LibJS: Fix this values in arrow functions

Also added a large this value test (and strict variant) to ensure this
values have no regressions.

4 files changed, 889 insertions, 4 deletions

diff --git a/Userland/Libraries/LibJS/Runtime/OrdinaryFunctionObject.cpp b/Userland/Libraries/LibJS/Runtime/OrdinaryFunctionObject.cpp
index 9fbff65da7..0f1ffc0b72 100644
--- a/Userland/Libraries/LibJS/Runtime/OrdinaryFunctionObject.cpp
+++ b/Userland/Libraries/LibJS/Runtime/OrdinaryFunctionObject.cpp
@@ -132,6 +132,7 @@ FunctionEnvironment* OrdinaryFunctionObject::create_environment(FunctionObject&
     auto* environment = heap().allocate<FunctionEnvironment>(global_object(), m_environment, variables);
     environment->set_function_object(function_being_invoked);
     if (m_is_arrow_function) {
+        environment->set_this_binding_status(FunctionEnvironment::ThisBindingStatus::Lexical);
         if (is<FunctionEnvironment>(m_environment))
             environment->set_new_target(static_cast<FunctionEnvironment*>(m_environment)->new_target());
     }
diff --git a/Userland/Libraries/LibJS/Runtime/VM.cpp b/Userland/Libraries/LibJS/Runtime/VM.cpp
index bcc012411f..8e993bcd7f 100644
--- a/Userland/Libraries/LibJS/Runtime/VM.cpp
+++ b/Userland/Libraries/LibJS/Runtime/VM.cpp
@@ -11,6 +11,7 @@
 #include <LibJS/Interpreter.h>
 #include <LibJS/Runtime/AbstractOperations.h>
 #include <LibJS/Runtime/Array.h>
+#include <LibJS/Runtime/BoundFunction.h>
 #include <LibJS/Runtime/Error.h>
 #include <LibJS/Runtime/FinalizationRegistry.h>
 #include <LibJS/Runtime/FunctionEnvironment.h>
@@ -471,7 +472,7 @@ Value VM::construct(FunctionObject& function, FunctionObject& new_target, Option
     if (auto* environment = callee_context.lexical_environment) {
         auto& function_environment = verify_cast<FunctionEnvironment>(*environment);
         function_environment.set_new_target(&new_target);
-        if (!this_argument.is_empty()) {
+        if (!this_argument.is_empty() && function_environment.this_binding_status() != FunctionEnvironment::ThisBindingStatus::Lexical) {
             function_environment.bind_this_value(global_object, this_argument);
             if (exception())
                 return {};
@@ -603,10 +604,9 @@ void VM::ordinary_call_bind_this(FunctionObject& function, ExecutionContext& cal
     auto* local_environment = callee_context.lexical_environment;
     auto& function_environment = verify_cast<FunctionEnvironment>(*local_environment);
 
-    // This is not completely as the spec describes it however without this stuff breaks
-    // (Could be related to the note at https://tc39.es/ecma262/#sec-runtime-semantics-instantiatearrowfunctionexpression )
+    // This almost as the spec describes it however we sometimes don't have callee_realm when dealing
+    // with proxies and arrow functions however this does seemingly achieve spec like behavior.
