fix(openapi-fetch): pass string bodies through without JSON serialization

.changeset/fix-string-body-serializer.md

@@ -0,0 +1,5 @@
+---
+"openapi-fetch": patch
+---
+
+Fix `defaultBodySerializer` double-encoding string bodies. Previously, passing a string as the request body (e.g. a pre-serialized JSON string) would result in it being wrapped in an extra layer of JSON quotes. Strings are now passed through as-is.

packages/openapi-fetch/biome.json

@@ -3,7 +3,7 @@
   "$schema": "https://biomejs.dev/schemas/2.3.14/schema.json",
   "extends": "//",
   "files": {
-    "includes": ["src/**", "test/**", "!**/examples/**", "!test/**/schemas/**", "!test/bench/**/*.min.js"]
+    "includes": ["src/**", "test/**", "!**/examples", "!test/**/schemas", "!test/bench/**/*.min.js"]
   },
   "linter": {
     "rules": {

packages/openapi-fetch/src/index.js

@@ -4,7 +4,7 @@ const PATH_PARAM_RE = /\{[^{}]+\}/g;
 const supportsRequestInitExt = () => {
   return (
     typeof process === "object" &&
-    Number.parseInt(process?.versions?.node?.substring(0, 2)) >= 18 &&
+    Number.parseInt(process?.versions?.node?.substring(0, 2), 10) >= 18 &&
     process.versions.undici
   );
 };
@@ -625,6 +625,9 @@ export function defaultBodySerializer(body, headers) {
   if (body instanceof FormData) {
     return body;
   }
+  if (typeof body === "string") {
+    return body;
+  }
   if (headers) {
     const contentType =
       headers.get instanceof Function

packages/openapi-fetch/test/common/create-client-e2e.test.js

@@ -67,7 +67,7 @@ const caToBuffer = (ca) => {
 const API_PORT = process.env.API_PORT || 4578;
 
 const app = express();
-app.get("/v1/foo", (req, res) => {
+app.get("/v1/foo", (_req, res) => {
   res.send("bar");
 });
 

packages/openapi-fetch/test/common/request.test.ts

@@ -236,7 +236,19 @@ describe("request", () => {
       });
 
       expect(bodyUsed).toBe(true);
-      expect(bodyText).toBe('""');
+      expect(bodyText).toBe("");
+    });
+
+    test.each(BODY_ACCEPTING_METHODS)("string body is passed through without JSON serialization - %s", async (method) => {
+      const { bodyUsed, bodyText } = await fireRequestAndGetBodyInformation({
+        method,
+        fetchOptions: {
+          body: "pre-serialized string",
+        },
+      });
+
+      expect(bodyUsed).toBe(true);
+      expect(bodyText).toBe("pre-serialized string");
     });
 
     test.each(BODY_ACCEPTING_METHODS)("`0` body (with body serializer) - %s", async (method) => {

packages/openapi-fetch/test/common/response.test.ts

@@ -127,7 +127,7 @@ describe("response", () => {
 
   describe("response object", () => {
     test.each([200, 404, 500] as const)("%s", async (status) => {
-      const client = createObservedClient<paths>({}, async (req) =>
+      const client = createObservedClient<paths>({}, async (_req) =>
         Response.json({ status, message: "OK" }, { status }),
       );
       const result = await client.GET(status === 200 ? "/resources" : `/error-${status}`);

packages/openapi-fetch/test/e2e/index.e2e.ts

@@ -1,4 +1,4 @@
-import { expect, type Page, test } from "@playwright/test";
+import { type Page, test } from "@playwright/test";
 
 // note: these tests load Chrome, Firefox, and Safari in Playwright to test a browser-realistic runtime.
 // the frontend is prepared via Vite to create a production-accurate app (and throw add’l type errors)

packages/openapi-fetch/test/http-methods/get.test.ts

@@ -1,6 +1,6 @@
 import { describe, expect, test } from "vitest";
 import { createObservedClient } from "../helpers.js";
-import type { components, paths } from "./schemas/get.js";
+import type { paths } from "./schemas/get.js";
 
 describe("GET", () => {
   test("sends correct method", async () => {

packages/openapi-fetch/test/middleware/middleware.test.ts

@@ -98,7 +98,7 @@ test("can modify response", async () => {
 
 test("returns original errors if nothing is returned", async () => {
   const actualError = new Error();
-  const client = createObservedClient<paths>({}, async (req) => {
+  const client = createObservedClient<paths>({}, async (_req) => {
     throw actualError;
   });
   client.use({