test,repl: add coverage for repl .clear+useGlobal

Add a test to cover situation where REPL is initialized with `useGlobal`
set to `true` and `.clear` is called. This adds coverage for code in
repl.js that is not currently covered.

Includes minor refactor of rocket functions in repl.js for concision.

PR-URL: https://github.com/nodejs/node/pull/10777
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/repl.js

@@ -721,9 +721,7 @@ REPLServer.prototype.createContext = function() {
 
   Object.defineProperty(context, '_', {
     configurable: true,
-    get: () => {
-      return this.last;
-    },
+    get: () => this.last,
     set: (value) => {
       this.last = value;
       if (!this.underscoreAssigned) {
@@ -1265,9 +1263,10 @@ function defineDefaultCommands(repl) {
     help: 'Print this help message',
     action: function() {
       const names = Object.keys(this.commands).sort();
-      const longestNameLength = names.reduce((max, name) => {
-        return Math.max(max, name.length);
-      }, 0);
+      const longestNameLength = names.reduce(
+        (max, name) => Math.max(max, name.length),
+        0
+      );
       names.forEach((name) => {
         const cmd = this.commands[name];
         const spaces = ' '.repeat(longestNameLength - name.length + 3);

test/parallel/test-repl-underscore.js

@@ -8,6 +8,7 @@ const stream = require('stream');
 testSloppyMode();
 testStrictMode();
 testResetContext();
+testResetContextGlobal();
 testMagicMode();
 
 function testSloppyMode() {
@@ -131,7 +132,28 @@ function testResetContext() {
   ]);
 }
 
-function initRepl(mode) {
+function testResetContextGlobal() {
+  const r = initRepl(repl.REPL_MODE_STRICT, true);
+
+  r.write(`_ = 10;     // explicitly set to 10
+          _;           // 10 from user input
+          .clear       // No output because useGlobal is true
+          _;           // remains 10
+          `);
+
+  assertOutput(r.output, [
+    'Expression assignment to _ now disabled.',
+    '10',
+    '10',
+    '10',
+  ]);
+
+  // delete globals leaked by REPL when `useGlobal` is `true`
+  delete global.module;
+  delete global.require;
+}
+
+function initRepl(mode, useGlobal) {
   const inputStream = new stream.PassThrough();
   const outputStream = new stream.PassThrough();
   outputStream.accum = '';
@@ -146,7 +168,8 @@ function initRepl(mode) {
     useColors: false,
     terminal: false,
     prompt: '',
-    replMode: mode
+    replMode: mode,
+    useGlobal: useGlobal
   });
 }