Modify environment binding behaviour of function (#1010)

* Modify environment binding behaviour of function

* Add basic test

boa/src/syntax/ast/node/declaration/function_decl/mod.rs

@@ -94,18 +94,23 @@ impl Executable for FunctionDecl {
 
         // Set the name and assign it in the current environment
         val.set_field("name", self.name(), context)?;
-        context
-            .realm_mut()
-            .environment
-            .create_mutable_binding(self.name().to_owned(), false, VariableScope::Function)
-            .map_err(|e| e.to_error(context))?;
 
-        context
-            .realm_mut()
-            .environment
-            .initialize_binding(self.name(), val)
-            .map_err(|e| e.to_error(context))?;
+        let environment = &mut context.realm_mut().environment;
+        if environment.has_binding(self.name()) {
+            environment
+                .set_mutable_binding(self.name(), val, true)
+                .map_err(|e| e.to_error(context))?;
+        } else {
+            environment
+                .create_mutable_binding(self.name().to_owned(), false, VariableScope::Function)
+                .map_err(|e| e.to_error(context))?;
 
+            context
+                .realm_mut()
+                .environment
+                .initialize_binding(self.name(), val)
+                .map_err(|e| e.to_error(context))?;
+        }
         Ok(Value::undefined())
     }
 }

boa/src/syntax/ast/node/declaration/mod.rs

@@ -19,3 +19,6 @@ pub use self::{
     let_decl_list::{LetDecl, LetDeclList},
     var_decl_list::{VarDecl, VarDeclList},
 };
+
+#[cfg(test)]
+mod tests;

boa/src/syntax/ast/node/declaration/tests.rs

@@ -0,0 +1,12 @@
+use crate::exec;
+
+#[test]
+fn duplicate_function_name() {
+    let scenario = r#"
+    function f () {}
+    function f () {return 12;}
+    f()
+    "#;
+
+    assert_eq!(&exec(scenario), "12");
+}