fix: add body stringification for application/json content-type

packages/better-fetch/src/test/fetch.test.ts

@@ -114,6 +114,18 @@ describe("fetch", () => {
 		expect(data?.body).to.deep.eq(message);
 	});
 
+	it("stringifies body when content-type is explicitly application/json", async () => {
+		const payload = { foo: "bar" };
+		const { data } = await $echo<any>("/echo", {
+			method: "POST",
+			body: payload,
+			headers: { "Content-Type": "application/json" },
+		});
+
+		expect(data?.body).toEqual(payload);
+		expect(data?.headers).to.include({ "content-type": "application/json" });
+	});
+
 	it("Bypass URLSearchParams body", async () => {
 		const data = new URLSearchParams({ foo: "bar" });
 		const { data: res } = await betterFetch<any>(getURL("post"), {

packages/better-fetch/src/utils.ts

@@ -212,19 +212,27 @@ export function getBody(options?: BetterFetchOption) {
 	if (!options?.body) {
 		return null;
 	}
-
-	let hasContentType = false;
+	let contentType: string | undefined = undefined;
 	if (options?.headers) {
 		if (options.headers instanceof Headers) {
-			hasContentType = options.headers.has("content-type");
+			contentType = options.headers.get("content-type") ?? undefined;
 		} else if (typeof options.headers === "object") {
-			hasContentType = "content-type" in options.headers && 
-				options.headers["content-type"] !== null && 
-				options.headers["content-type"] !== undefined;
+			if (
+				"content-type" in options.headers &&
+				options.headers["content-type"] !== null &&
+				options.headers["content-type"] !== undefined
+			) {
+				contentType = options.headers["content-type"];
+			}
 		}
 	}
-
-	if (isJSONSerializable(options.body) && !hasContentType) {
+
+	const hasContentType = contentType !== undefined;
+	const isJSONContentType = contentType !== undefined && JSON_RE.test(contentType);
+	if (
+		isJSONSerializable(options.body) &&
+		(!hasContentType || isJSONContentType)
+	) {
 		for (const [key, value] of Object.entries(options?.body)) {
 			if (value instanceof Date) {
 				options.body[key] = value.toISOString();