repl: assign underscore fix

doc/api/repl.markdown

@@ -86,6 +86,10 @@ The special variable `_` (underscore) contains the result of the last expression
 4
 ```
 
+*NOTE*: Explicitly assigning a value to `_` in the REPL can produce unexpected
+results. Also, attempting to create `_` as a `const` variable in the REPL will
+fail.
+
 The REPL provides access to any variables in the global scope. You can expose
 a variable to the REPL explicitly by assigning it to the `context` object
 associated with each `REPLServer`.  For example:

lib/repl.js

@@ -562,6 +562,12 @@ REPLServer.prototype.createContext = function() {
     });
   });
 
+  // Refer: https://github.com/nodejs/node/pull/3729#issuecomment-155460861
+  // The REPL stores the result of the last evaluated expression in context's _.
+  // But, in the REPL, if _ is defined as a const, then it will break the REPL.
+  // So, we define _ first, so that later redefiniitions will fail.
+  context._ = undefined;
+
   return context;
 };
 

test/parallel/test-repl.js

@@ -94,6 +94,14 @@ function error_test() {
   });
 
   send_expect([
+    // Ref: https://github.com/nodejs/node/pull/3729#issuecomment-155460861
+    // REPL stores the result of the last evaluated expression in _.
+    // This test makes sure that _ can not be redefined in REPL.
+    { client: client_unix, send: 'const _ = 1',
+      expect: /^TypeError: Identifier '_' has already been declared/ },
+    // `_` should still be assignable
+    { client: client_unix, send: '_ = 1\n_',
+      expect: `1\n${prompt_unix}1\n${prompt_unix}` },
     // Uncaught error throws and prints out
     { client: client_unix, send: 'throw new Error(\'test error\');',
       expect: /^Error: test error/ },