diff --git a/common/content/bookmarks.js b/common/content/bookmarks.js
index a95cc23e..09f088c5 100644
--- a/common/content/bookmarks.js
+++ b/common/content/bookmarks.js
@@ -628,10 +628,10 @@ var Bookmarks = Module("bookmarks", {
     },
 
     completion: function initCompletion() {
-        completion.bookmark = function bookmark(context, tags, extra) {
+        completion.bookmark = function bookmark(context, tags, extra = {}) {
             context.title = ["Bookmark", "Title"];
             context.format = bookmarks.format;
-            iter(extra || {}).forEach(function ([k, v]) {
+            iter(extra).forEach(function ([k, v]) {
                 if (v != null)
                     context.filters.push(function (item) item.item[k] != null && this.matchString(v, item.item[k]));
             });
diff --git a/common/content/commandline.js b/common/content/commandline.js
index e2252be2..de8d9c3c 100644
--- a/common/content/commandline.js
+++ b/common/content/commandline.js
@@ -866,9 +866,7 @@ var CommandLine = Module("commandline", {
      * @... {string} default - The initial value that will be returned
      *     if the user presses <CR> straightaway. @default ""
      */
-    input: function _input(prompt, callback, extra) {
-        extra = extra || {};
-
+    input: function _input(prompt, callback, extra = {}) {
         CommandPromptMode(prompt, update({ onSubmit: callback }, extra)).open();
     },
 
@@ -1396,10 +1394,8 @@ var CommandLine = Module("commandline", {
          *      @default {@link #selected}
          * @returns {object}
          */
-        getItem: function getItem(tuple) {
-            tuple = tuple || this.selected;
-            return tuple && tuple[0] && tuple[0].items[tuple[1]];
-        },
+        getItem: function getItem(tuple = this.selected)
+            tuple && tuple[0] && tuple[0].items[tuple[1]],
 
         /**
          * Returns a tuple representing the next item, at the given
@@ -1510,11 +1506,10 @@ var CommandLine = Module("commandline", {
          *      @default 1
          * @param {boolean} fromTab If true, this function was
          *      called by {@link #tab}.
+         *      @default false
          *      @private
          */
-        select: function select(idx, count, fromTab) {
-            count = count || 1;
-
+        select: function select(idx, count = 1, fromTab = false) {
             switch (idx) {
             case this.UP:
             case this.DOWN:
diff --git a/common/content/dactyl.js b/common/content/dactyl.js
index d5938625..6ad9e553 100644
--- a/common/content/dactyl.js
+++ b/common/content/dactyl.js
@@ -538,13 +538,11 @@ var Dactyl = Module("dactyl", XPCOM(Ci.nsISupportsWeakReference, ModuleBase), {
      * @param {boolean} silent Whether the command should be echoed on the
      *     command line.
      */
-    execute: function execute(str, modifiers, silent) {
+    execute: function execute(str, modifiers = {}, silent = false) {
         // skip comments and blank lines
         if (/^\s*("|$)/.test(str))
             return;
 
-        modifiers = modifiers || {};
-
         if (!silent)
             commands.lastCommand = str.replace(/^\s*:\s*/, "");
         let res = true;
@@ -892,7 +890,7 @@ var Dactyl = Module("dactyl", XPCOM(Ci.nsISupportsWeakReference, ModuleBase), {
      *     tabs.
      * @returns {boolean}
      */
-    open: function open(urls, params, force) {
+    open: function open(urls, params = {}, force = false) {
         if (typeof urls == "string")
             urls = dactyl.parseURLs(urls);
 
@@ -903,7 +901,6 @@ var Dactyl = Module("dactyl", XPCOM(Ci.nsISupportsWeakReference, ModuleBase), {
                         dactyl.open(urls, params, true);
                 });
 
-        params = params || {};
         if (isString(params))
             params = { where: params };
 
