http: enable call chaining with setHeader()

Make `response.setHeader` return the response object itself
so that multiple header setting can be chained.

Fixes: nodejs#33148

PR-URL: nodejs#35924
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ricky Zhou <0x19951125@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

doc/api/http.md

@@ -1597,13 +1597,17 @@ added: v0.4.0
 
 * `name` {string}
 * `value` {any}
+* Returns: {http.ServerResponse}
+
+Returns the response object.
 
 Sets a single header value for implicit headers. If this header already exists
 in the to-be-sent headers, its value will be replaced. Use an array of strings
 here to send multiple headers with the same name. Non-string values will be
 stored without modification. Therefore, [`response.getHeader()`][] may return
 non-string values. However, the non-string values will be converted to strings
-for network transmission.
+for network transmission. The same response object is returned to the caller,
+to enable call chaining.
 
 ```js
 response.setHeader('Content-Type', 'text/html');

lib/_http_outgoing.js

@@ -570,6 +570,7 @@ OutgoingMessage.prototype.setHeader = function setHeader(name, value) {
     this[kOutHeaders] = headers = ObjectCreate(null);
 
   headers[name.toLowerCase()] = [name, value];
+  return this;
 };
 
 

test/parallel/test-http-set-header-chain.js

@@ -0,0 +1,29 @@
+'use strict';
+const common = require('../common');
+const http = require('http');
+const assert = require('assert');
+const expected = {
+  '__proto__': null,
+  'testheader1': 'foo',
+  'testheader2': 'bar',
+  'testheader3': 'xyz'
+};
+const server = http.createServer(common.mustCall((req, res) => {
+  let retval = res.setHeader('testheader1', 'foo');
+
+  // Test that the setHeader returns the same response object.
+  assert.strictEqual(retval, res);
+
+  retval = res.setHeader('testheader2', 'bar').setHeader('testheader3', 'xyz');
+  // Test that chaining works for setHeader.
+  assert.deepStrictEqual(res.getHeaders(), expected);
+  res.end('ok');
+}));
+server.listen(0, () => {
+  http.get({ port: server.address().port }, common.mustCall((res) => {
+    res.on('data', () => {});
+    res.on('end', common.mustCall(() => {
+      server.close();
+    }));
+  }));
+});