     if (!callee_realm || this_mode == FunctionObject::ThisMode::Lexical) {
-        function_environment.bind_this_value(function.global_object(), callee_context.this_value);
         return;
     }
 
@@ -629,6 +629,20 @@ Value VM::call_internal(FunctionObject& function, Value this_value, Optional<Mar
     VERIFY(!exception());
     VERIFY(!this_value.is_empty());
 
+    if (is<BoundFunction>(function)) {
+        auto& bound_function = static_cast<BoundFunction&>(function);
+        auto bound_arguments = bound_function.bound_arguments();
+        if (arguments.has_value())
+            bound_arguments.extend(*arguments);
+
+        MarkedValueList with_bound_arguments { heap() };
+        with_bound_arguments.extend(bound_function.bound_arguments());
+        if (arguments.has_value())
+            with_bound_arguments.extend(*arguments);
+
+        return call_internal(bound_function.target_function(), bound_function.bound_this(), move(with_bound_arguments));
+    }
+
     ExecutionContext callee_context;
     prepare_for_ordinary_call(function, callee_context, js_undefined());
     if (exception())
diff --git a/Userland/Libraries/LibJS/Tests/this-value-strict.js b/Userland/Libraries/LibJS/Tests/this-value-strict.js
new file mode 100644
index 0000000000..8d9bf587aa
--- /dev/null
+++ b/Userland/Libraries/LibJS/Tests/this-value-strict.js
@@ -0,0 +1,424 @@
+"use strict";
+
+// Note the globalThisValue and globalObject do not need to be the same.
+const globalThisValue = this;
+const globalObject = (0, eval)("this");
+
+// These tests are done in global state to ensure that is possible
+const globalArrow = () => {
+    expect(this).toBe(globalThisValue);
+    return this;
+};
+
+function globalFunction() {
+    expect(this).toBe(undefined);
+
+    expect(globalArrow()).toBe(globalThisValue);
+
+    const arrowInGlobalFunction = () => this;
+    expect(arrowInGlobalFunction()).toBe(undefined);
+
+    return arrowInGlobalFunction;
+}
+
+expect(globalArrow()).toBe(globalThisValue);
+expect(globalFunction()()).toBe(undefined);
+
+const arrowFromGlobalFunction = globalFunction();
+
+const customThisValue = {
+    isCustomThis: true,
+    variant: 0,
+};
+
+const otherCustomThisValue = {
+    isCustomThis: true,
+    variant: 1,
+};
+
+describe("describe with arrow function", () => {
+    expect(this).toBe(globalThisValue);
+
+    test("nested test with normal function should get global object", function () {
+        expect(this).toBe(undefined);
+    });
+
+    test("nested test with arrow function should get same this value as enclosing function", () => {
+        expect(this).toBe(globalThisValue);
+    });
+});
+
+describe("describe with normal function", function () {
+    expect(this).toBe(undefined);
+    test("nested test with normal function should get global object", function () {
+        expect(this).toBe(undefined);
+    });
+
+    test("nested test with arrow function should get same this value as enclosing function", () => {
+        expect(this).toBe(undefined);
+    });
+});
+
+describe("basic behavior", () => {
+    expect(this).toBe(globalThisValue);
+
+    expect(arrowFromGlobalFunction()).toBeUndefined();
+
+    expect(customThisValue).not.toBe(otherCustomThisValue);
+
+    test("binding arrow function does not influence this value", () => {
+        const boundGlobalArrow = globalArrow.bind({ shouldNotBeHere: true });
+
+        expect(boundGlobalArrow()).toBe(globalThisValue);
+    });
+
+    function functionInArrow() {
+        expect(arrowFromGlobalFunction()).toBeUndefined();
+        return this;
+    }
+
+    function functionWithArrow() {
+        expect(arrowFromGlobalFunction()).toBeUndefined();
+        return () => {
+            expect(arrowFromGlobalFunction()).toBeUndefined();
+            return this;
+        };
+    }
+
+    function strictFunction() {
+        "use strict";
+        return this;
+    }
+
+    test("functions get globalObject as this value", () => {
+        expect(functionInArrow()).toBeUndefined();
+        expect(functionWithArrow()()).toBeUndefined();
+    });
+
+    test("strict functions get undefined as this value", () => {
+        expect(strictFunction()).toBeUndefined();
+    });
+
+    test("bound function gets overwritten this value", () => {
+        const boundFunction = functionInArrow.bind(customThisValue);
+        expect(boundFunction()).toBe(customThisValue);
+
+        const boundFunctionWithArrow = functionWithArrow.bind(customThisValue);
+        expect(boundFunctionWithArrow()()).toBe(customThisValue);
+
+        // However we cannot bind the arrow function itself
