debugger: run last command on presssing enter

Fixes: nodejs#2895

doc/api/debugger.markdown

@@ -84,6 +84,9 @@ The `repl` command allows code to be evaluated remotely. The `next` command
 steps over to the next line. Type `help` to see what other commands are
 available.
 
+Pressing `enter` without typing a command will repeat the previous debugger
+command.
+
 ## Watchers
 
 It is possible to watch expression and variable values while debugging. On

lib/_debugger.js

@@ -674,6 +674,9 @@ var helpMessage = 'Commands: ' + commands.map(function(group) {
   return group.join(', ');
 }).join(',\n');
 
+// Previous command received. Initialize to empty command.
+var lastCommand = '\n';
+
 
 function SourceUnderline(sourceText, position, repl) {
   if (!sourceText) return '';
@@ -945,10 +948,10 @@ Interface.prototype.requireConnection = function() {
 Interface.prototype.controlEval = function(code, context, filename, callback) {
   try {
     // Repeat last command if empty line are going to be evaluated
-    if (this.repl.rli.history && this.repl.rli.history.length > 0) {
-      if (code === '\n') {
-        code = this.repl.rli.history[0] + '\n';
-      }
+    if (code === '\n') {
+      code = lastCommand;
+    } else {
+      lastCommand = code;
     }
 
     // exec process.title => exec("process.title");

lib/repl.js

@@ -421,7 +421,10 @@ function REPLServer(prompt,
       }
     }
 
-    if (cmd || self.bufferedCommand) {
+    // self.context.setBreakpoint exists we are in the debugger.
+    // If we are in the debugger, then we should process empty lines as they are
+    // a shortcut for "repeat the previous debugging command".
+    if (cmd || self.bufferedCommand || self.context.setBreakpoint) {
       var evalCmd = self.bufferedCommand + cmd;
       if (/^\s*\{/.test(evalCmd) && /\}\s*$/.test(evalCmd)) {
         // It's confusing for `{ a : 1 }` to be interpreted as a block

test/fixtures/debugger-repeat-last.js

@@ -0,0 +1,7 @@
+var a = 1;
+
+var b = 2;
+
+var c = 3;
+
+b = c;

test/parallel/test-debugger-repeat-last.js

@@ -0,0 +1,46 @@
+'use strict';
+const path = require('path');
+const spawn = require('child_process').spawn;
+const assert = require('assert');
+
+const common = require('../common');
+
+const fixture = path.join(
+  common.fixturesDir,
+  'debugger-repeat-last.js'
+);
+
+const args = [
+  'debug',
+  fixture
+];
+
+const proc = spawn(process.execPath, args, { stdio: 'pipe' });
+proc.stdout.setEncoding('utf8');
+
+var stdout = '';
+
+var sentCommand = false;
+var sentEmpty = false;
+var sentExit = false;
+
+proc.stdout.on('data', (data) => {
+  stdout += data;
+  if (!sentCommand && stdout.includes('> 1')) {
+    setImmediate(() => {proc.stdin.write('n\n');});
+    return sentCommand = true;
+  }
+  if (!sentEmpty && stdout.includes('> 3')) {
+    setImmediate(() => {proc.stdin.write('\n');});
+    return sentEmpty = true;
+  }
+  if (!sentExit && sentCommand && sentEmpty) {
+    setTimeout(() => {proc.stdin.write('\n\n\n.exit\n\n\n');}, 1);
+    return sentExit = true;
+  }
+});
+
+process.on('exit', (exitCode) => {
+  assert.strictEqual(exitCode, 0);
+  console.log(stdout);
+});