Coding⏱️ 3 min read📅 2026-06-04

How to Fix: WebSocket connection failed: Error during WebSocket handshake: Unexpected response code: 400

Check the WebSocket connection settings in your client-side code.

Quick Answer: Ensure that the WebSocket connection is established correctly on the client-side, and verify that the server is listening for incoming connections.

📋 Table of Contents

  1. 🔍 Why This Happens
  2. 🛠️ Step-by-Step Verified Fixes
  3. 🎯 Final Words

The WebSocket connection failed error occurs when the client-side and server-side cannot establish a successful connection. This issue can affect anyone using your application, regardless of their device or browser.

This error is frustrating because it prevents users from accessing certain features or functionality within your application. Fortunately, this issue can be resolved with the right steps.

🔍 Why This Happens

  • The primary reason for this error is that the server-side code is not properly configured to handle WebSocket connections. In this case, the `http.createServer(app)` function is used instead of `express.listen()`, which can cause issues with establishing a secure connection.
  • Another possible cause is that the client-side JavaScript code is not correctly implementing the WebSocket protocol. Specifically, the `ws` library or Socket.io may not be properly initialized or configured.

🛠️ Step-by-Step Verified Fixes

Configuring Server-Side Code

  1. Step 1: Use `express.listen()` instead of `http.createServer(app)` to ensure that the server is listening on the correct port.
  2. Step 2: Verify that the `io` object is properly initialized and configured using Socket.io.
  3. Step 3: Check for any errors or warnings in the console output when attempting to establish a WebSocket connection.

Configuring Client-Side Code

  1. Step 1: Ensure that the client-side JavaScript code is correctly importing and initializing the `ws` library or Socket.io.
  2. Step 2: Verify that the `socket.io-client` library is properly configured with the correct server URL and port number.
  3. Step 3: Check for any syntax errors or typos in the client-side code that may be preventing the WebSocket connection from being established.

🎯 Final Words

To resolve the WebSocket connection failed error, you can try configuring your server-side code to use `express.listen()` and verifying that your client-side JavaScript code is correctly implementing the WebSocket protocol. If issues persist, consider seeking further assistance or consulting with a developer who has experience with Socket.io and Angular.

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