-License-Identifier: GPL-3.0
 * License-Filename: LICENSE
 */

// Mozilla WebExtension or Chromium Extension
if (typeof browser === 'undefined' && typeof chrome !== 'undefined') {
  browser = chrome;
}

browser.webRequest.onBeforeSendHeaders.addListener(
  function(details) {
    var headers = details.requestHeaders;
    
    // Check if browser or another extension has already enabled the header
    for (var index = 0; index < headers.length; ++index) {
      if ('name' in headers[index] &&
          headers[index].name.toLowerCase().indexOf('save-data') >= 0) {

        // Return the original headers
        return {
          requestHeaders: details.requestHeaders
    } } };

    // Append the actual header
    headers.push({
      'name': 'Save-Data',
      'value': 'on'
    });

    return {
      requestHeaders: details.requestHeaders
    };
  }, {
    urls: ["<all_urls>"]
  }, [
    "blocking",
    "requestHeaders"
  ]
);