Added MockServer class for tests. (#4843)

* Added a mock OpenAI-compatible endpoint at /v1/chat/completions.
Disabled by default.
Intended for debugging and e2e tests.

* Fixed empty prompts.

* Add mock server and example test

* Improve test

* Added `eslint-plugin-playwright`

* Removed `Date.now()` for reproducible responses.

* Ignore ERR_SERVER_NOT_RUNNING on close.

* Use MockServer in mock-openai.js

* Fixed error check.

* Removed mock server.

* Fix test name

---------

Co-authored-by: user <user@exmaple.com>
Co-authored-by: Cohee <18619528+Cohee1207@users.noreply.github.com>

tests/mock-server.test.js

@@ -0,0 +1,34 @@
+import { describe, test, expect, beforeAll, afterAll } from '@jest/globals';
+import { MockServer } from './util/mock-server.js';
+
+describe('MockServer tests', () => {
+    /** @type {MockServer} */
+    const mockServer = new MockServer({ port: 3000, host: '127.0.0.1' });
+
+    beforeAll(async () => {
+        await mockServer.start();
+    });
+
+    afterAll(async () => {
+        await mockServer.stop();
+    });
+
+    test('should provide OpenAI-compatible endpoint', async () => {
+        const requestBody = {
+            model: 'gpt-4o',
+            max_tokens: 400,
+            messages: [
+                { role: 'user', content: 'Hello, world!' },
+            ],
+        };
+        const response = await fetch('http://127.0.0.1:3000/v1/chat/completions', {
+            method: 'POST',
+            headers: { 'Content-Type': 'application/json' },
+            body: JSON.stringify(requestBody),
+        });
+        const expectedResponse = { 'choices': [{ 'finish_reason': 'stop', 'index': 0, 'message': { 'role': 'assistant', 'reasoning_content': 'gpt-4o\n1\n400', 'content': 'Hello, world!' } }], 'created': 0, 'model': 'gpt-4o' };
+        expect(response.status).toBe(200);
+        const json = await response.json();
+        expect(json).toEqual(expectedResponse);
+    });
+});

tests/util/mock-server.js

@@ -0,0 +1,101 @@
+import http from 'node:http';
+import { readAllChunks, tryParse } from '../../src/util.js';
+
+export class MockServer {
+    /** @type {string} */
+    host;
+    /** @type {number} */
+    port;
+    /** @type {import('http').Server} */
+    server;
+
+    /**
+     * Creates an instance of MockServer.
+     * @param {object} [param] Options object.
+     * @param {string} [param.host] The hostname or IP address to bind the server to.
+     * @param {number} [param.port] The port number to listen on.
+     */
+    constructor({ host, port } = {}) {
+        this.host = host ?? '127.0.0.1';
+        this.port = port ?? 3000;
+    }
+
+    /**
+     * Handles Chat Completions requests.
+     * @param {object} jsonBody The parsed JSON body from the request.
+     * @returns {object} Mock response object.
+     */
+    handleChatCompletions(jsonBody) {
+        const messages = jsonBody?.messages;
+        const lastMessage = messages?.[messages.length - 1];
+        const mockResponse = {
+            choices: [
+                {
+                    finish_reason: 'stop',
+                    index: 0,
+                    message: {
+                        role: 'assistant',
+                        reasoning_content: `${jsonBody?.model}\n${messages?.length}\n${jsonBody?.max_tokens}`,
+                        content: String(lastMessage?.content ?? 'No prompt messages.'),
+                    },
+                },
+            ],
+            created: 0,
+            model: jsonBody?.model,
+        };
+        return mockResponse;
+    }
+
+    /**
+     * Starts the mock server.
+     * @returns {Promise<void>}
+     */
+    async start() {
+        return new Promise((resolve, reject) => {
+            this.server = http.createServer(async (req, res) => {
+                try {
+                    const body = await readAllChunks(req);
+                    const jsonBody = tryParse(body.toString());
+                    if (req.method === 'POST' && req.url === '/v1/chat/completions') {
+                        const mockResponse = this.handleChatCompletions(jsonBody);
+                        res.writeHead(200, { 'Content-Type': 'application/json' });
+                        res.end(JSON.stringify(mockResponse));
+                    } else {
+                        res.writeHead(404);
+                        res.end();
+                    }
+                } catch (error) {
+                    res.writeHead(500);
+                    res.end();
+                }
+            });
+
+            this.server.on('error', (err) => {
+                reject(err);
+            });
+
+            this.server.listen(this.port, this.host, () => {
+                resolve();
+            });
+        });
+    }
+
+    /**
+     * Stops the mock server.
+     * @returns {Promise<void>}
+     */
+    async stop() {
+        return new Promise((resolve, reject) => {
+            if (!this.server) {
+                return reject(new Error('Server is not running.'));
+            }
+            this.server.closeAllConnections();
+            this.server.close(( /** @type {NodeJS.ErrnoException|undefined} */ err) => {
+                if (err && (err?.code !== 'ERR_SERVER_NOT_RUNNING')) {
+                    return reject(err);
+                }
+                resolve();
+            });
+        });
+    }
+}