LibJS: Don't assume a this argument for function.prototype.bind

Assuming we had at least one argument meant that the ...arg count would
underflow causing the bound function to have length 0 instead of the
given length when binding with no arguments.

2 files changed, 14 insertions, 1 deletions

diff --git a/Userland/Libraries/LibJS/Runtime/FunctionPrototype.cpp b/Userland/Libraries/LibJS/Runtime/FunctionPrototype.cpp
index 2fab7b2f4f..771a4e3f8d 100644
--- a/Userland/Libraries/LibJS/Runtime/FunctionPrototype.cpp
+++ b/Userland/Libraries/LibJS/Runtime/FunctionPrototype.cpp
@@ -103,7 +103,7 @@ JS_DEFINE_NATIVE_FUNCTION(FunctionPrototype::bind)
     auto* function = TRY(BoundFunction::create(global_object, target, this_argument, move(arguments)));
 
     // 4. Let argCount be the number of elements in args.
-    auto arg_count = vm.argument_count() - 1;
+    auto arg_count = vm.argument_count() > 0 ? vm.argument_count() - 1 : 0;
 
     // 5. Perform ? CopyNameAndLength(F, Target, "bound", argCount).
     TRY(copy_name_and_length(global_object, *function, target, "bound"sv, arg_count));
diff --git a/Userland/Libraries/LibJS/Tests/builtins/Function/Function.prototype.bind.js b/Userland/Libraries/LibJS/Tests/builtins/Function/Function.prototype.bind.js
index b121f6a888..3af8cb2b27 100644
--- a/Userland/Libraries/LibJS/Tests/builtins/Function/Function.prototype.bind.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Function/Function.prototype.bind.js
@@ -126,6 +126,19 @@ describe("bound function |this|", () => {
     test("arrow functions cannot be bound", () => {
         expect((() => this).bind("foo")()).toBe(globalThis);
     });
+
+    test("length of original function is used for bound function", () => {
+        [0, 1, 2147483647, 2147483648, 2147483649].forEach(value => {
+            function emptyFunction() {}
+
+            Object.defineProperty(emptyFunction, "length", { value });
+
+            expect(emptyFunction.bind().length).toBe(value);
+            expect(emptyFunction.bind(null).length).toBe(value);
+            expect(emptyFunction.bind(null, 0).length).toBe(Math.max(0, value - 1));
+            expect(emptyFunction.bind(null, 0, 1, 2).length).toBe(Math.max(0, value - 3));
+        });
+    });
 });
 
 describe("bound function constructors", () => {