I was trying to load a local file in a HTML page and got a Cross-Origin Request Blocked error in my browser. The solution was to start a Python web server with python3 -m http.server.

I had a JSON file I wanted to load into Javascript in a HTML page. Looking at StackOverflow I found I found fetch could do this

fetch("test.json")
    .then(response => response.json())
    .then(json => process(json))

Where process is some function that acts on the data; console.log is good for testing. Reading about using fetch shows that it works similarly to XMLHttpRequest, or jQuery's getJson, with slightly worse browser coverage. See the hospodarets article on fetch for more on using it.

When I tried to do this I opened the HTML file locally in Firefox and in the console got an error message:

Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at file:///...

Trying to request a file from HTTP gives this error because it has security implications.

The simplest workaround is to serve "test.json" from a server. This is trivial with Python's inbuilt HTTP server. Just go to the directory contianing the HTML and Json files in a shell and type python3 -m http.server to start a server. Then you can open the HTML file at http://localhost:8080/<filename.html> (or if you call it index.html you don't need the exentsion) and it will be able to load in the JSON file.

In production you would be running this behind a robust HTTP server like Nginx or Apache, but for quickly testing locally Python's inbuilt server is useful.