LibJS: Implement spec-compliant Object.prototype.toString

Author: https://github.com/mattco98
Commit: https://github.com/SerenityOS/serenity/commit/5ecd504f4e1
Pull-request: https://github.com/SerenityOS/serenity/pull/2765

Libraries/LibJS/Runtime/ObjectPrototype.cpp

@@ -68,10 +68,43 @@ JS_DEFINE_NATIVE_FUNCTION(ObjectPrototype::has_own_property)
 
 JS_DEFINE_NATIVE_FUNCTION(ObjectPrototype::to_string)
 {
-    auto* this_object = interpreter.this_value(global_object).to_object(interpreter, global_object);
+    auto this_value = interpreter.this_value(global_object);
+
+    if (this_value.is_undefined())
+        return js_string(interpreter, "[object Undefined]");
+    if (this_value.is_null())
+        return js_string(interpreter, "[object Null]");
+
+    auto* this_object = this_value.to_object(interpreter, global_object);
     if (!this_object)
         return {};
-    return js_string(interpreter, String::format("[object %s]", this_object->class_name()));
+
+    String tag;
+    auto to_string_tag = this_object->get(interpreter.well_known_symbol_to_string_tag());
+    
+    if (to_string_tag.is_string()) {
+        tag = to_string_tag.as_string().string();
+    } else if (this_object->is_array()) {
+        tag = "Array";
+    } else if (this_object->is_function()) {
+        tag = "Function";
+    } else if (this_object->is_error()) {
+        tag = "Error";
+    } else if (this_object->is_boolean_object()) {
+        tag = "Boolean";
+    } else if (this_object->is_number_object()) {
+        tag = "Number";
+    } else if (this_object->is_string_object()) {
+        tag = "String";
+    } else if (this_object->is_date()) {
+        tag = "Date";
+    } else if (this_object->is_regexp_object()) {
+        tag = "RegExp";
+    } else {
+        tag = "Object";
+    }
+
+    return js_string(interpreter, String::format("[object %s]", tag.characters()));
 }
 
 JS_DEFINE_NATIVE_FUNCTION(ObjectPrototype::to_locale_string)

Libraries/LibJS/Runtime/ProxyObject.h

@@ -64,8 +64,8 @@ public:
 private:
     virtual void visit_children(Visitor&) override;
     virtual bool is_proxy_object() const override { return true; }
-    virtual bool is_function() const override { return m_target.is_function(); }
 
+    virtual bool is_function() const override { return m_target.is_function(); }
     virtual bool is_array() const override { return m_target.is_array(); };
 
     Object& m_target;

Libraries/LibJS/Tests/builtins/Array/Array.prototype.toLocaleString.js

@@ -16,9 +16,7 @@ describe("normal behavior", () => {
     });
 
     test("number stringification differs from regular toString, for now", () => {
-        expect([1, 2, 3].toLocaleString()).toBe(
-            "[object NumberObject],[object NumberObject],[object NumberObject]"
-        );
+        expect([1, 2, 3].toLocaleString()).toBe("[object Number],[object Number],[object Number]");
     });
 
     test("null and undefined result in empty strings", () => {

Libraries/LibJS/Tests/builtins/Object/Object.prototype.toString.js

@@ -1,8 +1,18 @@
-test("basic functionality", () => {
+test("length", () => {
     expect(Object.prototype.toString).toHaveLength(0);
-    // FIXME: The tag is ObjectPrototype, but should be Object
-    // expect(Object.prototype.toString()).toBe("[object Object]");
-    expect({ foo: 1 }.toString()).toBe("[object Object]");
-    expect([].toString()).toBe("");
-    expect(Object.prototype.toString.call([])).toBe("[object Array]");
+});
+
+test("result for various object types", () => {
+    const oToString = o => Object.prototype.toString.call(o);
+
+    expect(oToString(undefined)).toBe("[object Undefined]");
+    expect(oToString(null)).toBe("[object Null]");
+    expect(oToString([])).toBe("[object Array]");
+    expect(oToString(function () {})).toBe("[object Function]");
+    expect(oToString(new Error())).toBe("[object Error]");
+    expect(oToString(new Boolean())).toBe("[object Boolean]");
+    expect(oToString(new Number())).toBe("[object Number]");
+    expect(oToString(new Date())).toBe("[object Date]");
+    expect(oToString(new RegExp())).toBe("[object RegExp]");
+    expect(oToString({})).toBe("[object Object]");
 });