repl: Add editor mode support

```js
> node
> .editor
// Entering editor mode (^D to finish, ^C to cancel)
function test() {
  console.log('tested!');
}

test();

// ^D
tested!
undefined
>
```

doc/api/repl.md

@@ -38,6 +38,7 @@ The following special commands are supported by all REPL instances:
   `> .save ./file/to/save.js`
 * `.load` - Load a file into the current REPL session.
   `> .load ./file/to/load.js`
+* `.editor` - Enter editor mode (`<ctrl>-D` to finish, `<ctrl>-C` to cancel)
 
 The following key combinations in the REPL have these special effects:
 

lib/repl.js

@@ -223,6 +223,7 @@ function REPLServer(prompt,
   self.underscoreAssigned = false;
   self.last = undefined;
   self.breakEvalOnSigint = !!breakEvalOnSigint;
+  self.editorMode = false;
 
   self._inTemplateLiteral = false;
 
@@ -394,7 +395,12 @@ function REPLServer(prompt,
   // Figure out which "complete" function to use.
   self.completer = (typeof options.completer === 'function')
     ? options.completer
-    : complete;
+    : completer;
+
+  function completer(text, cb) {
+    complete.call(self, text, self.editorMode
+      ? self.completeOnEditorMode(cb) : cb);
+  }
 
   Interface.call(this, {
     input: self.inputStream,
@@ -428,9 +434,11 @@ function REPLServer(prompt,
   });
 
   var sawSIGINT = false;
+  var sawCtrlD = false;
   self.on('SIGINT', function() {
     var empty = self.line.length === 0;
     self.clearLine();
+    self.turnOffEditorMode();
 
     if (!(self.bufferedCommand && self.bufferedCommand.length > 0) && empty) {
       if (sawSIGINT) {
@@ -454,6 +462,11 @@ function REPLServer(prompt,
     debug('line %j', cmd);
     sawSIGINT = false;
 
+    if (self.editorMode) {
+      self.bufferedCommand += cmd + '\n';
+      return;
+    }
+
     // leading whitespaces in template literals should not be trimmed.
     if (self._inTemplateLiteral) {
       self._inTemplateLiteral = false;
@@ -499,7 +512,8 @@ function REPLServer(prompt,
 
       // If error was SyntaxError and not JSON.parse error
       if (e) {
-        if (e instanceof Recoverable && !self.lineParser.shouldFail) {
+        if (e instanceof Recoverable && !self.lineParser.shouldFail &&
+            !sawCtrlD) {
           // Start buffering data like that:
           // {
           // ...  x: 1
@@ -515,6 +529,7 @@ function REPLServer(prompt,
       // Clear buffer if no SyntaxErrors
       self.lineParser.reset();
       self.bufferedCommand = '';
+      sawCtrlD = false;
 
       // If we got any output - print it (if no error)
       if (!e &&
@@ -555,9 +570,50 @@ function REPLServer(prompt,
   });
 
   self.on('SIGCONT', function() {
-    self.displayPrompt(true);
+    if (self.editorMode) {
+      self.outputStream.write(`${self._initialPrompt}.editor\n`);
+      self.outputStream.write(
+        '// Entering editor mode (^D to finish, ^C to cancel)\n');
+      self.outputStream.write(`${self.bufferedCommand}\n`);
+      self.prompt(true);
+    } else {
+      self.displayPrompt(true);
+    }
   });
 
+  // Wrap readline tty to enable editor mode
+  const ttyWrite = self._ttyWrite.bind(self);
+  self._ttyWrite = (d, key) => {
+    if (!self.editorMode || !self.terminal) {
+      ttyWrite(d, key);
+      return;
+    }
+
+    // editor mode
+    if (key.ctrl && !key.shift) {
+      switch (key.name) {
+        case 'd': // End editor mode
+          self.turnOffEditorMode();
+          sawCtrlD = true;
+          ttyWrite(d, { name: 'return' });
+          break;
+        case 'n': // Override next history item
+        case 'p': // Override previous history item
+          break;
+        default:
+          ttyWrite(d, key);
+      }
+    } else {
+      switch (key.name) {
+        case 'up':   // Override previous history item
+        case 'down': // Override next history item
+          break;
+        default:
+          ttyWrite(d, key);
+      }
+    }
+  };
+
   self.displayPrompt();
 }
 inherits(REPLServer, Interface);
@@ -680,6 +736,12 @@ REPLServer.prototype.setPrompt = function setPrompt(prompt) {
   REPLServer.super_.prototype.setPrompt.call(this, prompt);
 };
 
