Add stringifyJson option (#579)

Co-authored-by: Sindre Sorhus <sindresorhus@gmail.com>

Author: Reverier-Xu

readme.md

@@ -428,6 +428,31 @@ const json = await ky('https://example.com', {
 }).json();
 ```
 
+##### stringifyJson
+
+Type: `Function`\
+Default: `JSON.stringify()`
+
+User-defined JSON-stringifying function.
+
+Use-cases:
+1. Stringify JSON with a custom `replacer` function.
+
+```js
+import ky from 'ky';
+import {DateTime} from 'luxon';
+
+const json = await ky('https://example.com', {
+	stringifyJson: data => JSON.stringify(data, (key, value) => {
+		if (key.endsWith('_at')) {
+			return DateTime.fromISO(value).toSeconds();
+		}
+
+		return value;
+	})
+}).json();
+```
+
 ##### fetch
 
 Type: `Function`\

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.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 a 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, (key, value) => {
+			if (key.endsWith('_at')) {
+				return DateTime.fromISO(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 +733,23 @@ 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});
+
+	const json = {hello: 'world'};
+	const extra = 'extraValue';
+
+	server.post('/', async (request, response) => {
+		const body = await parseRawBody(request);
+		t.is(body, JSON.stringify({data: json, extra}));
+		response.end();
+	});
+
+	await ky.post(server.url, {
+		stringifyJson: data => JSON.stringify({data, extra}),
+		json,
+	});
+
+	await server.close();
+});