+        const failingArrowBound = boundFunctionWithArrow().bind(otherCustomThisValue);
+        expect(failingArrowBound()).toBe(customThisValue);
+
+        const boundStrictFunction = strictFunction.bind(customThisValue);
+        expect(boundStrictFunction()).toBe(customThisValue);
+    });
+});
+
+describe("functions on created objects", () => {
+    const obj = {
+        func: function () {
+            expect(arrowFromGlobalFunction()).toBeUndefined();
+            return this;
+        },
+
+        funcWithArrow: function () {
+            expect(arrowFromGlobalFunction()).toBeUndefined();
+            return () => this;
+        },
+
+        arrow: () => {
+            expect(arrowFromGlobalFunction()).toBeUndefined();
+            return this;
+        },
+        otherProperty: "yes",
+    };
+
+    test("function get this value of associated object", () => {
+        expect(obj.func()).toBe(obj);
+    });
+
+    test("arrow function on object get above this value", () => {
+        expect(obj.arrow()).toBe(globalThisValue);
+    });
+
+    test("arrow function from normal function from object has object as this value", () => {
+        expect(obj.funcWithArrow()()).toBe(obj);
+    });
+
+    test("bound overwrites value of normal object function", () => {
+        const boundFunction = obj.func.bind(customThisValue);
+        expect(boundFunction()).toBe(customThisValue);
+
+        const boundFunctionWithArrow = obj.funcWithArrow.bind(customThisValue);
+        expect(boundFunctionWithArrow()()).toBe(customThisValue);
+
+        const boundArrowFunction = obj.arrow.bind(customThisValue);
+        expect(boundArrowFunction()).toBe(globalThisValue);
+    });
+
+    test("also works for object defined in function", () => {
+        (function () {
+            expect(arrowFromGlobalFunction()).toBeUndefined();
+
+            // It is bound below
+            expect(this).toBe(customThisValue);
+
+            const obj2 = {
+                func: function () {
+                    expect(arrowFromGlobalFunction()).toBeUndefined();
+                    return this;
+                },
+
+                arrow: () => {
+                    expect(arrowFromGlobalFunction()).toBeUndefined();
+                    return this;
+                },
+                otherProperty: "also",
+            };
+
+            expect(obj2.func()).toBe(obj2);
+            expect(obj2.arrow()).toBe(customThisValue);
+        }.bind(customThisValue)());
+    });
+});
+
+describe("behavior with classes", () => {
+    class Basic {
+        constructor(value) {
+            expect(this).toBeInstanceOf(Basic);
+            this.arrowFunctionInClass = () => {
+                return this;
+            };
+
+            this.value = value;
+
+            expect(arrowFromGlobalFunction()).toBeUndefined();
+        }
+
+        func() {
+            expect(arrowFromGlobalFunction()).toBeUndefined();
+            return this;
+        }
+    }
+
+    const basic = new Basic(14);
+    const basic2 = new Basic(457);
+
+    expect(basic).not.toBe(basic2);
+
+    test("calling functions on class should give instance as this value", () => {
+        expect(basic.func()).toBe(basic);
+        expect(basic2.func()).toBe(basic2);
+    });
+
+    test("calling arrow function created in constructor should give instance as this value", () => {
+        expect(basic.arrowFunctionInClass()).toBe(basic);
+        expect(basic2.arrowFunctionInClass()).toBe(basic2);
+    });
+
+    test("can bind function in class", () => {
+        const boundFunction = basic.func.bind(customThisValue);
+        expect(boundFunction()).toBe(customThisValue);
+
+        const boundFunction2 = basic2.func.bind(otherCustomThisValue);
+        expect(boundFunction2()).toBe(otherCustomThisValue);
+    });
+});
+
+describe("derived classes behavior", () => {
+    class Base {
+        baseFunction() {
+            expect(arrowFromGlobalFunction()).toBeUndefined();
+
+            return this;
+        }
+    }
+
+    class Derived extends Base {
+        constructor(value) {
+            expect(arrowFromGlobalFunction()).toBeUndefined();
+            const arrowMadeBeforeSuper = () => {
+                expect(this).toBeInstanceOf(Derived);
+                return this;
+            };
+            super();
+            expect(arrowMadeBeforeSuper()).toBe(this);
+
+            this.arrowMadeBeforeSuper = arrowMadeBeforeSuper;
+            this.arrowMadeAfterSuper = () => {
+                expect(this).toBeInstanceOf(Derived);
+                return this;
+            };
+            this.value = value;
+        }
+
+        derivedFunction() {
