repl: don't terminate on null thrown

Previous behavior was to assume an error is a proper error in the
repl module. A check was added to not terminate the process on thrown
repl errors that are `null` or `undefined`.

PR-URL: https://github.com/nodejs/node/pull/14306
Fixes: https://github.com/nodejs/node/issues/12373
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com

lib/repl.js

@@ -283,17 +283,23 @@ function REPLServer(prompt,
   self._domain.on('error', function debugDomainError(e) {
     debug('domain error');
     const top = replMap.get(self);
+
     internalUtil.decorateErrorStack(e);
+    const isError = internalUtil.isError(e);
     if (e instanceof SyntaxError && e.stack) {
       // remove repl:line-number and stack trace
       e.stack = e.stack
                  .replace(/^repl:\d+\r?\n/, '')
                  .replace(/^\s+at\s.*\n?/gm, '');
-    } else if (e.stack && self.replMode === exports.REPL_MODE_STRICT) {
+    } else if (isError && self.replMode === exports.REPL_MODE_STRICT) {
       e.stack = e.stack.replace(/(\s+at\s+repl:)(\d+)/,
                                 (_, pre, line) => pre + (line - 1));
     }
-    top.outputStream.write((e.stack || e) + '\n');
+    if (isError && e.stack) {
+      top.outputStream.write(`${e.stack}\n`);
+    } else {
+      top.outputStream.write(`Thrown: ${String(e)}\n`);
+    }
     top.bufferedCommand = '';
     top.lines.level = [];
     top.displayPrompt();

test/parallel/test-repl-null-thrown.js

@@ -0,0 +1,24 @@
+'use strict';
+require('../common');
+const repl = require('repl');
+const assert = require('assert');
+const Stream = require('stream');
+
+const output = new Stream();
+let text = '';
+output.write = output.pause = output.resume = function(buf) {
+  text += buf.toString();
+};
+
+const replserver = repl.start({
+  output: output,
+  input: process.stdin
+});
+
+replserver.emit('line', 'process.nextTick(() => { throw null; })');
+replserver.emit('line', '.exit');
+
+setTimeout(() => {
+  console.log(text);
+  assert(text.includes('Thrown: null'));
+}, 0);