LibJS: Fix functions binding this to global object in strict mode

This fixes an issue where this would be bound to the global object
by default when operating in strict mode.

According to the specification, the expected value for |this| when
no binding is provided is undefined.

2 files changed, 15 insertions, 0 deletions

diff --git a/Userland/Libraries/LibJS/AST.cpp b/Userland/Libraries/LibJS/AST.cpp
index 1c2f0809b9..9e7edf8a40 100644
--- a/Userland/Libraries/LibJS/AST.cpp
+++ b/Userland/Libraries/LibJS/AST.cpp
@@ -158,6 +158,12 @@ CallExpression::ThisAndCallee CallExpression::compute_this_and_callee(Interprete
 
         return { this_value, callee };
     }
+
+    if (interpreter.vm().in_strict_mode()) {
+        // If we are in strict mode, |this| should never be bound to global object by default.
+        return { js_undefined(), m_callee->execute(interpreter, global_object) };
+    }
+
     return { &global_object, m_callee->execute(interpreter, global_object) };
 }
 
diff --git a/Userland/Libraries/LibJS/Tests/functions/function-strict-mode.js b/Userland/Libraries/LibJS/Tests/functions/function-strict-mode.js
index 99ec10b9a7..cf06b891a2 100644
--- a/Userland/Libraries/LibJS/Tests/functions/function-strict-mode.js
+++ b/Userland/Libraries/LibJS/Tests/functions/function-strict-mode.js
@@ -58,3 +58,12 @@ test('only the string "use strict" yields strict mode code', () => {
     "use stric";
     expect(isStrictMode()).toBeFalse();
 });
+
+test("strict mode does not apply global object to |this|", () => {
+    "use strict";
+    let functionThis;
+    (function () {
+        functionThis = this;
+    })();
+    expect(functionThis).toBeUndefined();
+});