Update body after beforeRequest

readme.md

@@ -740,6 +740,24 @@ Default: `[]`
 
 Called with [normalized](source/core/index.ts) [request options](#options). Got will make no further changes to the request before it is sent. This is especially useful in conjunction with [`got.extend()`](#instances) when you want to create an API client that, for example, uses HMAC-signing.
 
+**Note:** Changing `options.json` or `options.form` has no effect on the request, you should change `options.body` instead. If needed, update the `options.headers` accordingly. Example:
+
+```js
+const got = require('got');
+
+got.post({
+	json: {payload: 'old'},
+	hooks: {
+		beforeRequest: [
+			options => {
+				options.body = JSON.stringify({payload: 'new'});
+				options.headers['content-length'] = options.body.length.toString();
+			}
+		]
+	}
+});
+```
+
 **Tip:** You can override the `request` function by returning a [`ClientRequest`-like](https://nodejs.org/api/http.html#http_class_http_clientrequest) instance or a [`IncomingMessage`-like](https://nodejs.org/api/http.html#http_class_http_incomingmessage) instance. This is very useful when creating a custom cache mechanism.
 
 ###### hooks.beforeRedirect

source/core/index.ts

@@ -2308,6 +2308,10 @@ export default class Request extends Duplex implements RequestEvents<Request> {
 			}
 		}
 
+		if (options.body && this[kBody] !== options.body) {
+			this[kBody] = options.body;
+		}
+
 		const {agent, request, timeout, url} = options;
 
 		if (options.dnsCache && !('lookup' in options)) {

test/hooks.ts

@@ -17,6 +17,10 @@ const echoHeaders: Handler = (request, response) => {
 	response.end(JSON.stringify(request.headers));
 };
 
+const echoBody: Handler = async (request, response) => {
+	response.end(await getStream(request));
+};
+
 const echoUrl: Handler = (request, response) => {
 	response.end(request.url);
 };
@@ -1194,3 +1198,21 @@ test('no duplicate hook calls when returning original request options', withServ
 	t.is(beforeHookCount, 4);
 	t.is(afterHookCount, 4);
 });
+
+test('`beforeRequest` change body', withServer, async (t, server, got) => {
+	server.post('/', echoBody);
+
+	const response = await got.post({
+		json: {payload: 'old'},
+		hooks: {
+			beforeRequest: [
+				options => {
+					options.body = JSON.stringify({payload: 'new'});
+					options.headers['content-length'] = options.body.length.toString();
+				}
+			]
+		}
+	});
+
+	t.is(JSON.parse(response.body).payload, 'new');
+});