From 0560edf39095b9f87d837956a9418a28f74cd14e Mon Sep 17 00:00:00 2001
From: cjihrig <cjihrig@gmail.com>
Date: Thu, 25 Feb 2016 12:55:01 -0500
Subject: [PATCH] repl: warn on assignment to _

The REPL uses _ to store the result of the previous expression.
This can cause confusion for REPL users who assign libraries like
Underscore and Lodash to _. The documentation already mentions
this behavior. This commit adds a printed warning if the user
assigns to _.
---
 lib/repl.js                            | 19 ++++++++++--
 test/parallel/test-repl-last-result.js | 41 ++++++++++++++++++++++++++
 2 files changed, 58 insertions(+), 2 deletions(-)
 create mode 100644 test/parallel/test-repl-last-result.js

diff --git a/lib/repl.js b/lib/repl.js
index 7f913c0801738e..a97c346ac47b87 100644
--- a/lib/repl.js
+++ b/lib/repl.js
@@ -205,6 +205,7 @@ function REPLServer(prompt,
   self.useGlobal = !!useGlobal;
   self.ignoreUndefined = !!ignoreUndefined;
   self.replMode = replMode || exports.REPL_MODE_SLOPPY;
+  self._last = undefined;
 
   self._inTemplateLiteral = false;
 
@@ -467,7 +468,7 @@ function REPLServer(prompt,
           // the second argument to this function is there, print it.
           arguments.length === 2 &&
           (!self.ignoreUndefined || ret !== undefined)) {
-        self.context._ = ret;
+        self._last = ret;
         self.outputStream.write(self.writer(ret) + '\n');
       }
 
@@ -544,13 +545,15 @@ REPLServer.prototype.createContext = function() {
   this.lines = [];
   this.lines.level = [];
 
+  const self = this;
+
   // make built-in modules available directly
   // (loaded lazily)
   exports._builtinLibs.forEach(function(name) {
     Object.defineProperty(context, name, {
       get: function() {
         var lib = require(name);
-        context._ = context[name] = lib;
+        self._last = context[name] = lib;
         return lib;
       },
       // allow the creation of other globals with this name
@@ -562,11 +565,23 @@ REPLServer.prototype.createContext = function() {
     });
   });
 
+  Object.defineProperty(context, '_', {
+    configurable: true,
+    get: function() {
+      return self._last;
+    },
+    set: function(val) {
+      self._last = val;
+      self.outputStream.write('assignment to _ is reserved by the REPL\n');
+    }
+  });
+
   return context;
 };
 
 REPLServer.prototype.resetContext = function() {
   this.context = this.createContext();
+  this._last = undefined;
 
   // Allow REPL extensions to extend the new context
   this.emit('reset', this.context);
diff --git a/test/parallel/test-repl-last-result.js b/test/parallel/test-repl-last-result.js
new file mode 100644
index 00000000000000..3ffd7ec76fce1c
--- /dev/null
+++ b/test/parallel/test-repl-last-result.js
@@ -0,0 +1,41 @@
+'use strict';
+
+require('../common');
+const assert = require('assert');
+const repl = require('repl');
+const stream = require('stream');
+const inputStream = new stream.PassThrough();
+const outputStream = new stream.PassThrough();
+let output = '';
+
+outputStream.on('data', (data) => {
+  output += data;
+});
+
+process.on('exit', () => {
+  const lines = output.trim().split('\n');
+
+  assert.deepStrictEqual(lines, [
+    'undefined',
+    'assignment to _ is reserved by the REPL',
+    '42',
+    '42',
+    '85',
+    '85',
+    'undefined'
+  ]);
+});
+
+const r = repl.start({
+  prompt: '',
+  input: inputStream,
+  output: outputStream
+});
+
+r.write('_;\n');
+r.write('_ = 42;\n');
+r.write('_;\n');
+r.write('foo = 85;\n');
+r.write('_;\n');
+r.resetContext();
+r.write('_;\n');