Extract key from props in h() for UElement, keep key in props for URoot

src/core/h.ts

@@ -3,18 +3,19 @@ import type { UNode, UElement, URoot, UType, UComponent, UniversalProps } from "
 let _idCounter = 0;
 
 export function h(type: UType, props?: UniversalProps | null, ...children: UNode[]): UElement | URoot {
-  const { key, ...restProps } = props ?? {};
-  const resolvedProps: UniversalProps = restProps;
   const flatChildren = children.flat(Infinity as 1).filter((c: UNode) => c != null && c !== false) as UNode[];
 
   if (type === "root") {
     return {
       type: "root",
-      props: resolvedProps,
+      props: { ...(props ?? {}) },
       children: flatChildren,
     } as URoot;
   }
 
+  const { key, ...restProps } = props ?? {};
+  const resolvedProps: UniversalProps = restProps;
+
   return {
     type: type as string,
     props: resolvedProps,

test/mod.test.ts

@@ -100,6 +100,34 @@ describe("type guards", () => {
 });
 
 describe("UElement key field", () => {
+  it("h('div', { key: 'a' }) produces UElement with key: 'a' and no key in props", () => {
+    const el = h("div", { key: "a" });
+    expect(isUElement(el)).toBe(true);
+    if (isUElement(el)) {
+      expect(el.key).toBe("a");
+      expect(el.props.key).toBeUndefined();
+    }
+  });
+
+  it("h('div', { key: 'b', class: 'x' }) — key promoted, class remains in props", () => {
+    const el = h("div", { key: "b", class: "x" });
+    expect(isUElement(el)).toBe(true);
+    if (isUElement(el)) {
+      expect(el.key).toBe("b");
+      expect(el.props.key).toBeUndefined();
+      expect(el.props.class).toBe("x");
+    }
+  });
+
+  it("h('div', null) — key is undefined, no key in props", () => {
+    const el = h("div", null);
+    expect(isUElement(el)).toBe(true);
+    if (isUElement(el)) {
+      expect(el.key).toBeUndefined();
+      expect(el.props).toEqual({});
+    }
+  });
+
   it("h() extracts key from props and promotes to element level", () => {
     const el = h("div", { key: "item-1", class: "test" }, "hello");
     expect(isUElement(el)).toBe(true);
@@ -144,11 +172,11 @@ describe("UElement key field", () => {
   });
 
   it("h() with root type does not promote key to URoot", () => {
-    const root = h("root", { key: "should-not-appear", id: "test" }, "child");
+    const root = h("root", { key: "x", id: "test" }, "child");
     expect(isURoot(root)).toBe(true);
     if (isURoot(root)) {
       expect("key" in root).toBe(false);
-      expect(root.props.key).toBeUndefined();
+      expect(root.props.key).toBe("x");
     }
   });
 });