+            expect(arrowFromGlobalFunction()).toBeUndefined();
+            return this;
+        }
+    }
+
+    test("can create derived with arrow functions using this before super", () => {
+        const testDerived = new Derived(-89);
+        expect(testDerived.arrowMadeBeforeSuper()).toBe(testDerived);
+        expect(testDerived.arrowMadeAfterSuper()).toBe(testDerived);
+    });
+
+    test("base and derived functions get correct this values", () => {
+        const derived = new Derived(12);
+
+        expect(derived.derivedFunction()).toBe(derived);
+        expect(derived.baseFunction()).toBe(derived);
+    });
+
+    test("can bind derived and base functions", () => {
+        const derived = new Derived(846);
+
+        const boundDerivedFunction = derived.derivedFunction.bind(customThisValue);
+        expect(boundDerivedFunction()).toBe(customThisValue);
+
+        const boundBaseFunction = derived.baseFunction.bind(otherCustomThisValue);
+        expect(boundBaseFunction()).toBe(otherCustomThisValue);
+    });
+});
+
+describe("proxy behavior", () => {
+    test("with no handler it makes no difference", () => {
+        const globalArrowProxyNoHandler = new Proxy(globalArrow, {});
+        expect(globalArrowProxyNoHandler()).toBe(globalThisValue);
+    });
+
+    test("proxy around global arrow still gives correct this value", () => {
+        let lastThisArg = null;
+
+        const handler = {
+            apply(target, thisArg, argArray) {
+                expect(target).toBe(globalArrow);
+                lastThisArg = thisArg;
+                expect(this).toBe(handler);
+
+                return target(...argArray);
+            },
+        };
+
+        const globalArrowProxy = new Proxy(globalArrow, handler);
+        expect(globalArrowProxy()).toBe(globalThisValue);
+        expect(lastThisArg).toBeUndefined();
+
+        const boundProxy = globalArrowProxy.bind(customThisValue);
+        expect(boundProxy()).toBe(globalThisValue);
+        expect(lastThisArg).toBe(customThisValue);
+
+        expect(globalArrowProxy.call(15)).toBe(globalThisValue);
+        expect(lastThisArg).toBe(15);
+    });
+});
+
+describe("derived classes which access this before super should fail", () => {
+    class Base {}
+
+    test("direct access of this should throw reference error", () => {
+        class IncorrectConstructor extends Base {
+            constructor() {
+                this.something = "this will fail";
+                super();
+            }
+        }
+
+        expect(() => {
+            new IncorrectConstructor();
+        }).toThrowWithMessage(ReferenceError, "|this| has not been initialized");
+    });
+
+    test("access of this via a arrow function", () => {
+        class IncorrectConstructor extends Base {
+            constructor() {
+                const arrow = () => this;
+                arrow();
+                super();
+            }
+        }
+
+        expect(() => {
+            new IncorrectConstructor();
+        }).toThrowWithMessage(ReferenceError, "|this| has not been initialized");
+    });
+
+    test("access of this via a eval", () => {
+        class IncorrectConstructor extends Base {
+            constructor() {
+                eval("this.foo = 'bar'");
+                super();
+            }
+        }
+
+        expect(() => {
+            new IncorrectConstructor();
+        }).toThrowWithMessage(ReferenceError, "|this| has not been initialized");
+    });
+
+    test.skip("access of this via a eval in arrow function", () => {
+        class IncorrectConstructor extends Base {
+            constructor() {
+                const arrow = () => eval("() => this")();
+                arrow();
+                super();
+            }
+        }
+
+        expect(() => {
+            new IncorrectConstructor();
+        }).toThrowWithMessage(ReferenceError, "|this| has not been initialized");
+    });
+
+    test("access of this via arrow function even if bound with something else", () => {
+        class IncorrectConstructor extends Base {
+            constructor() {
+                const arrow = () => this;
+                const boundArrow = arrow.bind(customThisValue);
+                boundArrow();
+                super();
+            }
+        }
+
+        expect(() => {
+            new IncorrectConstructor();
+        }).toThrowWithMessage(ReferenceError, "|this| has not been initialized");
+    });
+});
+
+describe("in strict mode primitive this values are not converted to objects", () => {
+    const array = [true, false];
+
+    // Technically the comma is implementation defined here. (Also for tests below.)
