get name if not provided in `FormData.append` (#4434)

* get file name from blob if not provided

* add test

* another test

* format

2 files changed, 15 insertions, 4 deletions

diff --git a/src/bun.js/bindings/webcore/JSDOMFormData.cpp b/src/bun.js/bindings/webcore/JSDOMFormData.cpp
index 09a0a6c08..fdb71341c 100644
--- a/src/bun.js/bindings/webcore/JSDOMFormData.cpp
+++ b/src/bun.js/bindings/webcore/JSDOMFormData.cpp
@@ -286,6 +286,8 @@ static inline JSC::EncodedJSValue jsDOMFormDataPrototypeFunction_append1Body(JSC
     RELEASE_AND_RETURN(throwScope, JSValue::encode(toJS<IDLUndefined>(*lexicalGlobalObject, throwScope, [&]() -> decltype(auto) { return impl.append(WTFMove(name), WTFMove(value)); })));
 }
 
+extern "C" BunString Blob__getFileNameString(void* impl);
+
 static inline JSC::EncodedJSValue jsDOMFormDataPrototypeFunction_append2Body(JSC::JSGlobalObject* lexicalGlobalObject, JSC::CallFrame* callFrame, typename IDLOperation<JSDOMFormData>::ClassParameter castedThis)
 {
     auto& vm = JSC::getVM(lexicalGlobalObject);
@@ -298,10 +300,6 @@ static inline JSC::EncodedJSValue jsDOMFormDataPrototypeFunction_append2Body(JSC
     RETURN_IF_EXCEPTION(throwScope, encodedJSValue());
     EnsureStillAliveScope argument1 = callFrame->uncheckedArgument(1);
 
-    EnsureStillAliveScope argument2 = callFrame->argument(2);
-    auto filename = argument2.value().isUndefined() ? String() : convert<IDLUSVString>(*lexicalGlobalObject, argument2.value());
-    RETURN_IF_EXCEPTION(throwScope, encodedJSValue());
-
     RefPtr<Blob> blobValue = nullptr;
     if (argument1.value().inherits<JSBlob>()) {
         blobValue = Blob::create(argument1.value());
@@ -312,6 +310,10 @@ static inline JSC::EncodedJSValue jsDOMFormDataPrototypeFunction_append2Body(JSC
     }
     RETURN_IF_EXCEPTION(throwScope, encodedJSValue());
 
+    EnsureStillAliveScope argument2 = callFrame->argument(2);
+    auto filename = argument2.value().isUndefined() ? Bun::toWTFString(Blob__getFileNameString(blobValue->impl())) : convert<IDLUSVString>(*lexicalGlobalObject, argument2.value());
+    RETURN_IF_EXCEPTION(throwScope, encodedJSValue());
+
     RELEASE_AND_RETURN(throwScope, JSValue::encode(toJS<IDLUndefined>(*lexicalGlobalObject, throwScope, [&]() -> decltype(auto) { return impl.append(WTFMove(name), WTFMove(blobValue), WTFMove(filename)); })));
 }
 
diff --git a/src/bun.js/webcore/blob.zig b/src/bun.js/webcore/blob.zig
index 25f762e97..54c37e679 100644
--- a/src/bun.js/webcore/blob.zig
+++ b/src/bun.js/webcore/blob.zig
@@ -510,11 +510,20 @@ pub const Blob = struct {
         this.finalize();
     }
 
+    export fn Blob__getFileNameString(this: *Blob) callconv(.C) bun.String {
+        if (this.getFileName()) |filename| {
+            return bun.String.fromBytes(filename);
+        }
+
+        return bun.String.empty;
+    }
+
     comptime {
         _ = Blob__dupeFromJS;
         _ = Blob__destroy;
         _ = Blob__dupe;
         _ = Blob__setAsFile;
+        _ = Blob__getFileNameString;
     }
 
     pub fn writeFormatForSize(size: usize, writer: anytype, comptime enable_ansi_colors: bool) !void {