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Add-ons
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  6. Creating Reusable Modules

Creating Reusable Modules

In This Article
  1. A hashing add-on
    1. File picker
    2. Hash function
    3. Putting it together
  2. Creating separate modules
    1. filepicker.js
    2. md5.js
    3. index.js
  3. Distributing modules

Add-ons using the techniques described in this document are considered a legacy technology in Firefox. Don't use these techniques to develop new add-ons. Use WebExtensions instead. If you maintain an add-on which uses the techniques described here, consider migrating it to use WebExtensions.

From Firefox 53 onwards, no new legacy add-ons will be accepted on addons.mozilla.org (AMO).

From Firefox 57 onwards, WebExtensions will be the only supported extension type, and Firefox will not load other types.

Even before Firefox 57, changes coming up in the Firefox platform will break many legacy extensions. These changes include multiprocess Firefox (e10s), sandboxing, and multiple content processes. Legacy extensions that are affected by these changes should migrate to WebExtensions if they can. See the "Compatibility Milestones" document for more.

A wiki page containing resources, migration paths, office hours, and more, is available to help developers transition to the new technologies.

To follow this tutorial you'll need to have learned the basics of jpm.

With the SDK you don't have to keep all your add-on in a single "index.js" file. You can split your code into separate modules with clearly defined interfaces between them. You then import and use these modules from other parts of your add-on using the require() statement, in exactly that same way that you import core SDK modules like page-mod or panel.

It can often make sense to structure a larger or more complex add-on as a collection of modules. This makes the design of the add-on easier to understand and provides some encapsulation as each module will export only what it chooses to, so you can change the internals of the module without breaking its users.

Once you've done this, you can package the modules and distribute them independently of your add-on, making them available to other add-on developers and effectively extending the SDK itself.

In this tutorial we'll do exactly that with a module that calculates file hashes.

A hashing add-on

A hash function takes a string of bytes of any length, and produces a short, fixed length string of bytes as output. It's a useful way to create a "fingerprint" that can be used to identify a file. MD5 is a commonly used hash function: although it's no longer considered secure, it works fine outside a security context.

Here we'll write an add-on that lets the user select a file on disk and calculates its hash. For both these operations we'll use XPCOM interfaces.

File picker

To let the user select a file we'll use nsIFilePicker. The documentation for that interface includes an example which we can adapt like this:

var {Cc, Ci} = require("chrome");
function promptForFile() {
  const nsIFilePicker = Ci.nsIFilePicker;
  var fp = Cc["@mozilla.org/filepicker;1"]
           .createInstance(nsIFilePicker);
  var window = require("sdk/window/utils").getMostRecentBrowserWindow();
  fp.init(window, "Select a file", nsIFilePicker.modeOpen);
  fp.appendFilters(nsIFilePicker.filterAll | nsIFilePicker.filterText);
  var rv = fp.show();
  if (rv == nsIFilePicker.returnOK || rv == nsIFilePicker.returnReplace) {
    var file = fp.file;
    // Get the path as string. Note that you usually won't
    // need to work with the string paths.
    var path = fp.file.path;
    // work with returned nsILocalFile...
  }
  return path;
}

Hash function

Firefox has built-in support for hash functions, exposed via the nsICryptoHash XPCOM interface The documentation page for that interface includes an example of calculating an MD5 hash of a file's contents, given its path. We can adapt it like this:

var {Cc, Ci} = require("chrome");
// return the two-digit hexadecimal code for a byte
function toHexString(charCode) {
  return ("0" + charCode.toString(16)).slice(-2);
}
function md5File(path) {
  var f = Cc["@mozilla.org/file/local;1"]
          .createInstance(Ci.nsILocalFile);
  f.initWithPath(path);
  var istream = Cc["@mozilla.org/network/file-input-stream;1"]           
                .createInstance(Ci.nsIFileInputStream);
  // open for reading
  istream.init(f, 0x01, 0444, 0);
  var ch = Cc["@mozilla.org/security/hash;1"]
           .createInstance(Ci.nsICryptoHash);
  // we want to use the MD5 algorithm
  ch.init(ch.MD5);
  // this tells updateFromStream to read the entire file
  const PR_UINT32_MAX = 0xffffffff;
  ch.updateFromStream(istream, PR_UINT32_MAX);
  // pass false here to get binary data back
  var hash = ch.finish(false);
  // convert the binary hash data to a hex string.
  var s = Array.from(hash, (c, i) =>
      toHexString(hash.charCodeAt(i))).join("");
  return s;
}

Putting it together

The complete add-on adds a button to Firefox: when the user clicks the button, we ask them to select a file, compute the hash, and log the hash to the console:

var {Cc, Ci} = require("chrome");
// return the two-digit hexadecimal code for a byte
function toHexString(charCode) {
  return ("0" + charCode.toString(16)).slice(-2);
}
function md5File(path) {
  var f = Cc["@mozilla.org/file/local;1"]
          .createInstance(Ci.nsILocalFile);
  f.initWithPath(path);
  var istream = Cc["@mozilla.org/network/file-input-stream;1"]           
                .createInstance(Ci.nsIFileInputStream);
  // open for reading
  istream.init(f, 0x01, 0444, 0);
  var ch = Cc["@mozilla.org/security/hash;1"]
           .createInstance(Ci.nsICryptoHash);
  // we want to use the MD5 algorithm
  ch.init(ch.MD5);
  // this tells updateFromStream to read the entire file
  const PR_UINT32_MAX = 0xffffffff;
  ch.updateFromStream(istream, PR_UINT32_MAX);
  // pass false here to get binary data back
  var hash = ch.finish(false);
  // convert the binary hash data to a hex string.
  var s = Array.from(hash, (c, i) =>
      toHexString(hash.charCodeAt(i))).join("");
  return s;
}
function promptForFile() {
  var window = require("sdk/window/utils").getMostRecentBrowserWindow();
  const nsIFilePicker = Ci.nsIFilePicker;
  var fp = Cc["@mozilla.org/filepicker;1"]
          .createInstance(nsIFilePicker);
  fp.init(window, "Select a file", nsIFilePicker.modeOpen);
  fp.appendFilters(nsIFilePicker.filterAll | nsIFilePicker.filterText);
  var rv = fp.show();
  if (rv == nsIFilePicker.returnOK || rv == nsIFilePicker.returnReplace) {
    var file = fp.file;
    // Get the path as string. Note that you usually won't
    // need to work with the string paths.
    var path = fp.file.path;
    // work with returned nsILocalFile...
  }
  return path;
}
require("sdk/ui/button/action").ActionButton({
  id: "show-panel",
  label: "Show Panel",
  icon: {
    "16": "./icon-16.png"
  },
  onClick: function() {
    console.log(md5File(promptForFile()));
  }
});

This works , but index.js is now getting longer and its logic is harder to understand. Let's factor the file picker and hashing code into separate modules.

Creating separate modules

filepicker.js

First create a new file in "lib" called "filepicker.js". Copy the file picker code into this new file, and add the following line at the end:

exports.promptForFile = promptForFile;

This defines the public interface of the new module.

So "filepicker.js" should look like this:

var {Cc, Ci} = require("chrome");
function promptForFile() {
  var window = require("sdk/window/utils").getMostRecentBrowserWindow();
  const nsIFilePicker = Ci.nsIFilePicker;
  var fp = Cc["@mozilla.org/filepicker;1"]
           .createInstance(nsIFilePicker);
  fp.init(window, "Select a file", nsIFilePicker.modeOpen);
  fp.appendFilters(nsIFilePicker.filterAll | nsIFilePicker.filterText);
  var rv = fp.show();
  if (rv == nsIFilePicker.returnOK ||
      rv == nsIFilePicker.returnReplace) {
    var file = fp.file;
    // Get the path as string. Note that you usually won't
    // need to work with the string paths.
    var path = fp.file.path;
    // work with returned nsILocalFile...
  }
  return path;
}
exports.promptForFile = promptForFile;

md5.js

Next, create another file in "lib", called "md5.js". Copy the hashing code there, and add this line at the end:

exports.hashFile = md5File;