+    expect(array.toLocaleString()).toBe("true,false");
+
+    test("directly overwriting toString", () => {
+        let count = 0;
+        Boolean.prototype.toString = function () {
+            count++;
+            return typeof this;
+        };
+
+        expect(array.toLocaleString()).toBe("boolean,boolean");
+        expect(count).toBe(2);
+    });
+
+    test("overwriting toString with a getter", () => {
+        let count = 0;
+
+        Object.defineProperty(Boolean.prototype, "toString", {
+            get() {
+                count++;
+                const that = typeof this;
+                return function () {
+                    return that;
+                };
+            },
+        });
+
+        expect(array.toLocaleString()).toBe("boolean,boolean");
+        expect(count).toBe(2);
+    });
+});
diff --git a/Userland/Libraries/LibJS/Tests/this-value.js b/Userland/Libraries/LibJS/Tests/this-value.js
new file mode 100644
index 0000000000..73571e9e84
--- /dev/null
+++ b/Userland/Libraries/LibJS/Tests/this-value.js
@@ -0,0 +1,446 @@
+// Note the globalThisValue and globalObject do not need to be the same.
+const globalThisValue = this;
+const globalObject = (0, eval)("this");
+
+// These tests are done in global state to ensure that is possible
+const globalArrow = () => {
+    expect(this).toBe(globalThisValue);
+    return this;
+};
+
+function globalFunction() {
+    expect(this).toBe(globalObject);
+
+    expect(globalArrow()).toBe(globalThisValue);
+
+    const arrowInGlobalFunction = () => this;
+    expect(arrowInGlobalFunction()).toBe(globalObject);
+
+    return arrowInGlobalFunction;
+}
+
+expect(globalArrow()).toBe(globalThisValue);
+expect(globalFunction()()).toBe(globalObject);
+
+const arrowFromGlobalFunction = globalFunction();
+
+const customThisValue = {
+    isCustomThis: true,
+    variant: 0,
+};
+
+const otherCustomThisValue = {
+    isCustomThis: true,
+    variant: 1,
+};
+
+describe("describe with arrow function", () => {
+    expect(this).toBe(globalThisValue);
+
+    test("nested test with normal function should get global object", function () {
+        expect(this).toBe(globalObject);
+    });
+
+    test("nested test with arrow function should get same this value as enclosing function", () => {
+        expect(this).toBe(globalThisValue);
+    });
+});
+
+describe("describe with normal function", function () {
+    expect(this).toBe(globalObject);
+    test("nested test with normal function should get global object", function () {
+        expect(this).toBe(globalObject);
+    });
+
+    test("nested test with arrow function should get same this value as enclosing function", () => {
+        expect(this).toBe(globalObject);
+    });
+});
+
+describe("basic behavior", () => {
+    expect(this).toBe(globalThisValue);
+
+    expect(arrowFromGlobalFunction()).toBe(globalObject);
+
+    expect(customThisValue).not.toBe(otherCustomThisValue);
+
+    test("binding arrow function does not influence this value", () => {
+        const boundGlobalArrow = globalArrow.bind({ shouldNotBeHere: true });
+
+        expect(boundGlobalArrow()).toBe(globalThisValue);
+    });
+
+    function functionInArrow() {
+        expect(arrowFromGlobalFunction()).toBe(globalObject);
+        return this;
+    }
+
+    function functionWithArrow() {
+        expect(arrowFromGlobalFunction()).toBe(globalObject);
+        return () => {
+            expect(arrowFromGlobalFunction()).toBe(globalObject);
+            return this;
+        };
+    }
+
+    function strictFunction() {
+        "use strict";
+        return this;
+    }
+
+    test("functions get globalObject as this value", () => {
+        expect(functionInArrow()).toBe(globalObject);
+        expect(functionWithArrow()()).toBe(globalObject);
+    });
+
