feat: add support for stringifyJson

source/core/Ky.ts

@@ -203,7 +203,7 @@ export class Ky {
 		}
 
 		if (this._options.json !== undefined) {
-			this._options.body = JSON.stringify(this._options.json);
+			this._options.body = this._options.stringifyJson ? this._options.stringifyJson(this._options.json) : JSON.stringify(this._options.json);
 			this.request.headers.set('content-type', this._options.headers.get('content-type') ?? 'application/json');
 			this.request = new globalThis.Request(this.request, {body: this._options.body});
 		}

source/core/constants.ts

@@ -50,6 +50,7 @@ export const stop = Symbol('stop');
 export const kyOptionKeys: KyOptionsRegistry = {
 	json: true,
 	parseJson: true,
+	stringifyJson: true,
 	searchParams: true,
 	prefixUrl: true,
 	retry: true,

source/types/options.ts

@@ -58,6 +58,32 @@ export type KyOptions = {
 	*/
 	parseJson?: (text: string) => unknown;
 
+	/**
+	User-defined JSON-stringifying function.
+
+	Use-cases:
+	1. Stringify JSON with the custom `replacer` function.
+
+	@default JSON.stringify()
+
+	@example
+	```
+	import ky from 'ky';
+	import { DateTime } from 'luxon';
+
+	const json = await ky('https://example.com', {
+		stringifyJson: data => JSON.stringify(data, (_, value) => {
+			if (value instanceof DateTime) {
+				return value.toSeconds();
+			}
+
+			return value;
+		})
+	}).json();
+	```
+	 */
+	stringifyJson?: (data: unknown) => string;
+
 	/**
 	Search parameters to include in the request URL. Setting this will override all existing search parameters in the input URL.
 

test/main.ts

@@ -733,3 +733,24 @@ test('parseJson option with promise.json() shortcut', async t => {
 
 	await server.close();
 });
+
+test('stringifyJson option with request.json()', async t => {
+	const server = await createHttpTestServer({bodyParser: false});
+	server.post('/', async (request, response) => {
+		const body = await parseRawBody(request);
+		t.is(body, '{"data":{"hello":"world"},"extra":"extraValue"}');
+		response.end();
+	});
+
+	const json = {hello: 'world'};
+
+	await ky.post(server.url, {
+		stringifyJson: data => JSON.stringify({
+			data,
+			extra: 'extraValue',
+		}),
+		json,
+	});
+
+	await server.close();
+});