Update how REPLServer uses contexts

* Always use `this.context` or `self.context`.
* Move `resetContext` to `REPLServer.createContext`.
* Add `REPLServer.resetContext`, memoize `context` here.
* Memoize `exports.repl` in `start`.

Closes GH-851.

lib/repl.js

@@ -60,27 +60,14 @@ module.filename = process.cwd() + '/repl';
 // hack for repl require to work properly with node_modules folders
 module.paths = require('module')._nodeModulePaths(module.filename);
 
-
-function resetContext() {
-  context = vm.createContext();
-  for (var i in global) context[i] = global[i];
-  context.module = module;
-  context.require = require;
-  context.global = context;
-  context.global.global = context;
-  for (var i in require.cache) delete require.cache[i];
-}
-
-
 // Can overridden with custom print functions, such as `probe` or `eyes.js`
 exports.writer = util.inspect;
 
 
 function REPLServer(prompt, stream) {
   var self = this;
-  if (!context) resetContext();
-  if (!exports.repl) exports.repl = this;
-  self.context = context;
+
+  self.resetContext();
   self.bufferedCommand = '';
 
   if (stream) {
@@ -159,7 +146,7 @@ function REPLServer(prompt, stream) {
             // First we attempt to eval as expression with parens.
             // This catches '{a : 1}' properly.
             ret = vm.runInContext('(' + self.bufferedCommand + ')',
-                                  context,
+                                  self.context,
                                   'repl');
             if (typeof ret !== 'function') success = true;
           } catch (e) {
@@ -168,11 +155,11 @@ function REPLServer(prompt, stream) {
 
           if (!success) {
             // Now as statement without parens.
-            ret = vm.runInContext(self.bufferedCommand, context, 'repl');
+            ret = vm.runInContext(self.bufferedCommand, self.context, 'repl');
           }
 
           if (ret !== undefined) {
-            context._ = ret;
+            self.context._ = ret;
             self.outputStream.write(exports.writer(ret) + '\n');
           }
 
@@ -215,10 +202,33 @@ exports.REPLServer = REPLServer;
 // prompt is a string to print on each line for the prompt,
 // source is a stream to use for I/O, defaulting to stdin/stdout.
 exports.start = function(prompt, source) {
-  return new REPLServer(prompt, source);
+  var repl = new REPLServer(prompt, source);
+  if (!exports.repl) exports.repl = repl;
+  return repl;
 };
 
 
+REPLServer.prototype.createContext = function() {
+  var context = vm.createContext();
+
+  for (var i in global) context[i] = global[i];
+  context.module = module;
+  context.require = require;
+  context.global = context;
+  context.global.global = context;
+
+  return context;
+};
+
+REPLServer.prototype.resetContext = function(force) {
+  if (!context || force) {
+    context = this.createContext();
+    for (var i in require.cache) delete require.cache[i];
+  }
+
+  this.context = context;
+};
+
 REPLServer.prototype.displayPrompt = function() {
   this.rli.setPrompt(this.bufferedCommand.length ? '... ' : this.prompt);
   this.rli.prompt();
@@ -503,7 +513,7 @@ function defineDefaultCommands(repl) {
     action: function() {
       this.outputStream.write('Clearing context...\n');
       this.bufferedCommand = '';
-      resetContext();
+      this.resetContext(true);
       this.displayPrompt();
     }
   });