Flatten undefined properties correctly (#2276)

Author: ericallam

packages/core/src/v3/utils/flattenAttributes.ts

@@ -209,6 +209,10 @@ class AttributeFlattener {
       return;
     }
 
+    if (value === undefined) {
+      return;
+    }
+
     // Handle primitive values directly
     if (value === null) {
       this.addAttribute(prefix, NULL_SENTINEL);

packages/core/test/flattenAttributes.test.ts

@@ -61,6 +61,11 @@ describe("flattenAttributes", () => {
     expect(unflattenAttributes(result)).toEqual(input);
   });
 
+  it("flattens properties that are undefined correctly", () => {
+    const result = flattenAttributes({ foo: undefined, bar: "baz" });
+    expect(result).toEqual({ bar: "baz" });
+  });
+
   it("flattens complex objects correctly", () => {
     const obj = {
       level1: {