LibJS: Add length property to ScriptFunction

Author: https://github.com/linusg
Commit: https://github.com/SerenityOS/serenity/commit/4d931b524de
Pull-request: https://github.com/SerenityOS/serenity/pull/1622
Reviewed-by: https://github.com/awesomekling ✅

Libraries/LibJS/Runtime/ScriptFunction.cpp

@@ -24,8 +24,10 @@
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
+#include <AK/Function.h>
 #include <LibJS/AST.h>
 #include <LibJS/Interpreter.h>
+#include <LibJS/Runtime/Error.h>
 #include <LibJS/Runtime/ScriptFunction.h>
 #include <LibJS/Runtime/Value.h>
 
@@ -35,6 +37,7 @@ ScriptFunction::ScriptFunction(const ScopeNode& body, Vector<FlyString> paramete
     : m_body(body)
     , m_parameters(move(parameters))
 {
+    put_native_property("length", length_getter, length_setter);
 }
 
 ScriptFunction::~ScriptFunction()
@@ -60,4 +63,18 @@ Value ScriptFunction::construct(Interpreter& interpreter)
     return call(interpreter);
 }
 
+Value ScriptFunction::length_getter(Interpreter& interpreter)
+{
+    auto* this_object = interpreter.this_value().to_object(interpreter.heap());
+    if (!this_object)
+        return {};
+    if (!this_object->is_function())
+        return interpreter.throw_exception<Error>("TypeError", "Not a function");
+    return Value(static_cast<i32>(static_cast<const ScriptFunction*>(this_object)->parameters().size()));
+}
+
+void ScriptFunction::length_setter(Interpreter&, Value)
+{
+}
+
 }

Libraries/LibJS/Runtime/ScriptFunction.h

@@ -45,6 +45,9 @@ private:
     virtual bool is_script_function() const final { return true; }
     virtual const char* class_name() const override { return "ScriptFunction"; }
 
+    static Value length_getter(Interpreter&);
+    static void length_setter(Interpreter&, Value);
+
     NonnullRefPtr<ScopeNode> m_body;
     const Vector<FlyString> m_parameters;
 };

Libraries/LibJS/Tests/function-length.js

@@ -0,0 +1,17 @@
+function assert(x) { if (!x) throw 1; }
+
+try {
+    function foo() { }
+    assert(foo.length === 0);
+    assert((foo.length = 5) === 5);
+    assert(foo.length === 0);
+
+    function bar(a, b, c) {}
+    assert(bar.length === 3);
+    assert((bar.length = 5) === 5);
+    assert(bar.length === 3);
+
+    console.log("PASS");
+} catch (e) {
+    console.log("FAIL: " + e);
+}