Improve header handling (#2308)

Author: drwpow

.changeset/bumpy-parrots-lie.md

@@ -0,0 +1,5 @@
+---
+"openapi-fetch": patch
+---
+
+Improve header handling

package.json

@@ -30,7 +30,7 @@
     "size-limit": "^11.2.0",
     "turbo": "^2.5.2",
     "typescript": "^5.8.3",
-    "vitest": "^3.0.0"
+    "vitest": "^3.1.3"
   },
   "size-limit": [
     {

packages/openapi-fetch/src/index.js

@@ -73,24 +73,38 @@ export default function createClient(clientOptions) {
             });
     }
 
-    const serializedBody = body === undefined ? undefined : bodySerializer(body, headers);
-
-    const defaultHeaders =
+    const serializedBody =
+      body === undefined
+        ? undefined
+        : bodySerializer(
+            body,
+            // Note: we declare mergeHeaders() both here and below because it’s a bit of a chicken-or-egg situation:
+            // bodySerializer() needs all headers so we aren’t dropping ones set by the user, however,
+            // the result of this ALSO sets the lowest-priority content-type header. So we re-merge below,
+            // setting the content-type at the very beginning to be overwritten.
+            // Lastly, based on the way headers work, it’s not a simple “present-or-not” check becauase null intentionally un-sets headers.
+            mergeHeaders(baseHeaders, headers, params.header),
+          );
+    const finalHeaders = mergeHeaders(
       // with no body, we should not to set Content-Type
       serializedBody === undefined ||
-      // if serialized body is FormData; browser will correctly set Content-Type & boundary expression
-      serializedBody instanceof FormData
+        // if serialized body is FormData; browser will correctly set Content-Type & boundary expression
+        serializedBody instanceof FormData
         ? {}
         : {
             "Content-Type": "application/json",
-          };
+          },
+      baseHeaders,
+      headers,
+      params.header,
+    );
 
     const requestInit = {
       redirect: "follow",
       ...baseOptions,
       ...init,
       body: serializedBody,
-      headers: mergeHeaders(defaultHeaders, baseHeaders, headers, params.header),
+      headers: finalHeaders,
     };
 
     let id;

packages/openapi-metadata/package.json

@@ -68,7 +68,7 @@
   },
   "devDependencies": {
     "@types/node": "^22.15.3",
-    "@vitest/coverage-v8": "^3.0.0",
+    "@vitest/coverage-v8": "^3.1.3",
     "reflect-metadata": "^0.2.2",
     "typescript": "^5.8.3",
     "unbuild": "^3.5.0",