diff --git a/common/content/dactyl.js b/common/content/dactyl.js
index c3eee956..763e07d5 100644
--- a/common/content/dactyl.js
+++ b/common/content/dactyl.js
@@ -402,7 +402,7 @@ var Dactyl = Module("dactyl", XPCOM(Ci.nsISupportsWeakReference, ModuleBase), {
             if (info && info.file[0] !== "[")
                 ({ file: fileName, line: lineNumber, context: ctxt }) = info;
 
-        if (!context)
+        if (!context && fileName && fileName[0] !== "[")
             context = _userContext || ctxt;
 
         if (isinstance(context, ["Sandbox"]))
diff --git a/common/locale/en-US/eval.xml b/common/locale/en-US/eval.xml
index 40b059bc..a29e8f62 100644
--- a/common/locale/en-US/eval.xml
+++ b/common/locale/en-US/eval.xml
@@ -90,7 +90,6 @@
     <tags>:js :javas :javascript</tags>
     <spec>:javas<oa>cript</oa> <a>cmd</a></spec>
     <spec>:javascript &lt;&lt;<a>endpattern</a>\n<a>cmd</a>\n<a>endpattern</a></spec>
-    <spec>:javascript<oa>!</oa></spec>
     <description>
         <p>
             Evaluates the given <a>cmd</a> as JavaScript. Behaves exactly as
@@ -137,6 +136,32 @@
     </description>
 </item>
 
+<item>
+    <tags>REPL</tags>
+    <spec>:javascript<oa>!</oa> <oa>context</oa></spec>
+    <description>
+        <p>
+            Starts the JavaScript Read Eval Print Loop, where JavaScript
+            statements are entered and evaluated, their results printed, and the
+            input modified and entered again. Within the REPL, the results of a
+            given evaluation are available as variables named for the given
+            prompt.
+        </p>
+
+        <p>
+            If <oa>context</oa> is given, then statements are executed in that
+            global context.
+        </p>
+
+        <example><code><hl key="REPL">
+<hl key="REPL-E"><hl key="REPL-R">js1></hl> ({ <hl key="Key">foo</hl>: <str>bar</str> })</hl
+><hl key="REPL-P" style="display: block;"><hl key="Title Object">[object Object]</hl>::
+<hl key="Key">foo</hl>: <hl key="String">"bar"</hl></hl
+><hl key="REPL-E"><hl key="REPL-R">js2></hl> js1.foo</hl
+><hl key="REPL-P" style="display: block;"><str>bar</str></hl></hl></code></example>
+    </description>
+</item>
+
 <h2 tag="global-variables">Global Variables</h2>
 
 <item>
diff --git a/common/modules/javascript.jsm b/common/modules/javascript.jsm
index e8318182..b6abdb4d 100644
--- a/common/modules/javascript.jsm
+++ b/common/modules/javascript.jsm
@@ -12,6 +12,8 @@ defineModule("javascript", {
     use: ["services", "template", "util"]
 }, this);
 
+let isPrototypeOf = Object.prototype.isPrototypeOf;
+
 // TODO: Clean this up.
 
 var JavaScript = Module("javascript", {
@@ -40,6 +42,14 @@ var JavaScript = Module("javascript", {
         },
     }),
 
+    globals: Class.memoize(function () [
+       [this.modules.userContext, "Global Variables"],
+       [this.modules, "modules"],
+       [this.window, "window"]
+    ]),
+
+    toplevel: Class.memoize(function () this.modules.jsmodules),
+
     lazyInit: true,
 
     newContext: function () this.modules.newContext(this.modules.userContext),
@@ -83,7 +93,7 @@ var JavaScript = Module("javascript", {
             return [];
         if (isinstance(obj, ["Sandbox"]) && !toplevel) // Temporary hack.
             return [];
-        if (this.modules.jsmodules.isPrototypeOf(obj) && !toplevel)
+        if (isPrototypeOf.call(this.toplevel, obj) && !toplevel)
             return [];
 
         let completions = [k for (k in this.iter(obj, toplevel))];
@@ -310,11 +320,7 @@ var JavaScript = Module("javascript", {
         let end = (frame == -1 ? this._lastIdx : this._get(frame + 1).offset);
 
         this._cacheKey = null;
-        let obj = [[this.cache.evalContext, "Local Variables"],
-                   [this.replContext, "REPL Variables"],
-                   [this.modules.userContext, "Global Variables"],
-                   [this.modules, "modules"],
-                   [this.window, "window"]]; // Default objects;
+        let obj = [[this.cache.evalContext, "Local Variables"]].concat(this.globals);
         // Is this an object dereference?
         if (dot < statement) // No.
             dot = statement - 1;
@@ -751,15 +757,34 @@ var JavaScript = Module("javascript", {
         });
 
         modules.CommandREPLMode = Class("CommandREPLMode", modules.CommandMode, {
-            open: function open() {
-                let self = this;
+            init: function init(context) {
+                init.supercall(this);
 
-                this.context = modules.newContext(modules.userContext, true);
+                let self = this;
+                let sandbox = isinstance(context, ["Sandbox"]);
+
+                this.context = modules.newContext(context, !sandbox);
                 this.js = modules.JavaScript();
                 this.js.replContext = this.context;
-                this.js.newContext = function newContext() modules.newContext(self.context, true);
-                this.repl = REPL(this.context);
+                this.js.newContext = function newContext() modules.newContext(self.context, !sandbox);
 
+                this.js.globals = [
+                   [this.context, "REPL Variables"],
+                   [context, "REPL Global"]
+                ].concat(this.js.globals.filter(function ([global]) isPrototypeOf.call(global, context)));
+
+                if (!isPrototypeOf.call(modules.jsmodules, context))
+                    this.js.toplevel = context;
+
+                if (!isPrototypeOf.call(window, context))
+                    this.js.window = context;
+
+                if (this.js.globals.slice(2).some(function ([global]) global === context))
+                    this.js.globals.splice(1);
+
+                this.repl = REPL(this.context);
+            },
+            open: function open(context) {
                 this.updatePrompt();
 
                 modules.mow.echo(this.repl);
@@ -803,14 +828,14 @@ var JavaScript = Module("javascript", {
         commands.add(["javas[cript]", "js"],
             "Evaluate a JavaScript string",
             function (args) {
-                if (args.bang) // open JavaScript console
-                    dactyl.open("chrome://global/content/console.xul",
-                                { from: "javascript" });
-                else if (args[0])
+                modules.commandline;
+
+                if (args[0] && !args.bang)
                     dactyl.userEval(args[0]);
                 else {
                     modules.commandline;
-                    modules.CommandREPLMode().open();
+                    modules.CommandREPLMode(args[0] ? dactyl.userEval(args[0]) : modules.userContext)
+                           .open();
                 }
             }, {
                 bang: true,
diff --git a/pentadactyl/NEWS b/pentadactyl/NEWS
index b5bfedd0..e8d2ca69 100644
--- a/pentadactyl/NEWS
+++ b/pentadactyl/NEWS
@@ -86,6 +86,7 @@
         and linking to source code locations). [b4]
       - :downloads now opens a download list in the multi-line output
         buffer. [b6]
+      - :javascript! now opens a Read Eval Print Loop. [b6]
       - Added -arg flag to :map. [b6]
       - :extensions has been replaced with a more powerful :addons.
       - Added -literal flag to :command. [b6]