repl: Auto alignment for .editor mode

When in `.editor` mode, current line whitespace prefixes
are preserved in the subsequent line. User can hit backspace
to clean the whitespace

```js
node 🙈 ₹ node
> .editor
// Entering editor mode (^D to finish, ^C to cancel)
function test() {
  console.log('tested!'); //On enter, cursor will be after 2 spaces
  _
```

PR-URL: https://github.com/nodejs/node/pull/8241
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/readline.js

@@ -41,6 +41,7 @@ function Interface(input, output, completer, terminal) {
 
   this._sawReturn = false;
   this.isCompletionEnabled = true;
+  this._sawKeyPress = false;
   this._previousKey = null;
 
   EventEmitter.call(this);
@@ -247,6 +248,9 @@ Interface.prototype._addHistory = function() {
   // if the history is disabled then return the line
   if (this.historySize === 0) return this.line;
 
+  // if the trimmed line is empty then return the line
+  if (this.line.trim().length === 0) return this.line;
+
   if (this.history.length === 0 || this.history[0] !== this.line) {
     this.history.unshift(this.line);
 
@@ -947,6 +951,10 @@ function emitKeypressEvents(stream, iface) {
       if (r) {
         clearTimeout(timeoutId);
 
+        if (iface) {
+          iface._sawKeyPress = r.length === 1;
+        }
+
         for (var i = 0; i < r.length; i++) {
           if (r[i] === '\t' && typeof r[i + 1] === 'string' && iface) {
             iface.isCompletionEnabled = false;

lib/repl.js

@@ -471,6 +471,15 @@ function REPLServer(prompt,
 
     if (self.editorMode) {
       self.bufferedCommand += cmd + '\n';
+
+      // code alignment
+      const matches = self._sawKeyPress ? cmd.match(/^\s+/) : null;
+      if (matches) {
+        const prefix = matches[0];
+        self.inputStream.write(prefix);
+        self.line = prefix;
+        self.cursor = prefix.length;
+      }
       self.memory(cmd);
       return;
     }

test/parallel/test-repl-.editor.js

@@ -9,7 +9,7 @@ const repl = require('repl');
 // \u001b[3G - Moves the cursor to 3rd column
 const terminalCode = '\u001b[1G\u001b[0J> \u001b[3G';
 
-function run(input, output, event) {
+function run({input, output, event}) {
   const stream = new common.ArrayStream();
   let found = '';
 
@@ -49,7 +49,59 @@ const tests = [
     input: 'var i = 1;\ni + 3',
     output: '\n4',
     event: {ctrl: true, name: 'd'}
+  },
+  {
+    input: '  var i = 1;\ni + 3',
+    output: '\n4',
+    event: {ctrl: true, name: 'd'}
   }
 ];
 
-tests.forEach(({input, output, event}) => run(input, output, event));
+tests.forEach(run);
+
+// Auto code alignment for .editor mode
+function testCodeAligment({input, cursor = 0, line = ''}) {
+  const stream = new common.ArrayStream();
+
+  const replServer = repl.start({
+    prompt: '> ',
+    terminal: true,
+    input: stream,
+    output: stream,
+    useColors: false
+  });
+
+  stream.emit('data', '.editor\n');
+  input.split('').forEach((ch) => stream.emit('data', ch));
+  // Test the content of current line and the cursor position
+  assert.strictEqual(line, replServer.line);
+  assert.strictEqual(cursor, replServer.cursor);
+
+  replServer.write('', {ctrl: true, name: 'd'});
+  replServer.close();
+  // Ensure that empty lines are not saved in history
+  assert.notStrictEqual(replServer.history[0].trim(), '');
+}
+
+const codeAlignmentTests = [
+  {
+    input: 'var i = 1;\n'
+  },
+  {
+    input: '  var i = 1;\n',
+    cursor: 2,
+    line: '  '
+  },
+  {
+    input: '     var i = 1;\n',
+    cursor: 5,
+    line: '     '
+  },
+  {
+    input: ' var i = 1;\n var j = 2\n',
+    cursor: 2,
+    line: '  '
+  }
+];
+
+codeAlignmentTests.forEach(testCodeAligment);