Origin null is not allowed by Access-Control-Allow-Origin. Status code: 200 in JavaScript?

Question

I have been trying to access a local json file in javaScript using fetch (as shown below).

fetch('./js/data.json')
  .then(response => response.json())
  .then((data) => {
    console.log(data);
  })
  .then((error) => {
    console.log(error);
  });

I found out from this that I can not access the json file localy and not to access it from a https sever on localhost.

I set this up using ngrok.

fetch('https://a9f8-86-133-120-217.ngrok.io/js/data.json', {
    method: 'GET',
    headers: {
      'Accept': 'application/json',
      'Content-Type': 'application/json',
    }, 
  })  .then(response => response.json())
        .then((data) => {
            console.log(data);
        })
        .then((error) => {
            console.log(error);
        });

When I now run the code I get the following errors.

[Error] Preflight response is not successful. Status code: 501

[Error] Fetch API cannot load https://a9f8-86-133-120-217.ngrok.io/js/data.json due to access control checks.

After some research, I found this was due to working called cors.

I then added mode: 'no-cors' to my code. This then meant that the JSON file was being fetched but it was casing more error.

Unhandled Promise Rejection: SyntaxError: The string did not match the expected pattern.

The error is from the following line.

.then(response => response.json())

When I remove 'Content-Type': 'application/json' I get the following error.

Origin null is not allowed by Access-Control-Allow-Origin. Status code: 200

I have been trying to fix this for a few hours but have had no luck.

I feel this is something so simple that I'm missing.