The complete file looks like this:

var {Cc, Ci} = require("chrome");
// return the two-digit hexadecimal code for a byte
function toHexString(charCode) {
  return ("0" + charCode.toString(16)).slice(-2);
}
function md5File(path) {
  var f = Cc["@mozilla.org/file/local;1"]
          .createInstance(Ci.nsILocalFile);
  f.initWithPath(path);
  var istream = Cc["@mozilla.org/network/file-input-stream;1"]           
                .createInstance(Ci.nsIFileInputStream);
  // open for reading
  istream.init(f, 0x01, 0444, 0);
  var ch = Cc["@mozilla.org/security/hash;1"]
           .createInstance(Ci.nsICryptoHash);
  // we want to use the MD5 algorithm
  ch.init(ch.MD5);
  // this tells updateFromStream to read the entire file
  const PR_UINT32_MAX = 0xffffffff;
  ch.updateFromStream(istream, PR_UINT32_MAX);
  // pass false here to get binary data back
  var hash = ch.finish(false);
  // convert the binary hash data to a hex string.
  var s = Array.from(hash, (c, i) =>
      toHexString(hash.charCodeAt(i))).join("");
  return s;
}
exports.hashFile = md5File;

index.js

Finally, update index.js to import these two new modules and use them:

var filepicker = require("./filepicker.js");
var md5 = require("./md5.js");
require("sdk/ui/button/action").ActionButton({
  id: "show-panel",
  label: "Show Panel",
  icon: {
    "16": "./icon-16.png"
  },
  onClick: function() {
    console.log(md5.hashFile(filepicker.promptForFile()));
  }
});

Distributing modules

With jpm, we use npm as the package manager for SDK modules that don't ship inside Firefox. Module developers can publish SDK modules to npm, and add-on developers can install them from npm and build them into their add-ons.

To learn how to use third-party modules in your own code, see the tutorial on adding menu items.