@@ -1187,8 +1184,7 @@ var Dactyl = Module("dactyl", XPCOM(Ci.nsISupportsWeakReference, ModuleBase), {
             return [];
         }
     },
-    wrapCallback: function wrapCallback(callback, self) {
-        self = self || this;
+    wrapCallback: function wrapCallback(callback, self = this) {
         let save = ["forceOpen"];
         let saved = save.map(p => dactyl[p]);
         return function wrappedCallback() {
diff --git a/common/content/events.js b/common/content/events.js
index d86610ea..c264bfe6 100644
--- a/common/content/events.js
+++ b/common/content/events.js
@@ -212,8 +212,7 @@ var Events = Module("events", {
     /**
      * Wraps an event listener to ensure that errors are reported.
      */
-    wrapListener: function wrapListener(method, self) {
-        self = self || this;
+    wrapListener: function wrapListener(method, self = this) {
         method.wrapper = wrappedListener;
         wrappedListener.wrapped = method;
         function wrappedListener(event) {
diff --git a/common/content/hints.js b/common/content/hints.js
index f9ad6fbb..af09ad4d 100644
--- a/common/content/hints.js
+++ b/common/content/hints.js
@@ -12,11 +12,9 @@
 var HintSession = Class("HintSession", CommandMode, {
     get extendedMode() modes.HINTS,
 
-    init: function init(mode, opts) {
+    init: function init(mode, opts = {}) {
         init.supercall(this);
 
-        opts = opts || {};
-
         if (!opts.window)
             opts.window = modes.getStack(0).params.window;
 
@@ -1056,11 +1054,9 @@ var Hints = Module("hints", {
         return null;
     }, //}}}
 
-    open: function open(mode, opts) {
+    open: function open(mode, opts = {}) {
         this._extendedhintCount = opts.count;
 
-        opts = opts || {};
-
         mappings.pushCommand();
         commandline.input(["Normal", mode], null, {
             autocomplete: false,
diff --git a/common/content/history.js b/common/content/history.js
index 38f3424c..62aacf1b 100644
--- a/common/content/history.js
+++ b/common/content/history.js
@@ -13,9 +13,7 @@ var History = Module("history", {
 
     get service() services.history,
 
-    get: function get(filter, maxItems, sort) {
-        sort = sort || this.SORT_DEFAULT;
-
+    get: function get(filter, maxItems, sort = this.SORT_DEFAULT) {
         if (isString(filter))
             filter = { searchTerms: filter };
 
diff --git a/common/content/mappings.js b/common/content/mappings.js
index a71e2865..fa113b4d 100644
--- a/common/content/mappings.js
+++ b/common/content/mappings.js
@@ -187,9 +187,7 @@ var MapHive = Class("MapHive", Contexts.Hive, {
      * @param {Object} extra An optional extra configuration hash.
      * @optional
      */
-    add: function (modes, keys, description, action, extra) {
-        extra = extra || {};
-
+    add: function (modes, keys, description, action, extra = {}) {
         modes = Array.concat(modes);
         if (!modes.every(util.identity))
             throw TypeError(/*L*/"Invalid modes: " + modes);
@@ -814,9 +812,7 @@ var Mappings = Module("mappings", {
         });
     },
     completion: function initCompletion(dactyl, modules, window) {
-        completion.userMapping = function userMapping(context, modes_, hive) {
-            hive = hive || mappings.user;
-            modes_ = modes_ || [modes.NORMAL];
+        completion.userMapping = function userMapping(context, modes_ = [modes.NORMAL], hive = mappings.user) {
             context.keys = { text: function (m) m.names[0],
                              description: function (m) m.description + ": " + m.action };
             context.completions = hive.iterate(modes_);
diff --git a/common/content/marks.js b/common/content/marks.js
index caae980b..cfa27bf3 100644
--- a/common/content/marks.js
+++ b/common/content/marks.js
@@ -34,12 +34,10 @@ var Marks = Module("marks", {
 
     get localURI() buffer.focusedFrame.document.documentURI.replace(/#.*/, ""),
 
-    Mark: function Mark(params) {
+    Mark: function Mark(params = {}) {
         let win = buffer.focusedFrame;
         let doc = win.document;
 