+    test("strict functions get undefined as this value", () => {
+        expect(strictFunction()).toBeUndefined();
+    });
+
+    test("bound function gets overwritten this value", () => {
+        const boundFunction = functionInArrow.bind(customThisValue);
+        expect(boundFunction()).toBe(customThisValue);
+
+        const boundFunctionWithArrow = functionWithArrow.bind(customThisValue);
+        expect(boundFunctionWithArrow()()).toBe(customThisValue);
+
+        // However we cannot bind the arrow function itself
+        const failingArrowBound = boundFunctionWithArrow().bind(otherCustomThisValue);
+        expect(failingArrowBound()).toBe(customThisValue);
+
+        const boundStrictFunction = strictFunction.bind(customThisValue);
+        expect(boundStrictFunction()).toBe(customThisValue);
+    });
+});
+
+describe("functions on created objects", () => {
+    const obj = {
+        func: function () {
+            expect(arrowFromGlobalFunction()).toBe(globalObject);
+            return this;
+        },
+
+        funcWithArrow: function () {
+            expect(arrowFromGlobalFunction()).toBe(globalObject);
+            return () => this;
+        },
+
+        arrow: () => {
+            expect(arrowFromGlobalFunction()).toBe(globalObject);
+            return this;
+        },
+        otherProperty: "yes",
+    };
+
+    test("function get this value of associated object", () => {
+        expect(obj.func()).toBe(obj);
+    });
+
+    test("arrow function on object get above this value", () => {
+        expect(obj.arrow()).toBe(globalThisValue);
+    });
+
+    test("arrow function from normal function from object has object as this value", () => {
+        expect(obj.funcWithArrow()()).toBe(obj);
+    });
+
+    test("bound overwrites value of normal object function", () => {
+        const boundFunction = obj.func.bind(customThisValue);
+        expect(boundFunction()).toBe(customThisValue);
+
+        const boundFunctionWithArrow = obj.funcWithArrow.bind(customThisValue);
+        expect(boundFunctionWithArrow()()).toBe(customThisValue);
+
+        const boundArrowFunction = obj.arrow.bind(customThisValue);
+        expect(boundArrowFunction()).toBe(globalThisValue);
+    });
+
+    test("also works for object defined in function", () => {
+        (function () {
+            expect(arrowFromGlobalFunction()).toBe(globalObject);
+
+            // It is bound below
+            expect(this).toBe(customThisValue);
+
+            const obj2 = {
+                func: function () {
+                    expect(arrowFromGlobalFunction()).toBe(globalObject);
+                    return this;
+                },
+
+                arrow: () => {
+                    expect(arrowFromGlobalFunction()).toBe(globalObject);
+                    return this;
+                },
+                otherProperty: "also",
+            };
+
+            expect(obj2.func()).toBe(obj2);
+            expect(obj2.arrow()).toBe(customThisValue);
+        }.bind(customThisValue)());
+    });
+});
+
+describe("behavior with classes", () => {
+    class Basic {
+        constructor(value) {
+            expect(this).toBeInstanceOf(Basic);
+            this.arrowFunctionInClass = () => {
+                return this;
+            };
+
+            this.value = value;
+
+            expect(arrowFromGlobalFunction()).toBe(globalObject);
+        }
+
+        func() {
+            expect(arrowFromGlobalFunction()).toBe(globalObject);
+            return this;
+        }
+    }
+
+    const basic = new Basic(14);
+    const basic2 = new Basic(457);
+
+    expect(basic).not.toBe(basic2);
+
+    test("calling functions on class should give instance as this value", () => {
+        expect(basic.func()).toBe(basic);
+        expect(basic2.func()).toBe(basic2);
