fix: normalize headers reassigned in an onRequest hook

src/fetch.ts

@@ -109,6 +109,12 @@ export function createFetch(globalOptions: CreateFetchOptions = {}): $Fetch {
 
     if (context.options.onRequest) {
       await callHooks(context, context.options.onRequest);
+      // A hook may reassign `headers` to a plain object or array (a valid
+      // `HeadersInit`), so normalize it back to a `Headers` instance before the
+      // code below calls `.get()`/`.set()`/`.has()` on it.
+      if (!(context.options.headers instanceof Headers)) {
+        context.options.headers = new Headers(context.options.headers || {});
+      }
     }
 
     if (typeof context.request === "string") {
@@ -143,8 +149,6 @@ export function createFetch(globalOptions: CreateFetchOptions = {}): $Fetch {
 
         // Set Content-Type and Accept headers to application/json by default
         // for JSON serializable request bodies.
-        // Pass empty object as older browsers don't support undefined.
-        context.options.headers = new Headers(context.options.headers || {});
         if (!contentType) {
           context.options.headers.set("content-type", "application/json");
         }

test/index.test.ts

@@ -397,6 +397,32 @@ describe("ofetch", () => {
     ).toMatchObject({ foo: "2", bar: "3" });
   });
 
+  it("normalizes headers reassigned in onRequest hook", async () => {
+    // A `Headers` instance reassigned in the hook is sent as-is.
+    expect(
+      await $fetch(getURL("/echo"), {
+        method: "POST",
+        body: { num: 42 },
+        onRequest(ctx) {
+          ctx.options.headers = new Headers({ "x-foo": "bar" });
+        },
+      }).then((r) => r.headers)
+    ).toMatchObject({ "x-foo": "bar" });
+
+    // A plain object reassigned in the hook is normalized back to `Headers`
+    // instead of throwing `headers.get is not a function` before the request.
+    expect(
+      await $fetch(getURL("/echo"), {
+        method: "POST",
+        body: { num: 42 },
+        onRequest(ctx) {
+          // @ts-expect-error backwards compatibility for object headers
+          ctx.options.headers = { "x-foo": "bar" };
+        },
+      }).then((r) => r.headers)
+    ).toMatchObject({ "x-foo": "bar" });
+  });
+
   it("hook errors", async () => {
     // onRequest
     await expect(