Do not set content-type on body-less requests (#1826)

* Do not set content-type on body-less requests

* Add docs

* Remove commented code

* Implement CR suggestions

* Add change-set

* Fix change-set

* Post-rebase fix

Author: goce-cz

Diff for: .changeset/chilled-forks-rush.md

@@ -0,0 +1,5 @@
+---
+"openapi-fetch": minor
+---
+
+Do not set content-type on body-less requests

Diff for: docs/openapi-fetch/api.md

@@ -181,6 +181,17 @@ const { data, error } = await client.PUT("/submit", {
 });
 ```
 
+::: tip
+
+For convenience, `openapi-fetch` sets `Content-Type` to `application/json` automatically
+for any request that provides value for the `body` parameter. When the `bodySerializer` returns an instance of `FormData`,
+`Content-Type` is omitted, allowing the browser to set it automatically with the correct message part boundary.
+
+You can also set `Content-Type` manually through `headers` object either in the fetch options,
+or when instantiating the client.
+
+:::
+
 ## Path serialization
 
 openapi-fetch supports path serialization as [outlined in the 3.1 spec](https://swagger.io/docs/specification/serialization/#path). This happens automatically, based on the specific format in your OpenAPI schema:

Diff for: packages/openapi-fetch/src/index.js

@@ -1,8 +1,4 @@
 // settings & const
-const DEFAULT_HEADERS = {
-  "Content-Type": "application/json",
-};
-
 const PATH_PARAM_RE = /\{[^{}]+\}/g;
 
 /** Add custom parameters to Request object */
@@ -41,7 +37,6 @@ export default function createClient(clientOptions) {
     ...baseOptions
   } = { ...clientOptions };
   baseUrl = removeTrailingSlash(baseUrl);
-  baseHeaders = mergeHeaders(DEFAULT_HEADERS, baseHeaders);
   const middlewares = [];
 
   /**
@@ -58,6 +53,7 @@ export default function createClient(clientOptions) {
       parseAs = "json",
       querySerializer: requestQuerySerializer,
       bodySerializer = globalBodySerializer ?? defaultBodySerializer,
+      body,
       ...init
     } = fetchOptions || {};
     if (localBaseUrl) {
@@ -78,19 +74,25 @@ export default function createClient(clientOptions) {
             });
     }
 
+    const serializedBody = body === undefined ? undefined : bodySerializer(body);
+
+    const defaultHeaders =
+      // 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
+        ? {}
+        : {
+            "Content-Type": "application/json",
+          };
+
     const requestInit = {
       redirect: "follow",
       ...baseOptions,
       ...init,
-      headers: mergeHeaders(baseHeaders, headers, params.header),
+      body: serializedBody,
+      headers: mergeHeaders(defaultHeaders, baseHeaders, headers, params.header),
     };
-    if (requestInit.body !== undefined) {
-      requestInit.body = bodySerializer(requestInit.body);
-      // remove `Content-Type` if serialized body is FormData; browser will correctly set Content-Type & boundary expression
-      if (requestInit.body instanceof FormData) {
-        requestInit.headers.delete("Content-Type");
-      }
-    }
 
     let id;
     let options;

Diff for: packages/openapi-fetch/test/index.test.ts

@@ -4,14 +4,15 @@ import createClient, {
   type BodySerializer,
   type FetchOptions,
   type MethodResponse,
+  type HeadersOptions,
   type Middleware,
   type MiddlewareCallbackParams,
   type QuerySerializerOptions,
   type Client,
   type PathBasedClient,
   createPathBasedClient,
 } from "../src/index.js";
-import { server, baseUrl, useMockRequestHandler, toAbsoluteURL } from "./fixtures/mock-server.js";
+import { baseUrl, server, toAbsoluteURL, useMockRequestHandler } from "./fixtures/mock-server.js";
 import type { paths } from "./fixtures/api.js";
 
 beforeAll(() => {
@@ -819,12 +820,7 @@ describe("client", () => {
         await client.GET("/self");
 
         // assert default headers were passed
-        expect(getRequest().headers).toEqual(
-          new Headers({
-            ...headers, // assert new header got passed
-            "Content-Type": "application/json", //  probably doesn’t need to get tested, but this was simpler than writing lots of code to ignore these
-          }),
-        );
+        expect(getRequest().headers).toEqual(new Headers(headers));
       });
 
       it("can be overridden", async () => {
@@ -850,7 +846,6 @@ describe("client", () => {
         expect(getRequest().headers).toEqual(
           new Headers({
             "Cache-Control": "no-cache",
-            "Content-Type": "application/json",
           }),
         );
       });
@@ -894,6 +889,139 @@ describe("client", () => {
       });
     });
 
+    describe("content-type", () => {
