Jetpack module for searching and executing browser commands

commands.js

/*

Module for searching available commands in the browser.

Allows searching and execution of commands in Firefox, such as
'Back', 'Zoom In', 'Reload'.

Example that adds buttons to the add-on bar for every command,
which trigger the command when clicked:

const widget = require("widget");
const commands = require("commands");
commands.search("", function(command) {
  widget.Widget({
    label: command.alias,
    content: command.alias,
    onClick: command.execute
  });
});

API

The 'search' method takes two parameters. the first is a string of text
to match against command names. For example, "zo" will match the commands
"Zoom In" and "Zoom Out". The second parameter is a callback that'll be
passed the command objects for all commands matching the search. The command
object has an 'alias' property, which contains a label used in the application
to refer to the command. The command object has an 'execute' method which
will trigger the command. 

Implementation

The module enumerates all <command> elements found in the active browser
window, and then searches for any element in the browser that has a
command attribute with that command's id, storing the item's label
attribute value. The DOM traveral is only done once, on the first invocation
of the 'search' method.

*/

let commands = {};
let aliases = {};
let cached = false;

function cacheCommands() {
  let window = require("window-utils").activeBrowserWindow;
  let document = window.document;
  let commandEls = document.querySelectorAll("command");
  for (let i = 0; i < commandEls.length; i++) {
    let commandEl = commandEls[i];
    let labels = [];
    let els = document.querySelectorAll("*[command=\"" + commandEl.id + "\"]");
    if (els) {
      for (let i = 0; i < els.length; i++) {
        if (els[i].label) {
          labels.push(els[i].label);
        }
      }
    }
    if (labels.length) {
      commands[commandEl.id] = commandEl.id;
      labels.forEach(function(label) aliases[label] = commandEl.id);
    }
  }
}

function command(id, alias) {
  this.alias = alias;
  this.execute = function() {
    if (commands[id]) {
      let document = require("window-utils").activeBrowserWindow.document
      let el = document.getElementById(id);
      if (el)
        el.doCommand();
    }
  };
}

exports.search = function(text, callback) {
  if (!cached) {
    cacheCommands();
    cached = true;
  }

  for (let [alias, id] in Iterator(aliases)) {
    if (!text.length || (new RegExp(text, "i")).test(alias))
      callback(new command(id, alias));
  }
};