Document Tags and Contributors

Tags: 
  • Add-on SDK
 Contributors to this page: wbamberg, Sebastianz, arai, pc.wiz.tt, zombie, maybe, Canuckistani
 Last updated by: wbamberg, Dec 1, 2016, 10:52:32 AM
See also
  1. WebExtensions
  2. Getting started
    1. What are WebExtensions?
    2. Your first WebExtension
    3. Your second WebExtension
    4. Anatomy of a WebExtension
    5. Example WebExtensions
  3. How to
    1. Intercept HTTP requests
    2. Modify a web page
    3. Add a button to the toolbar
    4. Implement a settings page
  4. Concepts
    1. Using the JavaScript APIs
    2. User interface components
    3. Content scripts
    4. Match patterns
    5. Internationalization
    6. Content Security Policy
    7. Native messaging
  5. Porting
    1. Porting a Google Chrome extension
    2. Porting a legacy Firefox add-on
    3. Embedded WebExtensions
    4. Comparison with the Add-on SDK
    5. Comparison with XUL/XPCOM extensions
    6. Chrome incompatibilities
  6. Firefox workflow
    1. Temporary Installation in Firefox
    2. Debugging
    3. Getting started with web-ext
    4. web-ext command reference
    5. WebExtensions and the Add-on ID
    6. Publishing your WebExtension
  7. JavaScript APIs
    1. Browser support for JavaScript APIs
    2. alarms
    3. bookmarks
    4. browserAction
    5. browsingData
    6. commands
    7. contextMenus
    8. contextualIdentities
    9. cookies
    10. downloads
    11. events
    12. extension
    13. extensionTypes
    14. history
    15. i18n
    16. identity
    17. idle
    18. management
    19. notifications
    20. omnibox
    21. pageAction
    22. runtime
    23. sessions
    24. sidebarAction
    25. storage
    26. tabs
    27. topSites
    28. webNavigation
    29. webRequest
    30. windows
  8. Manifest keys
    1. applications
    2. author
    3. background
    4. browser_action
    5. chrome_url_overrides
    6. commands
    7. content_scripts
    8. content_security_policy
    9. default_locale
    10. description
    11. developer
    12. homepage_url
    13. icons
    14. manifest_version
    15. name
    16. omnibox
    17. options_ui
    18. page_action
    19. permissions
    20. short_name
    21. sidebar_action
    22. version
    23. web_accessible_resources
  9. Add-on SDK
  10. Getting started
    1. Installation
    2. Getting started
    3. Troubleshooting
  11. High-Level APIs
    1. addon-page
    2. base64
    3. clipboard
    4. context-menu
    5. hotkeys
    6. indexed-db
    7. l10n
    8. notifications
    9. page-mod
    10. page-worker
    11. panel
    12. passwords
    13. private-browsing
    14. querystring
    15. request
    16. selection
    17. self
    18. simple-prefs
    19. simple-storage
    20. system
    21. tabs
    22. timers
    23. ui
    24. url
    25. webextension
    26. widget
    27. windows
  12. Low-Level APIs
    1. /loader
    2. chrome
    3. console/plain-text
    4. console/traceback
    5. content/content
    6. content/loader
    7. content/mod
    8. content/symbiont
    9. content/worker
    10. core/heritage
    11. core/namespace
    12. core/promise
    13. dev/panel
    14. event/core
    15. event/target
    16. frame/hidden-frame
    17. frame/utils
    18. fs/path
    19. io/byte-streams
    20. io/file
    21. io/text-streams
    22. lang/functional
    23. lang/type
    24. loader/cuddlefish
    25. loader/sandbox
    26. net/url
    27. net/xhr
    28. places/bookmarks
    29. places/favicon
    30. places/history
    31. platform/xpcom
    32. preferences/event-target
    33. preferences/service
    34. remote/child
    35. remote/parent
    36. stylesheet/style
    37. stylesheet/utils
    38. system/child_process
    39. system/environment
    40. system/events
    41. system/runtime
    42. system/unload
    43. system/xul-app
    44. tabs/utils
    45. test/assert
    46. test/harness
    47. test/httpd
    48. test/runner
    49. test/utils
    50. ui/button/action
    51. ui/button/toggle
    52. ui/frame
    53. ui/id
    54. ui/sidebar
    55. ui/toolbar
    56. util/array
    57. util/collection
    58. util/deprecate
    59. util/list
    60. util/match-pattern
    61. util/object
    62. util/uuid
    63. window/utils
  13. Firefox for Android
  14. Getting started
    1. Walkthrough
    2. Debugging
    3. Code snippets
  15. APIs
    1. Accounts.jsm
    2. BrowserApp
    3. HelperApps.jsm
    4. Home.jsm
    5. HomeProvider.jsm
    6. JavaAddonManager.jsm
    7. NativeWindow
    8. Notifications.jsm
    9. PageActions.jsm
    10. Prompt.jsm
    11. RuntimePermissions.jsm
    12. Snackbars.jsm
    13. Sound.jsm
    14. Tab
  16. Legacy
  17. Restartless extensions
    1. Overview
  18. Overlay extensions
    1. Overview
  19. Themes
  20. Lightweight themes
    1. Overview
  21. Complete themes
    1. Overview
  22. Publishing add-ons
  23. Guides
    1. Signing and distribution overview
    2. Submit an add-on
    3. Review policies
    4. Developer agreement
    5. Featured add-ons
    6. Contact addons.mozilla.org
  24. Community and support
  25. Channels
    1. Add-ons blog
    2. Add-on forums
    3. Stack Overflow
    4. Development newsgroup
    5. IRC Channel