From a517bf27210fdea4e2be905dcb7d7879fdcfcaa5 Mon Sep 17 00:00:00 2001
From: 54k1 <54k1@protonmail.com>
Date: Tue, 29 Dec 2020 21:24:28 +0530
Subject: [PATCH 1/2] Modify environment binding behaviour of function

---
 .../ast/node/declaration/function_decl/mod.rs | 28 +++++++++++--------
 1 file changed, 17 insertions(+), 11 deletions(-)

diff --git a/boa/src/syntax/ast/node/declaration/function_decl/mod.rs b/boa/src/syntax/ast/node/declaration/function_decl/mod.rs
index 5838cf49615..a292fbcbcf7 100644
--- a/boa/src/syntax/ast/node/declaration/function_decl/mod.rs
+++ b/boa/src/syntax/ast/node/declaration/function_decl/mod.rs
@@ -94,17 +94,23 @@ impl Executable for FunctionDecl {
 
         // Set the name and assign it in the current environment
         val.set_field("name", self.name());
-        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())
     }

From 5b5d83516c2a84de6c689dd409bd26dc3c4bcfff Mon Sep 17 00:00:00 2001
From: 54k1 <54k1@protonmail.com>
Date: Sun, 3 Jan 2021 09:56:14 +0530
Subject: [PATCH 2/2] Add basic test

---
 boa/src/syntax/ast/node/declaration/mod.rs   |  3 +++
 boa/src/syntax/ast/node/declaration/tests.rs | 12 ++++++++++++
 2 files changed, 15 insertions(+)
 create mode 100644 boa/src/syntax/ast/node/declaration/tests.rs

diff --git a/boa/src/syntax/ast/node/declaration/mod.rs b/boa/src/syntax/ast/node/declaration/mod.rs
index a256a75e87f..ac5a0dbbb92 100644
--- a/boa/src/syntax/ast/node/declaration/mod.rs
+++ b/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;
diff --git a/boa/src/syntax/ast/node/declaration/tests.rs b/boa/src/syntax/ast/node/declaration/tests.rs
new file mode 100644
index 00000000000..a98ac86f523
--- /dev/null
+++ b/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");
+}