+REPLServer.prototype.turnOffEditorMode = function() {
+  this.editorMode = false;
+  this.setPrompt(this._initialPrompt);
+};
+
+
 // A stream to push an array into a REPL
 // used in REPLServer.complete
 function ArrayStream() {
@@ -987,6 +1049,39 @@ function complete(line, callback) {
   }
 }
 
+REPLServer.prototype.completeOnEditorMode = (callback) => (err, results) => {
+  if (err) return callback(err);
+
+  const [completions, completeOn] = results;
+  const prefixLength = completeOn.length;
+
+  if (prefixLength === 0) return callback(null, [[], completeOn]);
+
+  const isNotEmpty = (v) => v.length > 0;
+  const trimCompleteOnPrefix = (v) => v.substring(prefixLength);
+  const data = completions.filter(isNotEmpty).map(trimCompleteOnPrefix);
+
+  callback(null, [[`${completeOn}${longestCommonPrefix(data)}`], completeOn]);
+
+  function longestCommonPrefix(arr = []) {
+    const cnt = arr.length;
+    if (cnt === 0) return '';
+    if (cnt === 1) return arr[0];
+
+    const first = arr[0];
+    // complexity: O(m * n)
+    for (let m = 0; m < first.length; m++) {
+      const c = first[m];
+      for (let n = 1; n < cnt; n++) {
+        const entry = arr[n];
+        if (m >= entry.length || c !== entry[m]) {
+          return first.substring(0, m);
+        }
+      }
+    }
+    return first;
+  }
+};
 
 /**
  * Used to parse and execute the Node REPL commands.
@@ -1189,6 +1284,17 @@ function defineDefaultCommands(repl) {
       this.displayPrompt();
     }
   });
+
+  repl.defineCommand('editor', {
+    help: 'Entering editor mode (^D to finish, ^C to cancel)',
+    action() {
+      if (!this.terminal) return;
+      this.editorMode = true;
+      REPLServer.super_.prototype.setPrompt.call(this, '');
+      this.outputStream.write(
+        '// Entering editor mode (^D to finish, ^C to cancel)\n');
+    }
+  });
 }
 
 function regexpEscape(s) {

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

@@ -0,0 +1,55 @@
+'use strict';
+
+const common = require('../common');
+const assert = require('assert');
+const repl = require('repl');
+
+// \u001b[1G - Moves the cursor to 1st column
+// \u001b[0J - Clear screen
+// \u001b[3G - Moves the cursor to 3rd column
+const terminalCode = '\u001b[1G\u001b[0J> \u001b[3G';
+
+function run(input, output, event) {
+  const stream = new common.ArrayStream();
+  let found = '';
+
+  stream.write = (msg) => found += msg.replace('\r', '');
+
+  const expected = `${terminalCode}.editor\n` +
+                   '// Entering editor mode (^D to finish, ^C to cancel)\n' +
+                   `${input}${output}\n${terminalCode}`;
+
+  const replServer = repl.start({
+    prompt: '> ',
+    terminal: true,
+    input: stream,
+    output: stream,
+    useColors: false
+  });
+
+  stream.emit('data', '.editor\n');
+  stream.emit('data', input);
+  replServer.write('', event);
+  replServer.close();
+  assert(found === expected, `Expected: ${expected}, Found: ${found}`);
+}
+
+const tests = [
+  {
+    input: '',
+    output: '\n(To exit, press ^C again or type .exit)',
+    event: {ctrl: true, name: 'c'}
+  },
+  {
+    input: 'var i = 1;',
+    output: '',
+    event: {ctrl: true, name: 'c'}
+  },
+  {
+    input: 'var i = 1;\ni + 3',
+    output: '\n4',
+    event: {ctrl: true, name: 'd'}
+  }
+];
+
+tests.forEach(({input, output, event}) => run(input, output, event));

test/parallel/test-repl-tab-complete.js

@@ -348,3 +348,25 @@ testCustomCompleterAsyncMode.complete('a', common.mustCall((error, data) => {
     'a'
   ]);
 }));
+
+// tab completion in editor mode
+const editorStream = new common.ArrayStream();
+const editor = repl.start({
+  stream: editorStream,
+  terminal: true,
+  useColors: false
+});
+
+editorStream.run(['.clear']);
+editorStream.run(['.editor']);
+
+editor.completer('co', common.mustCall((error, data) => {
+  assert.deepStrictEqual(data, [['con'], 'co']);
+}));
+
+editorStream.run(['.clear']);
+editorStream.run(['.editor']);
+
+editor.completer('var log = console.l', common.mustCall((error, data) => {
+  assert.deepStrictEqual(data, [['console.log'], 'console.l']);
+}));