+    });
+
+    test("calling arrow function created in constructor should give instance as this value", () => {
+        expect(basic.arrowFunctionInClass()).toBe(basic);
+        expect(basic2.arrowFunctionInClass()).toBe(basic2);
+    });
+
+    test("can bind function in class", () => {
+        const boundFunction = basic.func.bind(customThisValue);
+        expect(boundFunction()).toBe(customThisValue);
+
+        const boundFunction2 = basic2.func.bind(otherCustomThisValue);
+        expect(boundFunction2()).toBe(otherCustomThisValue);
+    });
+});
+
+describe("derived classes behavior", () => {
+    class Base {
+        baseFunction() {
+            expect(arrowFromGlobalFunction()).toBe(globalObject);
+
+            return this;
+        }
+    }
+
+    class Derived extends Base {
+        constructor(value) {
+            expect(arrowFromGlobalFunction()).toBe(globalObject);
+            const arrowMadeBeforeSuper = () => {
+                expect(this).toBeInstanceOf(Derived);
+                return this;
+            };
+            super();
+            expect(arrowMadeBeforeSuper()).toBe(this);
+
+            this.arrowMadeBeforeSuper = arrowMadeBeforeSuper;
+            this.arrowMadeAfterSuper = () => {
+                expect(this).toBeInstanceOf(Derived);
+                return this;
+            };
+            this.value = value;
+        }
+
+        derivedFunction() {
+            expect(arrowFromGlobalFunction()).toBe(globalObject);
+            return this;
+        }
+    }
+
+    test("can create derived with arrow functions using this before super", () => {
+        const testDerived = new Derived(-89);
+        expect(testDerived.arrowMadeBeforeSuper()).toBe(testDerived);
+        expect(testDerived.arrowMadeAfterSuper()).toBe(testDerived);
+    });
+
+    test("base and derived functions get correct this values", () => {
+        const derived = new Derived(12);
+
+        expect(derived.derivedFunction()).toBe(derived);
+        expect(derived.baseFunction()).toBe(derived);
+    });
+
+    test("can bind derived and base functions", () => {
+        const derived = new Derived(846);
+
+        const boundDerivedFunction = derived.derivedFunction.bind(customThisValue);
+        expect(boundDerivedFunction()).toBe(customThisValue);
+
+        const boundBaseFunction = derived.baseFunction.bind(otherCustomThisValue);
+        expect(boundBaseFunction()).toBe(otherCustomThisValue);
+    });
+});
+
+describe("proxy behavior", () => {
+    test("with no handler it makes no difference", () => {
+        const globalArrowProxyNoHandler = new Proxy(globalArrow, {});
+        expect(globalArrowProxyNoHandler()).toBe(globalThisValue);
+    });
+
+    test("proxy around global arrow still gives correct this value", () => {
+        let lastThisArg = null;
+
+        const handler = {
+            apply(target, thisArg, argArray) {
+                expect(target).toBe(globalArrow);
+                lastThisArg = thisArg;
+                expect(this).toBe(handler);
+
+                return target(...argArray);
+            },
+        };
+
+        const globalArrowProxy = new Proxy(globalArrow, handler);
+        expect(globalArrowProxy()).toBe(globalThisValue);
+        expect(lastThisArg).toBeUndefined();
+
+        const boundProxy = globalArrowProxy.bind(customThisValue);
+        expect(boundProxy()).toBe(globalThisValue);
+        expect(lastThisArg).toBe(customThisValue);
+
+        expect(globalArrowProxy.call(15)).toBe(globalThisValue);
+        expect(lastThisArg).toBe(15);
+    });
+});
+
+describe("derived classes which access this before super should fail", () => {
+    class Base {}
+
+    test("direct access of this should throw reference error", () => {
+        class IncorrectConstructor extends Base {
