LibJS: Add spec comments to SymbolConstructor

Userland/Libraries/LibJS/Runtime/Symbol.cpp

@@ -34,4 +34,20 @@ ErrorOr<String> Symbol::descriptive_string() const
     return String::formatted("Symbol({})", description);
 }
 
+// 20.4.5.1 KeyForSymbol ( sym ), https://tc39.es/ecma262/#sec-keyforsymbol
+Optional<String> Symbol::key() const
+{
+    // 1. For each element e of the GlobalSymbolRegistry List, do
+    //    a. If SameValue(e.[[Symbol]], sym) is true, return e.[[Key]].
+    if (m_is_global) {
+        // NOTE: Global symbols should always have a description string
+        VERIFY(m_description.has_value());
+        return m_description;
+    }
+
+    // 2. Assert: GlobalSymbolRegistry does not currently contain an entry for sym.
+    // 3. Return undefined.
+    return {};
+}
+
 }

Userland/Libraries/LibJS/Runtime/Symbol.h

@@ -24,6 +24,7 @@ public:
     bool is_global() const { return m_is_global; }
 
     ErrorOr<String> descriptive_string() const;
+    Optional<String> key() const;
 
 private:
     Symbol(Optional<String>, bool);

Userland/Libraries/LibJS/Runtime/SymbolConstructor.cpp

@@ -42,14 +42,22 @@ ThrowCompletionOr<void> SymbolConstructor::initialize(Realm& realm)
 ThrowCompletionOr<Value> SymbolConstructor::call()
 {
     auto& vm = this->vm();
-    if (vm.argument(0).is_undefined())
-        return Symbol::create(vm, {}, false);
-    return Symbol::create(vm, TRY(vm.argument(0).to_string(vm)), false);
+    auto description = vm.argument(0);
+
+    // 2. If description is undefined, let descString be undefined.
+    // 3. Else, let descString be ? ToString(description).
+    auto description_string = description.is_undefined()
+        ? Optional<String> {}
+        : TRY(description.to_string(vm));
+
+    // 4. Return a new Symbol whose [[Description]] is descString.
+    return Symbol::create(vm, move(description_string), false);
 }
 
 // 20.4.1.1 Symbol ( [ description ] ), https://tc39.es/ecma262/#sec-symbol-description
 ThrowCompletionOr<NonnullGCPtr<Object>> SymbolConstructor::construct(FunctionObject&)
 {
+    // 1. If NewTarget is not undefined, throw a TypeError exception.
     return vm().throw_completion<TypeError>(ErrorType::NotAConstructor, "Symbol");
 }
 
@@ -83,16 +91,16 @@ JS_DEFINE_NATIVE_FUNCTION(SymbolConstructor::for_)
 JS_DEFINE_NATIVE_FUNCTION(SymbolConstructor::key_for)
 {
     auto argument = vm.argument(0);
+
+    // 1. If sym is not a Symbol, throw a TypeError exception.
     if (!argument.is_symbol())
         return vm.throw_completion<TypeError>(ErrorType::NotASymbol, TRY_OR_THROW_OOM(vm, argument.to_string_without_side_effects()));
 
-    auto& symbol = argument.as_symbol();
-    if (symbol.is_global()) {
-        // NOTE: Global symbols should always have a description string
-        return PrimitiveString::create(vm, *symbol.description());
-    }
-
-    return js_undefined();
+    // 2. Return KeyForSymbol(sym).
+    auto key = argument.as_symbol().key();
+    return key.has_value()
+        ? PrimitiveString::create(vm, key.release_value())
+        : js_undefined();
 }
 
 }