-        params = params || {};
-
         params.location = doc.documentURI.replace(/#.*/, ""),
         params.offset = buffer.scrollPosition;
         params.path = DOM(buffer.findScrollable(0, false)).xpath;
diff --git a/common/content/modes.js b/common/content/modes.js
index e5d4a9d3..838eab65 100644
--- a/common/content/modes.js
+++ b/common/content/modes.js
@@ -502,11 +502,10 @@ var Modes = Module("modes", {
         return StackElement;
     })(),
     cacheId: 0,
-    boundProperty: function BoundProperty(desc) {
+    boundProperty: function BoundProperty(desc = {}) {
         let id = this.cacheId++;
         let value;
 
-        desc = desc || {};
         return Class.Property(update({
             configurable: true,
             enumerable: true,
diff --git a/common/content/tabs.js b/common/content/tabs.js
index 0dffdc6b..3d33972c 100644
--- a/common/content/tabs.js
+++ b/common/content/tabs.js
@@ -408,8 +408,7 @@ var Tabs = Module("tabs", {
      * @param {number} count How many tabs to remove.
      * @param {boolean} focusLeftTab Focus the tab to the left of the removed tab.
      */
-    remove: function remove(tab, count, focusLeftTab) {
-        count = count || 1;
+    remove: function remove(tab, count = 1, focusLeftTab = false) {
         let res = this.count > count;
 
         let tabs = this.visibleTabs;
diff --git a/common/modules/base.jsm b/common/modules/base.jsm
index 14805265..e0f0eaad 100644
--- a/common/modules/base.jsm
+++ b/common/modules/base.jsm
@@ -1094,8 +1094,7 @@ let stub = Class.Property({
  */
 var ErrorBase = Class("ErrorBase", Error, {
     level: 2,
-    init: function EB_init(message, level) {
-        level = level || 0;
+    init: function EB_init(message, level = 0) {
         let error = Error(message);
         update(this, error);
         this.stack = error.stack;
@@ -1269,10 +1268,10 @@ var StructBase = Class("StructBase", Array, {
 });
 
 var Timer = Class("Timer", {
-    init: function init(minInterval, maxInterval, callback, self) {
+    init: function init(minInterval, maxInterval, callback, self = this) {
         this._timer = services.Timer();
         this.callback = callback;
-        this.self = self || this;
+        this.self = self;
         this.minInterval = minInterval;
         this.maxInterval = maxInterval;
         this.doneAt = 0;
diff --git a/common/modules/buffer.jsm b/common/modules/buffer.jsm
index 8339bdc6..2350e3b7 100644
--- a/common/modules/buffer.jsm
+++ b/common/modules/buffer.jsm
@@ -784,11 +784,10 @@ var Buffer = Module("Buffer", {
      * @param {number} count The multiple of 'scroll' lines to scroll.
      * @optional
      */
-    scrollByScrollSize: function scrollByScrollSize(direction, count) {
+    scrollByScrollSize: function scrollByScrollSize(direction, count = 1) {
         let { options } = this.modules;
 
         direction = direction ? 1 : -1;
-        count = count || 1;
 
         if (options["scroll"] > 0)
             this.scrollVertical("lines", options["scroll"] * direction);
diff --git a/common/modules/commands.jsm b/common/modules/commands.jsm
index dc9215b6..0e0d58fc 100644
--- a/common/modules/commands.jsm
+++ b/common/modules/commands.jsm
@@ -155,15 +155,13 @@ var Command = Class("Command", {
      * @param {Args} args The Args object passed to {@link #action}.
      * @param {Object} modifiers Any modifiers to be passed to {@link #action}.
      */
-    execute: function execute(args, modifiers) {
+    execute: function execute(args, modifiers = {}) {
         const { dactyl } = this.modules;
 
         let context = args.context;
         if (this.deprecated)
             this.warn(context, "deprecated", _("warn.deprecated", ":" + this.name, this.deprecated));
 
