override request dispatcher from init (#2928)

lib/web/fetch/request.js

@@ -103,7 +103,7 @@ class Request {
       // 5. Set fallbackMode to "cors".
       fallbackMode = 'cors'
     } else {
-      this[kDispatcher] = input[kDispatcher]
+      this[kDispatcher] = init.dispatcher || input[kDispatcher]
 
       // 6. Otherwise:
 

test/fetch/issue-2898-comment.js

@@ -0,0 +1,42 @@
+'use strict'
+
+const { once } = require('node:events')
+const { createServer } = require('node:http')
+const { test } = require('node:test')
+const { Agent, Request, fetch } = require('../..')
+const { tspl } = require('@matteo.collina/tspl')
+
+test('issue #2828, RequestInit dispatcher options overrides Request input dispatcher', async (t) => {
+  const { strictEqual } = tspl(t, { plan: 2 })
+
+  class CustomAgentA extends Agent {
+    dispatch (options, handler) {
+      options.headers['x-my-header-a'] = 'hello'
+      return super.dispatch(...arguments)
+    }
+  }
+
+  class CustomAgentB extends Agent {
+    dispatch (options, handler) {
+      options.headers['x-my-header-b'] = 'world'
+      return super.dispatch(...arguments)
+    }
+  }
+
+  const server = createServer((req, res) => {
+    strictEqual(req.headers['x-my-header-a'], undefined)
+    strictEqual(req.headers['x-my-header-b'], 'world')
+    res.end()
+  }).listen(0)
+
+  t.after(server.close.bind(server))
+  await once(server, 'listening')
+
+  const request = new Request(`http://localhost:${server.address().port}`, {
+    dispatcher: new CustomAgentA()
+  })
+
+  await fetch(request, {
+    dispatcher: new CustomAgentB()
+  })
+})