js: Add REPL pretty-printing handler for Promises

Userland/Utilities/js.cpp

@@ -1,6 +1,6 @@
 /*
  * Copyright (c) 2020, Andreas Kling <kling@serenityos.org>
- * Copyright (c) 2020, Linus Groh <mail@linusgroh.de>
+ * Copyright (c) 2020-2021, Linus Groh <mail@linusgroh.de>
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
@@ -47,6 +47,7 @@
 #include <LibJS/Runtime/NumberObject.h>
 #include <LibJS/Runtime/Object.h>
 #include <LibJS/Runtime/PrimitiveString.h>
+#include <LibJS/Runtime/Promise.h>
 #include <LibJS/Runtime/ProxyObject.h>
 #include <LibJS/Runtime/RegExpObject.h>
 #include <LibJS/Runtime/ScriptFunction.h>
@@ -271,6 +272,32 @@ static void print_proxy_object(const JS::Object& object, HashTable<JS::Object*>&
     print_value(&proxy_object.handler(), seen_objects);
 }
 
+static void print_promise(const JS::Object& object, HashTable<JS::Object*>& seen_objects)
+{
+    auto& promise = static_cast<const JS::Promise&>(object);
+    print_type("Promise");
+    switch (promise.state()) {
+    case JS::Promise::State::Pending:
+        out("\n  state: ");
+        out("\033[36;1mPending\033[0m");
+        break;
+    case JS::Promise::State::Fulfilled:
+        out("\n  state: ");
+        out("\033[32;1mFulfilled\033[0m");
+        out("\n  result: ");
+        print_value(promise.result(), seen_objects);
+        break;
+    case JS::Promise::State::Rejected:
+        out("\n  state: ");
+        out("\033[31;1mRejected\033[0m");
+        out("\n  result: ");
+        print_value(promise.result(), seen_objects);
+        break;
+    default:
+        VERIFY_NOT_REACHED();
+    }
+}
+
 static void print_array_buffer(const JS::Object& object, HashTable<JS::Object*>& seen_objects)
 {
     auto& array_buffer = static_cast<const JS::ArrayBuffer&>(object);
@@ -367,6 +394,8 @@ static void print_value(JS::Value value, HashTable<JS::Object*>& seen_objects)
             return print_regexp_object(object, seen_objects);
         if (is<JS::ProxyObject>(object))
             return print_proxy_object(object, seen_objects);
+        if (is<JS::Promise>(object))
+            return print_promise(object, seen_objects);
         if (is<JS::ArrayBuffer>(object))
             return print_array_buffer(object, seen_objects);
         if (object.is_typed_array())