-        modifiers = modifiers || {};
-
         if (args.count != null && !this.count)
             throw FailedAssertion(_("command.noCount"));
         if (args.bang && !this.bang)
@@ -543,21 +541,21 @@ var CommandHive = Class("CommandHive", Contexts.Hive, {
     repeat: null,
 
     /**
-     * Adds a new command to the builtin hive. Accessible only to core
-     * dactyl code. Plugins should use group.commands.add instead.
+     * Adds a new command to the builtin hive. Accessible only to core dactyl
+     * code. Plugins should use group.commands.add instead.
      *
-     * @param {[string]} specs The names by which this command can be
-     *     invoked. The first name specified is the command's canonical
-     *     name.
+     * @param {[string]} specs The names by which this command can be invoked.
+     *     The first name specified is the command's canonical name.
      * @param {string} description A description of the command.
      * @param {function} action The action invoked by this command.
      * @param {Object} extra An optional extra configuration hash.
-     * @optional
+     *     @optional
+     * @param {boolean} replace Replace an existing command of the same name.
+     *     @optional
      */
-    add: function add(specs, description, action, extra, replace) {
+    add: function add(specs, description, action, extra = {}, replace = false) {
         const { commands, contexts } = this.modules;
 
-        extra = extra || {};
         if (!extra.definedAt)
             extra.definedAt = contexts.getCaller(Components.stack.caller);
         if (!extra.sourceModule)
@@ -594,10 +592,8 @@ var CommandHive = Class("CommandHive", Contexts.Hive, {
         return name;
     },
 
-    _add: function _add(names, description, action, extra, replace) {
+    _add: function _add(names, description, action, extra = {}, replace = false) {
         const { contexts } = this.modules;
-
-        extra = extra || {};
         extra.definedAt = contexts.getCaller(Components.stack.caller.caller);
         return this.add.apply(this, arguments);
     },
@@ -968,7 +964,7 @@ var Commands = Module("commands", {
      *     Args object.
      * @returns {Args}
      */
-    parseArgs: function parseArgs(str, params) {
+    parseArgs: function parseArgs(str, params = {}) {
         const self = this;
 
         function getNextArg(str, _keepQuotes=keepQuotes) {
@@ -990,7 +986,7 @@ var Commands = Module("commands", {
 
         try {
 
-            var { allowUnknownOptions, argCount, complete, extra, hereDoc, literal, options, keepQuotes } = params || {};
+            var { allowUnknownOptions, argCount, complete, extra, hereDoc, literal, options, keepQuotes } = params;
 
             if (!options)
                 options = [];
@@ -1776,8 +1772,7 @@ var Commands = Module("commands", {
     }
 });
 
-let quote = function quote(q, list, map) {
-    map = map || Commands.quoteMap;
+let quote = function quote(q, list, map = Commands.quoteMap) {
     let re = RegExp("[" + list + "]", "g");
     function quote(str) (q + String.replace(str, re, $0 => ($0 in map ? map[$0] : ("\\" + $0)))
                            + q);
diff --git a/common/modules/completion.jsm b/common/modules/completion.jsm
index 1aa27c46..001b267b 100644
--- a/common/modules/completion.jsm
+++ b/common/modules/completion.jsm
@@ -33,10 +33,7 @@ lazyRequire("template", ["template"]);
  * @constructor
  */
 var CompletionContext = Class("CompletionContext", {
-    init: function cc_init(editor, name, offset) {
-        if (!name)
-            name = "";
-
+    init: function cc_init(editor, name = "", offset = 0) {
         let self = this;
         if (editor instanceof this.constructor) {
             let parent = editor;
@@ -68,7 +65,7 @@ var CompletionContext = Class("CompletionContext", {
             self.__defineGetter__("value", function get_value() this.top.value);
 
             self.offset = parent.offset;
-            self.advance(offset || 0);
+            self.advance(offset);
 