+      const BODY_ACCEPTING_METHODS = [["PUT"], ["POST"], ["DELETE"], ["OPTIONS"], ["PATCH"]] as const;
+      const ALL_METHODS = [...BODY_ACCEPTING_METHODS, ["GET"], ["HEAD"]] as const;
+
+      const fireRequestAndGetContentType = async (options: {
+        defaultHeaders?: HeadersOptions;
+        method: (typeof ALL_METHODS)[number][number];
+        fetchOptions: FetchOptions<any>;
+      }) => {
+        const client = createClient<any>({ baseUrl, headers: options.defaultHeaders });
+        const { getRequest } = useMockRequestHandler({
+          baseUrl,
+          method: "all",
+          path: "/blogposts-optional",
+          status: 200,
+        });
+        await client[options.method]("/blogposts-optional", options.fetchOptions as any);
+
+        const request = getRequest();
+        return request.headers.get("content-type");
+      };
+
+      it.each(ALL_METHODS)("no content-type for body-less requests - %s", async (method) => {
+        const contentType = await fireRequestAndGetContentType({
+          method,
+          fetchOptions: {},
+        });
+
+        expect(contentType).toBe(null);
+      });
+
+      it.each(ALL_METHODS)("no content-type for `undefined` body requests - %s", async (method) => {
+        const contentType = await fireRequestAndGetContentType({
+          method,
+          fetchOptions: {
+            body: undefined,
+          },
+        });
+
+        expect(contentType).toBe(null);
+      });
+
+      const BODIES = [{ prop: "a" }, {}, "", "str", null, false, 0, 1, new Date("2024-08-07T09:52:00.836Z")] as const;
+      const METHOD_BODY_COMBINATIONS = BODY_ACCEPTING_METHODS.flatMap(([method]) =>
+        BODIES.map((body) => [method, body] as const),
+      );
+
+      it.each(METHOD_BODY_COMBINATIONS)(
+        "implicit default content-type for body-full requests - %s, %j",
+        async (method, body) => {
+          const contentType = await fireRequestAndGetContentType({
+            method,
+            fetchOptions: {
+              body,
+            },
+          });
+
+          expect(contentType).toBe("application/json");
+        },
+      );
+
+      it.each(METHOD_BODY_COMBINATIONS)(
+        "provided default content-type for body-full requests - %s, %j",
+        async (method, body) => {
+          const contentType = await fireRequestAndGetContentType({
+            defaultHeaders: {
+              "content-type": "application/my-json",
+            },
+            method,
+            fetchOptions: {
+              body,
+            },
+          });
+
+          expect(contentType).toBe("application/my-json");
+        },
+      );
+
+      it.each(METHOD_BODY_COMBINATIONS)(
+        "native-fetch default content-type for body-full requests, when default is suppressed - %s, %j",
+        async (method, body) => {
+          const contentType = await fireRequestAndGetContentType({
+            defaultHeaders: {
+              "content-type": null,
+            },
+            method,
+            fetchOptions: {
+              body,
+            },
+          });
+          // the fetch implementation won't allow sending a body without content-type,
+          // and it defaults to `text/plain;charset=UTF-8`, however the actual default value
+          // is irrelevant and might be flaky across different fetch implementations
+          // for us, it's important that it's not `application/json`
+          expect(contentType).not.toBe("application/json");
+        },
+      );
+
+      it.each(METHOD_BODY_COMBINATIONS)(
+        "specified content-type for body-full requests - %s, %j",
+        async (method, body) => {
+          const contentType = await fireRequestAndGetContentType({
+            method,
+            fetchOptions: {
+              body,
+              headers: {
+                "content-type": "application/my-json",
+              },
+            },
+          });
+
+          expect(contentType).toBe("application/my-json");
+        },
+      );
+
+      it.each(METHOD_BODY_COMBINATIONS)(
+        "specified content-type for body-full requests, even when default is suppressed - %s, %j",
+        async (method, body) => {
+          const contentType = await fireRequestAndGetContentType({
+            method,
+            fetchOptions: {
+              body,
+              headers: {
+                "content-type": "application/my-json",
+              },
+            },
+          });
+
+          expect(contentType).toBe("application/my-json");
+        },
+      );
+    });
+
     describe("fetch", () => {
       it("createClient", async () => {
         function createCustomFetch(data: any) {