util: inspect arguments properly

Right now it is not possible to distinguish arguments from a regular
object. This adds a arguments indicator.

PR-URL: #19467
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/util.js

@@ -49,6 +49,7 @@ const types = internalBinding('types');
 Object.assign(types, require('internal/util/types'));
 const {
   isAnyArrayBuffer,
+  isArgumentsObject,
   isDataView,
   isExternal,
   isMap,
@@ -538,9 +539,13 @@ function formatValue(ctx, value, recurseTimes, ln) {
   if (noIterator) {
     braces = ['{', '}'];
     if (prefix === 'Object ') {
-      // Object fast path
-      if (keyLength === 0)
+      if (isArgumentsObject(value)) {
+        braces[0] = '[Arguments] {';
+        if (keyLength === 0)
+          return '[Arguments] {}';
+      } else if (keyLength === 0) {
         return '{}';
+      }
     } else if (typeof value === 'function') {
       const name =
         `${constructor || tag}${value.name ? `: ${value.name}` : ''}`;

test/parallel/test-util-inspect.js

@@ -1399,3 +1399,8 @@ util.inspect(process);
                     'extra: true }';
   assert(out === expect || out === expectAlt);
 }
+
+{ // Test argument objects.
+  const args = (function() { return arguments; })('a');
+  assert.strictEqual(util.inspect(args), "[Arguments] { '0': 'a' }");
+}