How do you set up a local testing server?

Throughout most of the learning area, we tell you to just open your examples directly in a browser — this can be done by double-clicking the HTML file, dragging and dropping it into the browser window, or choosing File > Open… and navigating to the HTML file. There are many ways to achieve this.

If the web address path starts with file:// followed by the path to the file on your local hard drive, a local file is being used. In contrast, if you view one of our examples hosted on GitHub (or an example on some other remote server), the web address will start with http:// or https://, to show that the file has been received via HTTP.

Some examples won't run if you open them as local files. This can be due to a variety of reasons, the most likely being:

• They feature asynchronous requests. Some browsers (including Chrome) will not run async requests (see Fetching data from the server) if you just run the example from a local file. This is because of security restrictions (for more on web security, read Website security).
• They feature a server-side language. Server-side languages (such as PHP or Python) require a special server to interpret the code and deliver the results.
• They include other files. Browsers commonly treat requests to load resources using the file:// schema as cross-origin requests. So if you load a local file that includes other local files, this may trigger a CORS error.

To get around the problem of async requests, we need to test such examples by running them through a local web server.

If you only need HTML, CSS and JavaScript, and no server-side language, the easiest way may be to check for extensions in your code editor. As well as automating installation and set-up for your local HTTP server, they also integrate nicely with your code editors. Testing local files in an HTTP server may be one click away.

For VSCode, you can check the following free extension:

• vscode-preview-server. You can check it on its home page.

The Node.js http-server module is an easiest way to host HTML files in any directory.

To use the module:

1. Run the following commands to check if Node.js is already installed:
   bash

   node -v
   npm -v
   npx -v
   

2. If Node.js is not installed, you need to install it. Follow the download instructions in the Node.js docs, then run the above commands again to check if the installation is successful.
3. Let's assume the directory is /path/to/project. Run the following command to start the server:
   bash

   npx http-server /path/to/project -o -p 9999
   

   This hosts all files in the /path/to/project directory on localhost:9999. The option -o will open the index.html page in a web browser. If index.html doesn't exist, then the directory is displayed instead.

Another way to achieve this is to use Python's http.server module.

To do this:

1. Install Python. If you are using Linux or macOS, it should be available on your system already. If you are a Windows user, you can get an installer from the Python homepage and follow the instructions to install it:
   • Go to python.org
   • Under the Download section, click the link for Python "3.xxx".
   • At the bottom of the page, click the Windows Installer link to download the installer file.
   • When it has downloaded, run it.
   • On the first installer page, make sure you check the "Add Python 3.xxx to PATH" checkbox.
   • Click Install, then click Close when the installation has finished.
2. Open your command prompt (Windows) / terminal (macOS/ Linux). To check if Python is installed, enter the following command:
   bash

   python -V
   # If the above fails, try:
   python3 -V
   # Or, if the "py" command is available, try:
   py -V
   

3. This should return a version number. If this is OK, navigate to the directory that contains the website code you want to test, using the cd command.
   bash

   # include the directory name to enter it, for example
   cd Desktop
   # use two dots to jump up one directory level if you need to
   cd ..
   

4. Enter the command to start up the server in that directory:
   bash

   # If Python version returned above is 3.X
   # On Windows, try "python -m http.server" or "py -3 -m http.server"
   python3 -m http.server
   # If Python version returned above is 2.X
   python -m SimpleHTTPServer
   

5. By default, this will run the contents of the directory on a local web server, on port 8000. You can go to this server by going to the URL localhost:8000 in your web browser. Here you'll see the contents of the directory listed — click the HTML file you want to run.

The best approach for working with server side languages, such as Python, PHP, or JavaScript, depends on the server-side language you are using, and whether you're working with a web framework or "stand-alone" code.

If you're working with a web framework, usually the framework will provide its own development server. For example, the following languages/frameworks come with a development server:

• Python web frameworks, such as Django, Flask, and Pyramid.
• Node/JavaScript frameworks such as Express Web Framework (Node.js/JavaScript)
• PHP has its own built-in development server:
  bash

  cd path/to/your/php/code
  php -S localhost:8000
  

If you're not working directly with a server-side framework or a programming language that provides a development server, Python's http.server module can also be used to test server-side code written in languages such as Python, PHP, JavaScript, and so on, by invoking server-side Common Gateway Interface (CGI) scripts. For examples of how to use this feature see Execute a Script Remotely Through the Common Gateway Interface (CGI) in How to Launch an HTTP Server in One Line of Python Code on realpython.com.