+            constructor() {
+                this.something = "this will fail";
+                super();
+            }
+        }
+
+        expect(() => {
+            new IncorrectConstructor();
+        }).toThrowWithMessage(ReferenceError, "|this| has not been initialized");
+    });
+
+    test("access of this via a arrow function", () => {
+        class IncorrectConstructor extends Base {
+            constructor() {
+                const arrow = () => this;
+                arrow();
+                super();
+            }
+        }
+
+        expect(() => {
+            new IncorrectConstructor();
+        }).toThrowWithMessage(ReferenceError, "|this| has not been initialized");
+    });
+
+    test("access of this via a eval", () => {
+        class IncorrectConstructor extends Base {
+            constructor() {
+                eval("this.foo = 'bar'");
+                super();
+            }
+        }
+
+        expect(() => {
+            new IncorrectConstructor();
+        }).toThrowWithMessage(ReferenceError, "|this| has not been initialized");
+    });
+
+    test.skip("access of this via a eval in arrow function", () => {
+        class IncorrectConstructor extends Base {
+            constructor() {
+                const arrow = () => eval("() => this")();
+                arrow();
+                super();
+            }
+        }
+
+        expect(() => {
+            new IncorrectConstructor();
+        }).toThrowWithMessage(ReferenceError, "|this| has not been initialized");
+    });
+
+    test("access of this via arrow function even if bound with something else", () => {
+        class IncorrectConstructor extends Base {
+            constructor() {
+                const arrow = () => this;
+                const boundArrow = arrow.bind(customThisValue);
+                boundArrow();
+                super();
+            }
+        }
+
+        expect(() => {
+            new IncorrectConstructor();
+        }).toThrowWithMessage(ReferenceError, "|this| has not been initialized");
+    });
+});
+
+describe("with statements", () => {
+    test("this value is still the global object", () => {
+        const obj = { haveValue: true, hello: "friends" };
+        with (obj) {
+            expect(this).toBe(globalThisValue);
+            expect(hello).toBe("friends");
+        }
+    });
+
+    test("with gets this value form outer scope", () => {
+        const obj = { haveValue: true, hello: "friends" };
+
+        function callme() {
+            with (obj) {
+                expect(this).toBe(customThisValue);
+                expect(hello).toBe("friends");
+            }
+        }
+
+        const boundMe = callme.bind(customThisValue);
+        boundMe();
+    });
+});
+
+describe("in non strict mode primitive this values are converted to objects", () => {
+    const array = [true, false];
+
+    // Technically the comma is implementation defined here. (Also for tests below.)
+    expect(array.toLocaleString()).toBe("true,false");
+
+    test("directly overwriting toString", () => {
+        let count = 0;
+        Boolean.prototype.toString = function () {
+            count++;
+            return typeof this;
+        };
+
+        expect(array.toLocaleString()).toBe("object,object");
+        expect(count).toBe(2);
+    });
+
+    test("overwriting toString with a getter", () => {
+        let count = 0;
+
+        Object.defineProperty(Boolean.prototype, "toString", {
+            get() {
+                count++;
+                const that = typeof this;
+                return function () {
+                    return that;
+                };
+            },
+        });
+
+        expect(array.toLocaleString()).toBe("object,object");
+        expect(count).toBe(2);
+    });
+});