LibJS: Implement Symbol.toStringTag

Author: https://github.com/mattco98
Commit: https://github.com/SerenityOS/serenity/commit/43d955014d3
Pull-request: https://github.com/SerenityOS/serenity/pull/2765

Libraries/LibJS/Runtime/ArrayIteratorPrototype.cpp

@@ -42,6 +42,8 @@ void ArrayIteratorPrototype::initialize(Interpreter& interpreter, GlobalObject&
 {
     Object::initialize(interpreter, global_object);
     define_native_function("next", next, 0, Attribute::Writable | Attribute::Configurable);
+
+    define_property(interpreter.well_known_symbol_to_string_tag(), js_string(interpreter, "Array Iterator"), Attribute::Configurable);
 }
 
 ArrayIteratorPrototype::~ArrayIteratorPrototype()

Libraries/LibJS/Runtime/BigIntPrototype.cpp

@@ -44,6 +44,8 @@ void BigIntPrototype::initialize(Interpreter& interpreter, GlobalObject& global_
     define_native_function("toString", to_string, 0, attr);
     define_native_function("toLocaleString", to_locale_string, 0, attr);
     define_native_function("valueOf", value_of, 0, attr);
+
+    define_property(interpreter.well_known_symbol_to_string_tag(), js_string(interpreter, "BigInt"), Attribute::Configurable);
 }
 
 BigIntPrototype::~BigIntPrototype()

Libraries/LibJS/Runtime/JSONObject.cpp

@@ -48,6 +48,8 @@ void JSONObject::initialize(Interpreter& interpreter, GlobalObject& global_objec
     u8 attr = Attribute::Writable | Attribute::Configurable;
     define_native_function("stringify", stringify, 3, attr);
     define_native_function("parse", parse, 2, attr);
+
+    define_property(interpreter.well_known_symbol_to_string_tag(), js_string(interpreter, "JSON"), Attribute::Configurable);
 }
 
 JSONObject::~JSONObject()

Libraries/LibJS/Runtime/MathObject.cpp

@@ -74,6 +74,8 @@ void MathObject::initialize(Interpreter& interpreter, GlobalObject& global_objec
     define_property("PI", Value(M_PI), 0);
     define_property("SQRT1_2", Value(M_SQRT1_2), 0);
     define_property("SQRT2", Value(M_SQRT2), 0);
+
+    define_property(interpreter.well_known_symbol_to_string_tag(), js_string(interpreter, "Math"), Attribute::Configurable);
 }
 
 MathObject::~MathObject()

Libraries/LibJS/Runtime/SymbolPrototype.cpp

@@ -50,6 +50,8 @@ void SymbolPrototype::initialize(Interpreter& interpreter, GlobalObject& global_
     define_native_property("description", description_getter, nullptr, Attribute::Configurable);
     define_native_function("toString", to_string, 0, Attribute::Writable | Attribute::Configurable);
     define_native_function("valueOf", value_of, 0, Attribute::Writable | Attribute::Configurable);
+
+    define_property(interpreter.well_known_symbol_to_string_tag(), js_string(interpreter, "Symbol"), Attribute::Configurable);
 }
 
 SymbolPrototype::~SymbolPrototype()

Libraries/LibJS/Tests/builtins/BigInt/BigInt.prototype.@@toStringTag.js

@@ -0,0 +1,3 @@
+test("basic functionality", () => {
+    expect(BigInt.prototype[Symbol.toStringTag]).toBe("BigInt");
+});

Libraries/LibJS/Tests/builtins/JSON/JSON.@@toStringTag.js

@@ -0,0 +1,4 @@
+test("basic functionality", () => {
+    expect(JSON[Symbol.toStringTag]).toBe("JSON");
+    expect(JSON.toString()).toBe("[object JSON]");
+});

Libraries/LibJS/Tests/builtins/Math/Math.@@toStringTag.js

@@ -0,0 +1,4 @@
+test("basic functionality", () => {
+    expect(Math[Symbol.toStringTag]).toBe("Math");
+    expect(Math.toString()).toBe("[object Math]");
+});

Libraries/LibJS/Tests/builtins/Symbol/Symbol.prototype.@@toStringTag.js

@@ -0,0 +1,3 @@
+test("basic functionality", () => {
+    expect(Symbol.prototype[Symbol.toStringTag]).toBe("Symbol");
+});

Libraries/LibJS/Tests/custom-@@toStringTag.js

@@ -0,0 +1,26 @@
+test("inside objects", () => {
+    const o = {
+        [Symbol.toStringTag]: "hello friends",
+    };
+
+    expect(o.toString()).toBe("[object hello friends]");
+});
+
+test("inside classes", () => {
+    class A {
+        constructor() {
+            this[Symbol.toStringTag] = "hello friends";
+        }
+    }
+
+    const a = new A();
+    expect(a.toString()).toBe("[object hello friends]");
+});
+
+test("non-string values are ignored", () => {
+    const o = {
+        [Symbol.toStringTag]: [1, 2, 3],
+    };
+
+    expect(o.toString()).toBe("[object Object]");
+});

Libraries/LibJS/Tests/iterators/array-iterator.js

@@ -2,6 +2,11 @@ test("length", () => {
     expect(Array.prototype[Symbol.iterator].length).toBe(0);
 });
 
+test("@@toStringTag", () => {
+    expect([].values()[Symbol.toStringTag]).toBe("Array Iterator");
+    expect([].values().toString()).toBe("[object Array Iterator]");
+});
+
 test("same function as Array.prototype.values", () => {
     expect(Array.prototype[Symbol.iterator]).toBe(Array.prototype.values);
 });