             /**
              * @property {boolean} Specifies that this context is not finished
@@ -156,7 +153,7 @@ var CompletionContext = Class("CompletionContext", {
              * @property {number} This context's offset from the beginning of
              *     {@link #editor}'s value.
              */
-            this.offset = offset || 0;
+            this.offset = offset;
             /**
              * @property {function} A function which is called when any subcontext
              *     changes its completion list. Only called when
diff --git a/common/modules/io.jsm b/common/modules/io.jsm
index 6a997350..4a61d673 100644
--- a/common/modules/io.jsm
+++ b/common/modules/io.jsm
@@ -241,11 +241,13 @@ var IO = Module("io", {
     /**
      * Sets the current working directory.
      *
-     * @param {string} newDir The new CWD. This may be a relative or
+     * @param {File|string} newDir The new CWD. This may be a relative or
      *     absolute path and is expanded by {@link #expandPath}.
+     *     @optional
+     *     @default = "~"
      */
-    set cwd(newDir) {
-        newDir = newDir && newDir.path || newDir || "~";
+    set cwd(newDir = "~") {
+        newDir = newDir.path || newDir;
 
         if (newDir == "-") {
             util.assert(this._oldcwd != null, _("io.noPrevDir"));
diff --git a/common/modules/messages.jsm b/common/modules/messages.jsm
index 5c9045a4..36fead2c 100644
--- a/common/modules/messages.jsm
+++ b/common/modules/messages.jsm
@@ -11,9 +11,9 @@ defineModule("messages", {
 
 var Messages = Module("messages", {
 
-    init: function init(name) {
+    init: function init(name = "messages") {
         let self = this;
-        this.name = name || "messages";
+        this.name = name;
 
         this._ = Class("_", String, {
             init: function _(message) {
@@ -97,10 +97,9 @@ var Messages = Module("messages", {
         let { Buffer, commands, hints, io, mappings, modes, options, sanitizer } = overlay.activeModules;
         file = io.File(file);
 
-        function properties(base, iter_, prop) iter(function _properties() {
+        function properties(base, iter_, prop = "description") iter(function _properties() {
             function key(...args) [base, obj.identifier || obj.name].concat(args).join(".").replace(/[\\:=]/g, "\\$&");
 
-            prop = prop || "description";
             for (var obj in iter_) {
                 if (!obj.hive || obj.hive.name !== "user") {
                     yield key(prop) + " = " + obj[prop];
diff --git a/common/modules/styles.jsm b/common/modules/styles.jsm
index fc466c7f..6584a7ef 100644
--- a/common/modules/styles.jsm
+++ b/common/modules/styles.jsm
@@ -382,8 +382,7 @@ var Styles = Module("Styles", {
         return val;
     },
 
-    completeSite: function (context, content, group) {
-        group = group || styles.user;
+    completeSite: function (context, content, group = styles.user) {
         context.anchored = false;
         try {
             context.fork("current", 0, this, function (context) {
diff --git a/common/modules/template.jsm b/common/modules/template.jsm
index 99e4f947..5b89f187 100644
--- a/common/modules/template.jsm
+++ b/common/modules/template.jsm
@@ -467,9 +467,7 @@ var Template = Module("Template", {
                         ["td", { style: style[i] || "" }, d])])];
     },
 
-    usage: function usage(iter, format) {
-
-        format = format || {};
+    usage: function usage(iter, format = {}) {
         let desc = format.description || (item => this.linkifyHelp(item.description));
         let help = format.help || (item => item.name);
         let sourceLink = (frame) => {